Building ventilation diagnosis, improvement, repair, inspection, standards & specification questions & answers.

FAQs about Building Ventilation Design, Inspection, Diagnosis, Cure - articles: How to Inspect, Diagnose, & Repair Ventilation & Fresh Air (or Stale Air) Problems in buildings.

Building Ventilation Design, Inspection, Diagnosis, Cure FAQs

These questions & answers were posted originally at VENTILATION in BUILDINGS - home. Be sure to see that list of building ventilation problem diagnosis and cure articles.

Our sketch illustrating a wind-washing effect on attic insulation is provided courtesy of Carson Dunlop Associates, a Toronto home inspection, education, & report writing firm.

On 2020-07-21 by Dan K. - problems with my un-vented "hot roof"

I will try to clarify

Current System (pre-demo):

-Unvented Rafters consisting of 2 x 12 boards spaced 24" on center.

-R38 Kraft faced fiberglass insulation was used and it filled the entire cavity. I did not provide a space for venting (top or bottom).

-I then put a 6 Mil poly vapor barrier under the insulation (against bottom of rafters making contact with kraft faced paper on insulation).

-5/8" drywall taped and finished.

The problem:

-roof leak at ridge due to storm

-condensation coming through vapor barrier leaks near recessed lights and ridge beam.

Current status:

-pulling down all drywall and 6 mil poly vapor barrier.
-drying out all insulation (only moisture found other than at roof leak area was located between 6 mil poly and kraft faced paper. The pink fiberglass insulation was completely dry). Plan on buying new what doesn't look re-usable per your below comments.

Current plan for re-install (Unvented System):

-Install 4" rigid insulation board (R-20 vapor impermeable) against OSB roof sheathing. Tape edges and seams (Nebraska IRC 2018)

-Re-install R38 kraft faced fiberglass insulation

-Install 1/2" Rigid insulation board on bottom side of rafters taped at seams to create interior air/vapor barrier. (IRC will not allow Class I barrier on unvented rafter systems in my climate zone. If I read correctly, the rigid insulation is a class II barrier that if taped can perform the same as the drywall without the labor of tape and mud.

-Install T&G Pine on top of 1/2" rigid insulation. The pine would be stained and coated with polyurethane.

Questions I have prior to re-install:

1. Will the 1/2" rigid insulation perform as an air/vapor barrier the same way drywall would? If I could skip the painful labor process of mudding and taping I would like to.

2. Can I install the T&G Pine directly against the 1/2" rigid insulation?

To clarify my understanding of a Vented Rafter System I offer the following. Correct me if I am wrong:

I 100% understand that majority of moisture will enter the cavity from the inside; but on a vented system my understanding is that you still install an air/vapor barrier on the inside to reduce as much of this as possible. If moisture were to breach the interior air/vapor barrier and enter the cavity it would be extremely slow to dry through the top vent as the rafter baffles are vapor impermeable and taped at seams (Nebraska IRC).

Only way for interior moisture to vent would be to slowly absorb into 2 x 12 rafters; which may be too slow to prevent mold/rot. In the event moisture entered from the roof, it would vent rapidly and could not damage insulation below as the rafter baffle would keep the moisture contained until it has time to dry. My conclusion to this is that the Vented Rafter System only really provides moisture venting from roof leaks. Any moisture coming from interior would vent so slowly the mold/rot would still likely occur.

Any corrections/input on my questions about 1/2" rigid board would be appreciated.

R/,
Dan

On 2020-07-14 - by (mod) -

I don't think we've got this exactly right nor completely sorted out but I have a couple of comments to offer.

One is that I don't have the same understanding of venting as you.

Roof cavity venting is intended to remove moisture and that may enter the roof cavity from the interior not only from the exterior of the building

in fact moisture is more likely to enter the roof cavity from the interior from air leaks where is water or moisture leaking into the roof cavity from the exterior is a roof leak.

Second I see what looks like a lot of moldy material.

If that were my home I would continue removing insulation and drywall until I had at least two feet of clean material or one rafter bay margin of mo li d free material around the suspect area.

Also in one of your photos it appears to me that the vent cavity was on the wrong side of the ceiling cavity,

that is, it was located just above the drywall instead of between the top of the insulation and the underside of the OSB roof deck. If the insulation was carried all the way to the ridge there's no way that that your vent path would work even if there were air flow entering at the eaves.

I would not reuse installation that was removed if it was visibly moldy or even close to other moldy materials.

I would hate the thought of having to do the job all over again because two years from now the space smells moldy. Insulation is cheap by comparison.

On 2020-07-14 by Dan K.

On 2020-07-14 by Dan K.

On 2020-07-14 by Dan K.

On 2020-07-14 by Dan K.

Ok. So I think you have convinced me to stick with the unvented roof. I was thinking the vent would have been my failsafe if interior condensation were to get into the cavity; however, after reading this and further research realized the vent is only for moisture coming from the roof exterior and interior condensation is unlikely to be vented out. Therefore ridge and soffit vents not worth the extra $.

Following your suggestions and going through Nebraska residential code I think I would like to do the following but have some more questions:

-4" Rigid Foam (R-20) against bottom side of OSB roof sheathing (Owens Corning Foamular Polystyrene)
-Re-install my R38 fiberglass insulation below that
-Install a continuous layer of 1/2" foil faced or rigid foam board that is rated as Class II vapor retarder on bottom side of rafters. Taping all seams to act as my interior air/vapor barrier.

My additional questions are:
1. Should I remove the kraft paper backing on the R38 when reinstalling since it is also a Class II vapor retarder? In other words, is it bad to have the kraft backing paper touching the 1"2 rigid foam board I'd like to install on the bottom side of rafters
2. I'd really like to not do drywall. Is it possible to install the T&G pine directly on top of the 1/2" rigid insulation board? Is this a bad idea?

I'll post some pictures so everyone can see my mistake as I believe the moisture entered in several places. First being the ridge that had metal shake up and tear in membrane (high wind storm in May with 6" of rain that followed), interior condensation at ridge beam as the tape that sealed the 6 mil poly came loose, interior condensation around recessed lights, and interior condensation that I believe came in around the upstairs bathroom vent.

On 2020-07-06 - by (mod) -

Dan: at

INSULATION LOCATION for CATHEDRAL CEILINGS

we describe a high-labor method that combines providing an air barrier above fiberglass insulation in a cathedral ceiling to avoid heat loss by air movement in the roof, combined with the addition of solid foam insulation to provide a much higher R-value for the completed cathedral ceiling structure. -- DF

I built this design first in the 1970s - and can say that the building worked so well that we had to get rid of our Jotul woodstove as the heat drove us out of the insulated room. (I traded the Jotul to Paul Galow for a cool wristwatch.)

If you want a still higher R-value ceiling you can add another layer of Hi-R foam on the ceiling side (I did that), taking great care that the ceiling is sealed - no leaky penetrations - since any air leak into the cavity brings moisture that will accumulate.

Also see

CATHEDRAL CEILING VENTILATION

HOT ROOF DESIGN PROBLEMS

I am familiar with the 'leave out the plastic' argument. In my opinion and experience, there is almost no leakage through drywall on a ceiling; nearly all of the air leakage that brings in and traps moisture from the occupied space comes at penetrations, with a bit at seams if they're not well sealed.

So you don't "need" the plastic by that account. And lots of us leave it off in modern construction, though 50 years ago the poly was de rigeur.

There is a claim that moisture entering the wall under some conditions will exit the wall under others- the wall breathes. In my experience walls choke more than they breathe. If air pressure and penetrations push moisture into a wall or ceiling cavity, say around an electrical box, moisture accumulates there and does not simply exhale soon enough to avoid any risk of mold or other moisture-issues. Plastic won't prevent that as the plastic is going to be cut around every electrical box or other penetration. Sealing at those penetrations can help.

In my 1970's super hi-r cathedral ceiling we added a layer of foil faced foam on the room side of the rafters, foil taped the joints - so our ceiling, sans penetrations, had a perm rate close to zero. The trick was to mark the rafters with care and use long drywall screws to install the ceiling drywall screwing through the Hi-R foam into the rafter bottoms.

Over the ensuing decades until I sold that house a few years back, we took care to inspect the roof, tolerate no roof leaks, and we never had a problem with moisture in the roof cavity - not from either side of the roof.

OPINION:

There is no benefit to ridge and soffit venting if you do NOT have sufficient under-roof air space to actually move air up from eaves to ridge.

1 1/2" baffles often don't give you 1 1/2" of air space - pushing insulation into the cavity squashes the center of the baffle, and because the baffle has flanges you're not venting the whole cavity anyway - out of a 15 1/2" wide space between rafters you're losing maybe 4" at baffle sides as well as the squash problem.

That's why some of us used the DIY labor-intensive furring strips and solid foam "baffle" design I sketched for you below. You get more air space (I used a 1x2 and today might use a1x3 to get a true 1.5" or 2.5" space) and you get added R-value. The gripe that you're making a moisture trap sandwich is a risk with all of these designs - those thin flexible styrofoam baffles don't pass moisture either.

Taping the baffle sides is not useful; any moisture getting into the cavity will do well if it can find its way up and out through the vent space.

On 2020-07-05 by Dan K.

Mr. Friedman,

Thanks for your input. I'll have to put Castle Danger Ale on my short list of beers to try next time I'm up that way. So I've pulled down drywall and insulation to dry out. . Aside from the roof leak at the ridge, I think I confirmed the condensation leaking through vapor barrier in several areas (most notably near the upstairs shower). I'm still struggling with vented/unvented. I think I went wrong by installing the vapor barrier in the first place. Reading up on Nebraska residential code, it specifically calls out not to install one with an unvented rafter. It reads as follows:

Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met:

2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.

Because I now am invested in taking down all drywall to remove vapor barrier I am considering a vented system, but I cannot find a definitive answer on how to go about insulating a vented rafter in climate zone 5 (Nebraska). Here is what I am thinking and looking for the right design if this is not it:

Vented Rafter System - From Top Down

-Metal Roof (ridge vent and vented soffits)
-Underlayment
-OSB Roof Sheathing
-Minimum 1.5" Rafter Vents/Baffles taped on edges and seams (any suggestions on brands would be helpful)
-R38 unfaced fiberglass pink insulation
-1" Foil Faced Ridged insulation taped on edges and seams
-1/2" drywall taped and painted.

Can you/someone verify this is a correct design for my climate zone (5)? I've seen so many designs I am unable to confirm what is correct. I've also seen what I described above with a 1 x 3" spacer used between the Foil Faced Rigid Insulation and the drywall to establish a 1" gap between them.

On 2020-07-01 - by (mod) - unvented roof problems & solutions

Dan

Yep that's what I'd call a "hot" roof or "un-vented" roof design.

I'd have completely filled the roof cavity with insulation and I'd have taken care to seal any penetrations into the ceiling to avoid air leaks - that's the No. 1 source of moisture trouble in un-vented roof cavities. The poly barrier is ok though since learning that most of the air leaks are at openings not "through" the drywall, it's less critical.

When you have water dripping from the fan I'd bet a 6-pack of Minnesota's Castle Danger Ale (the best) that there are air leaks into the roof cavity, with a second possibility of a leak from above.

What can happen in either case is water accumulates in the cavity on the cavity side of the poly barrier, until it finds a way to drip back out. Just as you surmise.

In my OPINION there's no value in adding ridge and soffit vents as there is no venting air space above the insulation - you'll get almost no air movement through the roof cavity.

I would remove wet insulation and wet drywall and toss them out.

Let the roof cavity dry.

Be sure the roof isn't leaking from above.

Restore the insulation.

Seal the penetrations like around that fan.

Put off the nice looking T&G pine until you've been through 4 seasons and are sure there's no more air leaks into the roof nor water leaks in or out - lest you have to do the whole demo and repair again.

On 2020-07-01 by Dan K.

Needing some assistance with my unvented roof. First of all, the roof is a sealed rafter system made of 2x12's. There is no attic. I think this would be considered a "hot roof" design if I followed some of your other articles on this website. Anyways, I built the house and may have made a DIY rookie mistake.

After having a roofing company install a metal roof and soffits (no ridge vent or soffit vent), I filled the inside rafter cavity with fiberglass batt insulation with backing paper on one side (R38 completely filled space, touching roof sheathing OSB), put a 6 mil poly vapor barrier on ceiling side, and then drywalled and painted ceiling. I've lived in the house for almost 5 years, never noticed any issues.

This year we had water dripping from the ceiling fan. I had the roofing company come inspect the roof and there was a ridge cap shingle up a little bit and a tear in the underlayment near it right in line with the ceiling fan.

Problem is that the underlayment tear has been there since installation 4 years ago. I removed a piece of the ceiling drywall and found that there was water between the vapor barrier and insulation backing paper near the ridge - insulation was dry. I'd like to assume that water had been leaking in for quite awhile and filled the vapor barrier slowly from the roof leak.

I'm fearful that the water leak was actually condensation build-up that finally leaked out through a weak spot in the vapor barrier (ceiling fan). I don't think I sealed the vapor barrier very well at the ridge beam.

Either way, I have a problem now. The moisture build up needs to vented. I think I've decided on a painful course of action and need someone knowledgeable to confirm it will work. here is my plan:

1. Have the roofing company come back and vent the ridge and soffits

2. Remove sections of my ceiling drywall and remove vapor barrier in its entirety

3. Remove wet insulation to dry.

4. Install plastic (Provent from Home Depot) rafter baffles against underside of roof sheathing (OSB) to create a 1.5" airway.

5. Re-install R38 fiberglass with backing paper insulation.

6. Re-install drywall.

7. Install 1 x 6 T&G Pine (what I was going to do when building but ran out of time and had to settle with drywall).

Will this work? I am essentially venting the rafters and removing the vapor barrier. Still using the same R38 fiberglass insulation with backing paper.

After reading some posts, I feel I made a bad design choice.

On 2020-01-10 - by (mod) -

Anon

To discuss CO or carbon monoxide levels in flue gas we need to start with

1. what fuel is being burned - LP gas, natural gas, or No. 2 home heating oil?

2. what type of equipment: conventional draft, forced draft, condensing furnace or boiler?

Bottom line: the actual CO or carbon dioxide level could be quite high, well over 400 ppm, a level at which a 3 hour exposure may be fatal,

Watch out: such equipment should be shut down immediately as it is unsafe.

The CO level continues to increase if a fuel is being burned and oxygen is decreased below the required level. I can't give an absolute upper limit on the CO that might be reached - as I said in starting this reply.

We have published a series of article giving details about carbon monoxide;
a good place to start, found by using the InspectApedia.com on-page search box to search for "Carbon Monoxide"

is CARBON MONOXIDE - CO

where we discuss CO exposure limits

On 2020-01-09 by Anonymous

maximum concentration of Carbon Monoxide in a vent of a boiler

On 2019-10-11 by (mod) -

Patricia

Vent must be extended to the building exterior, it may not terminate in the Attic.

On 2019-10-11 by Patricia

This is not an individual home. It is a three-story building

This is a condo setting, therefore there is a crawl attic

Is it legal to vent a bathroom into an attic that is a common area ie: third floor condo

On 2019-08-24 by (mod) - whole house fan/venting

Mike:

Is your whole house fan installed in the top floor ceiling & thus will vent out of any roof vents available: both ridge and soffits?

Or is it a roof exhaust vent fan installed in the roof itself - in which case when it's on the fan will suck air in at ridge and soffits and wont' be so great at venting the house ?

On 2019-08-24 by mike maruska

whole house fan/venting
Today, 10:04 AM
New build for son . I plumber 50 yrs. Industrial. 785sq ft ranch on crawl. roof has 12 ft ridge vent. Spec says vented sofetts-looks like 112' linear .

Builder installed home depot cheap 200$ whole house fan (kid smokes.) Sounds like helicopter-wife left house when he turned on. "he can turn it on to let het out then shut down" builder says trying to sell install-
I found a quiet cool fan for 450 dollars with no sound (43 dba) I want instead of his cheapo oversized 3000 plus cfm fan.
Issue now is venting. I need 2.5 cfm per sq ft as per recommended 3.0 is better. 780 x 2.5= 1950 cfm ---so the quiet cool 2250 cfm is the plan if possible.

My ridge vent is 18sq nfa inches per foot or 216 sq inches (12 ft long). soffet vent is ventilated dont know wha he will use there but 125 linear ft.

will this whole house fan even work absent holes cut in roof for vents to accomadate the 2250 quiet fan altrnative to his loud oversized cheapo?
thought i read 1/2 ridge vent 1/2 sofitt vent somewhere.
advise best way to go. paid 950$ for cheapo installed and would like to save whole house fan since he smokes -if possible.

On 2019-05-28 by ruth dickey

we have 1904 sq. ft. crawl space, doublewide, , we have custom wood skirting, , we have the original vapor barrier on the ground over the clay dirt floor, the top overhead is all buttoned up nice, , however, we have added numerous passive vents, and we get a lot of rain and humidity, , , there are noi leaks, , and rain water doeds not get in, we have gutters with extenders, to trenches, away from foundation, plus a dith for raian water in the rear, extending around one side of property for , so it does not get under crawl, however, also our hvac does not work, we do not use heat or air, we have wood stove in winter, and fans in summer,

so that is not a factor in the moisture of the crawlspace, m,, this wood skirting does not breathe, and the 8x14 inch or so soffit vents, they give you are not really 1 sq. ft. of passive vent air, that's recommended for every 150 sq. ft.

so we are going to add more passive air vents, and if we have to, , we ll add a new 6 mil plastic floor vapor barrier over the old one, a job that I dread, because the skirting has to stay on the whole time, and , in spots, its less than 15 inches high, with the anchor straps there and everything, we cannot get into a dehumidifier, they are very expensive, and this ,

vapor barrier under there, has sharp rocks all over under it, , its like Vietnam under there, with full knee and arm pads, ect., , I believr its just not getting enough venting, mainly, down the wide middle part, , and the jerks whpo installed it, blocked off two of the rear vents, , too low to the ground, where the metal frame is blocking them, ,

plus the wood skirting does not breathe, we chodse wood over other types for , strength, weed eaters ect., and for the cold winter months, , please advise., , will we be better off with steel skirting, and if so even though we cannot afford that for quite a while, what about in the winter?, we have heatape for pipes, but, again these 8x14 or so soffit type vents they give you are only half of the recommended 1 ft to 150 ft of space, thank you for your answers.

On 2019-05-15 by (mod) -

You should indeed follow the advice of your inspector. Even a smsll change in conditions can produce fatal carbon monoxide.

On 2019-05-15 by ken

My merchantal vent system on my oil boiler looks to be to low. Although I had no problems with it for 20 years an inspector came in and said I need it a
foot high off the ground. Right now it is only a few inches of the ground. Never had any carbin monixide problems. Can you help me ?

On 2019-01-06 by (mod) -

Jerry:

Thanks for asking about cold air falling into the bath from a ceiling exhaust vent: it's probably an event that irks lots of people, especially in winter.

Cold air will fall down out of a ceiling exhaust vent fan for at least the following reasons:

1. there is a source of air that can enter the fan and duct system, such as a wall terminating fan vent that is not closing

2. cold air is more-dense, or heavier, than warm air, so if there is enough cold air supply in the fan exhaust duct, once it's in the duct system it can fall "down" in some locations, overpowering the natural wish of warm air to rise and escape out of the same path.

3. Wind or outdoor air currents blowing air into the exhaust fan terminating opening can be a big contributor to cold air backflow through the exhaust fan opening.

Take a look at the bath exhaust vent fan articles in the series starting at

BATHROOM VENTILATION CODES SPECS - home - https://inspectapedia.com/ventilation/Bathroom_Ventilation.php

for details on proper exhaust vent fan ducting, location, etc.

On 2019-01-04 by Jerry

What would cause cold air to come out of fan vent in ceiling of bathroom? stays cold you can feel a draft also. Thanks,

On 2018-03-22 by (mod) -

I have not seen such a requirement but I will research your question further.

There is a basic assumption with exhaust vents that any adjacent vent is working correctly, that is, it closes when it's not active. That's to assure that there's not a backdraft from another nearby exhaust nor from any other source.

On 2018-03-22 by Geode

I would like to know if there is a code requirement for minimum distance between two bathroom exhausts through an exterior wall, each separate ducts and separate exhausts.

Both would run through attic space thru gable end. They would be well away from windows and no other exhaust or intakes located in same wall. I don't see an issue especially if both had baffles.

 

On 2018-03-22 by (mod) - code requirement for minimum distance between two bathroom exhausts

I have not seen such a requirement but I will research your question further.

There is a basic assumption with exhaust vents that any adjacent vent is working correctly, that is, it closes when it's not active. That's to assure that there's not a backdraft from another nearby exhaust nor from any other source.

On 2018-03-22 by Geode

I would like to know if there is a code requirement for minimum distance between two bathroom exhausts through an exterior wall, each separate ducts and separate exhausts. Both would run through attic space thru gable end. They would be well away from windows and no other exhaust or intakes located in same wall. I don't see an issue especially if both had baffles.

On 2017-11-30 by (mod) - any benefit to having bigger duct

Ted,

I'm pretty sure we answered this question on another page. If I'm wrong you're welcome to use the page bottom contact link to shoot me an email.

The basic concept is that increasing the diameter of piping anywhere in a piping system will improve the overall flow rate, all else being unchanged.

On 2017-11-30 by Ted Taala

Hi all,

Question #1: If most of a bathroom exhaust duct run is 3" diameter steel pipe, is there any benefit to having bigger duct (4"-6" round galvanized steel) at either end, and/or a bigger roof cap? I'm guessing that there is a benefit to the average values of the run as a whole, but what would be the point of diminishing returns?

Question #2: How does one calculate air volume, flow, etc., when a run consists of differing diameter duct (i.e. 2' of 4" duct to 18' of 3" duct to 5' of 4" duct? I'm guessing you calculate the values for each section, then average for total length. For EDL, figure for each section then total?

Question #3: Aside from increased noise, what are the major drawbacks to reducing a 4" fan exhaust to a 3" duct? Or a 6" fan exhaust to a 3" duct?

I'm installing a bathroom exhaust fan in my master bathroom and retrofitting two other fans. For the master and hallway bath, both on 2nd flr, I'm installing 6" galvanized round duct and roof jacks. No issues there.

The problem is the guest 3/4 bathroom on the ground floor. I haven't traced out the duct run yet, as access is very limited, but based on house dimensions, room locations and what I can see through the ceiling opening and in the attic, it looks like it has a short length (maybe 2') of 4" vinyl flex attached to 3" steel pipe (real pipe, not thin-walled duct). I estimate the pipe runs about 8' horizontal, then about 10' vertical into the attic, with at least one, probably two, 90 degree elbows, or one 90 and two 45s. In the attic, 4" vinyl flex is attached to the pipe end and runs vertical around 5' to a 4" roof jack.

My first choice would be to replace the current stuff with 4" galvanized duct directly to the nearest exterior wall (~14' straight run) and exhaust through a wall cap, but I think this may not be feasible without tearing out major stretches of walls and ceiling. If I'm stuck with the 3" pipe, I'm exploring what I can do to make the best of it.

The bathroom is small (47 sqft) with no exterior wall. The current fan (Utilitech model #7108-03-L) is 110 cfm and airflow is strong at the roof jack. House framing is steel, and I live in Hawaii so there are no insulation concerns.

The fan I'm looking to install is a Panasonic WhisperGreen Select FV-05-11VKSL1, which will run constant at 30-50CFM and kick up to 110CFM with a wall switch. I would actually prefer the 150CFM model due to the length of duct run but I think that may not work well with 3" pipe.

The shower in this bathroom doesn't get much use but my cat's litter box lives in there and the toilet gets used a lot, so fan is mostly for odor-control. The bathroom opens on to a juncture of guest room, dining room and kitchen, so bathroom smells are very unwelcome.

Besides answers to my questions at top of post, any ideas or recommendations would be very welcome. Thanks much!

On 2017-03-28 by (mod) -

Super. We also welcome your questions, critique, content suggestions.

On 2017-03-28 by Sheila - my fresh air intake ducts were too small

Thanks

I truly appreciate your help. I will let you know what I learn and I will read those articles you suggested. Thanks again. Sheila

On 2017-03-28 by (mod) -

Sheila

Do keep in touch and let me know what you're told: I may be able to suggest questions to ask.

Just to avoid confusion about what sort of air is needed at an hvac system, you might want to read

RETURN AIR, INCREASE

or see the whole series of articles beginning at

RETURN AIR REGISTERS & DUCTS - home

Inadequate return air, regardless of the reason for it, will mean inadequate air flow out of the air handler and increased heating or cooling costs.

We do NOT want to over-do the use of outdoor intake into a residential HVAC system. Take the extreme case as a clear example:

if ALL of the air entering the air handler were coming from outdoors we'd have a "one-way" heating or cooling system: we get an armload of new air, heat or cool it, then blow it into the living area. That's the most-expensive-possible way to operate a system.

Daniel

On 2017-03-28 2 by Sheila

Thanks so much for that advice. It gives me the direction of who to ask. I didn't know where to begin before. I appreciate the help.

On 2017-03-27 by (mod) -

Sheila

I think you're asking a reasonable question but not one that can be answered by e-text. Certainly there are plenty of residential HVAC systems with no fresh air intake - an HVAC design professional would perhaps take a look at your building and your requirements - and could give an impartial, expert plan.

On 2017-03-27 by Sheila

Hello, I recently had a high efficiency furnace installed and I asked the installer to also sell me a central air conditioning system, but he refused saying my fresh air intake ducts were too small, and that's why there was a clicking or tapping sound in the basement duct work when the furnace finished a cycle.

Do I need to expand my two fresh air intake vents that are only four inches by 8 inches and located in the front hall and in the dining room or is there a better way? The furnace now pulls air from outside through a PVC system. I live in a 100-plus-year-old house that is 2 1/2 storeys (finished attic with ducts that supply heat to the top floor).

I've used window air conditioners in the past, but would like something more efficient. Many thanks for any advice you can offer.

On 2016-12-31 by (mod) - My eyes watered and my nose started running after floors were re-finished

Donna

You need to find the brand and specific substances used to clean, prep, seal, and coat the flooring. That will allow you or us to find the MSDS for the product.

On 2016-12-31 by Donna

I work part time at a fitness center for our local town in NC and they just refinished the floors the night before.

No windows to open and when I came in to work the next day the place the odor was strong. My eyes watered and my nose started running.

The full time staff are off for the weekend but the part time staff had to work. I opened the doors and turned on a fan when I came in but the fumes are overwhelming. How can I find out if this stuff is toxic?

Question: How to fix "Leaks" or condensation problem in roof at un-vented valley rafters?

I just built a new house consisting of cathedral ceilings throughout. I installed perforated soffits along the bottom of the eaves, however, I mistakenly forgot to vent the roof along the valley rafters spanning approximately 30' in length. I have discovered a leak at both valley only occurring in the spring time when the temperature finally rises above freezing.

I strip back both valleys and re shingled using snow and ice shield as well as torch down roofing, it still leaked at first snow. Is my roof sweating from improper ventilation?

I also ran the ridge vent continuously along the peak, should I only have this where the soffit is vented?(not above the valley). I ran styrofoam baffles under all of the sheathing even at the valley rafter, could this be warming the roof where they are not vented to the soffit? Can somebody offer some helpful advice? - Scott

Reply:

Scott, the ice and water shield approach can stop leaks around valleys - and is a good practice.

The fact that you observed leaks when temperature rises above freezing suggests that a pile-up of snow or ice in the roof valleys is melting, that the water load is exceeding the present roof valley's ability to drain without leakage, and that thus water is finding its way under the shingles past the valley and into the building.

Waterproofing three feet on either side of most valleys will seem to "fix" this leak, but we really don't want to leave water trapped under the roof shingles either - that can be a source of roof damage and reduced roof life.

So in addition to the ice and water shield membrane improvement, take a look at roof valley design details including bends and sealants that prevent water from passing out of the valley and into the roof structure.

These three articles should be helpful in curing leaks at your roof valleys for all weather conditions:

• VENTILATION, ROOF, BEST PRACTICES
• ASPHALT SHINGLE VALLEY FLASHING
• FLASHING MEMBRANES PEEL & STICK

Question: how do you control air flow from one room to another

how can you control air flow to the next room - George 4/19/11

Reply: Tips for Controlling airflow between rooms in a building

George:

Assuming that your rooms have doors, a simple test can sometimes give us a clue about relative air pressure and air movement between building rooms. With the HVAC system or ventilation fans of interest running for five or ten minutes, and with all room doors shut, we open a door between rooms about one inch.

Even without using instruments we might feel air movement between the rooms, and in some cases we've actually seen the door blown further open or shut depending on the direction of air movement.

If we find that a room is pressurized by the HVAC system when the door is shut, in particular when the room is heated or cooled only by supply ducts (without a local return air duct), we know that providing a method for air to flow out of the room (and presumably on towards the central air returns) will increase the inflow of conditioned air into the room - better heating or cooling performance will be the result.

Airflow between rooms as well as control over that air flow is made possible by:

• doors remaining open or shut
• doors that are undercut (or overcut) to provide 3/4" to 1" of open space for air movement
• a through wall opening, usually near wall top, in a partition between the rooms, covered with louvered grille
• a through wall opening, usually near wall top, in a partition between the rooms, covered with louvered grille and equipped with a circulating fan that blows air in the desired direction.

If your rooms have no doors to close between them you may still have some impact on air flow from "room A" into "room B" by studying how air leaves "B". If room "B" has a window, vent, or door that is providing an exit path for airflow, closing that can slow air entry into that room from "A".

Take a look at SMOKE PENCIL / SMOKE GUN SOURCES

and at BLOWER DOORS & AIR INFILTRATION for tools and methods for tracing air flow in and out of rooms.

Question: Perforated soffits, no ridge vent, cathedral ceilings: condensation or roof leaks?

I just built a new house consisting of cathedral ceilings throughout. I installed perforated soffits along the bottom of the eaves, however, I mistakenly forgot to vent the roof along the valley rafters spanning approximately 30' in length. I have discovered a leak at both valley only occuring in the spring time when the temperature finally rises above freezing.

I strip back both valleys and re shingled using snow and ice shield as well as torch down roofing, it still leaked at first snow. Is my roof sweating from improper ventilation?

I also ran the ridge vent continuously along the peak, should I only have this where the soffit is vented?(not above the valley). I ran styrofoam baffles under all of the sheathing even at the valley rafter, could this be warming the roof where they are not vented to the soffit?

Can somebody offer some helpful advice? - Scott 11/11/11

Reply:

Seems to me that if the problem were really a roof leak it would not occur only in spring. So yes, I would start looking for an indoor moisture source and condensation trap. Your valley rafter roof section has no intake venting so that could be related.

Look for ceiling penetrations sending moisture into the roof cavity. I would consider making an inspection cit into the ceiling under the "leak" area to inspect the roof cavity avd insulation for trapped moisture, stains that may help diagnose water passage or sources, and for a hidden mold or rot problem.

Let us know what you find, and send photos of the roof problem areas outside and inside and we may be able to offer more useful comments.

Check for ice dam leakage as well.

Question: driving rain leaks into dormers?

I have old fashioned dormers. The driving rain and snow accumulation seems to get into dormers and cause leaks down below. Can I construct a shield in front of dormer that will permit air flow but shield against the driving rain and especialy built up snow that melts into the dormer? - Ed Harrison 8/13/12

Reply:

Ed, first we need to identify where driving rain or snow are leaking into your dormers.

Posting your question here suggests you think it's entering through a roof or ridge vent? If so, check that the vents have not lost the usual baffles or flashing that is designed specifically for this protection.

Question/Reader comment:

Thank you for your sharing, it was really helpful to solve the ant problem in my house.

By the way do you think is it good idea? Because actually I already tried it before.
But I will try again according your article saying. Thanks! Have good one! Shafiqur

Question:

(May 29, 2014) Pete said:
Does the cold air return duct placed in the marriage wall of my modular home need to be moved and the marriage wall resealed?

Reply:

I can't really say, Pete. You probably have a reason for asking but a duct problem was not stated.

Question:

(June 22, 2014) Victor said:
No ridge vent. Two louvers in gable ends. Should I use perforated soffit under the front and back overhangs?

Reply:

Victor in North America and some other countries gable-end-only venting was a traditional practice into the 1960's. It is not effective at cooling the attic except under a few wind conditions combined with large opening and exit vents, and it won't cool and dry lower roof edges.

If you add soffit intake venting that will help by allowing air to enter at those lower roof edges, moving by convection upwards as the attic air is heated by sunlight; that opening will also reduce the tendency of exit-only attic vents to suck heat and conditioned air out of the building (increasing energy costs).

But optimum would be to install soffit and ridge vents and then close off the gable end vents.

Question: We temporarily closed the gable vents

(Aug 22, 2014) Anonymous said:
We have a static vent on the roof of our mobile home (a #301 MAXIMUM vent - since July). We have soffits (10'') - (this also since July) as well as gable vents. We are having ventilation problems. The hot air from the attic is entering our living space.

We temporarily closed the gable vents (as best we could) and this seems to have solved the problem. However when we open the windows there is a smell as if the air from the attic is coming down the soffits and entering the house. Is this possible. Our mobile home is 66' x 14' and we are in Canada.

We are wondering if the MAXIMUM vent on the roof is doing its job considering that the house is 66' long. D.S.

Reply:

Anon

If the roof intake venting is insufficient then it's quite possible that the negative pressure created by your roof exit vent is increasing air leakage into the attic space.

Question:

(Mar 30, 2015) Laurie said:
Who is responsible for installing a gas dryer with no exterior ventilation? My home warrenty did this and I was told it was illegal and very dangerous

Reply:

The danger depends on more than no exterior venting: is this a gas dryer or electric?

In any case, the appliance installer would be expected to either connect the dryer to an exterior vent or to tell you that you need one.

...

Continue reading at VENTILATION in BUILDINGS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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