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Wind washing (C) Carson Dunlop Associates Building Ventilation

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Here is our index to key articles discussing building ventilation diagnosis, improvement, repair, inspection, standards & specifications:

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

These articles explain how to inspect, diagnose and repair ventilation problems in buildings.

We suggest building ventilation methods and we describe the pros and cons of different ventilation approaches used to assure good indoor air quality, low building energy usage, and minimal building moisture problems.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Building Ventilation Design, Inspection, Diagnosis, Cure: key articles

Window condensation (C) Daniel FriedmanInformation is provided about visual clues of building condition as well as direct inspection for the presence or absence of proper building ventilation systems, pathways, and moisture and air barriers to stop leaks where we do not want ventilation.

Our page top sketch explaining the wind-washing effect on attic insulation is provided courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

At left we show an example of severe indoor condensation at a window.

In the articles listed here we explain how to identify and correct various building leaks, moisture, and venting problems such as ice dams, blocked attic ventilation, excessive indoor humidity, how to prevent indoor mold, and how to respond to building floods and similar emergencies.

Our photo (above-left) shows extreme condensation at a building basement window.

Moisture at this level risks severe mold growth which can be costly to clean up, and over a longer time this condition invites rot or insect attack on the building.

Below is our list of articles on the diagnosis and cure of building ventilation problems.

To find what you need quickly, if you don't want to scroll through this ventilation design, equipment, installation & troubleshooting ARTICLE INDEX you are welcome to use the page top or bottom SEARCH BOX to search InspectApedia for specific articles and information.

Research & References for Building Ventilation

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

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On 2022-11-23 by InspectApedia (Editor) - AirCycler FR-V was never hooked up

@Jim Ryan,

Before we connect any wires on one end you need to know where those wires go on the other end of that run.

Otherwise the connections could be arbitrary and potentially damage the equipment.

I'm saying that I don't assume that the color codes that you see at the end of the wires that were Twisted together tell us correctly how those wires actually are routed and I suspect that they're not connected at the other end either since if they were, twisting them all together would certainly blow a control board.

On 2022-11-23 by Jim Ryan

Hi. Our AirCycler FR-V was never hooked up. Both the furnace and thermostat have Red-Blue-White-Yellow and Green wire nutted together.

Do I place each pair together in the Air-Cycler Box which has the color codes placement screws. The box does have the red and white wires running to the power damper. Thank you. Jim Ryan

On 2020-08-21 by Garry

Hello,

I live in Edmonton, Alberta, Canada and I have a bathroom fan vent and a natural gas meter close together on the side of my house.

The bathroom fan vent is about two feet away from the natural gas regulator and everything was installed by the builder when the house was built.

Do you know if this is code compliant? How close to a natural gas regulator can a bathroom fan vent be installed?

Thank you very much for your assistance.

Garry

On 2020-07-21 by Dan K.

Moldy insulation due to inadequately-vented roof (C) InspectApedia.com DanKI 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) - 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

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. - photos of mold- or water- damaged insulation?

Moldy insulation due to inadequately-vented roof (C) InspectApedia.com DanK


Moldy insulation due to inadequately-vented roof (C) InspectApedia.com DanK


Moldy insulation due to inadequately-vented roof (C) InspectApedia.com DanK

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) -

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.


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