Attic ventilation fixes: here we explain how to correct improper or inadequate attic or under-roof ventilation in buildings.

This article series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. It describes proper roof ventilation placement, amounts, and other details.

Roof Ventilation Choices, Design & Specifications

These attic or roof ventilation specification or standards questions and answers were posted originally at ROOF VENTILATION SPECIFICATIONS - be sure to review that article.

[Click any image for a detailed, enlarged version.]

Question: how to ventilate an attic to stop condensation & mold

(Mar 23, 2015) Anonymous said:
We have an attic that is molding up due to humidity.We thought the roof leaks so we replaced the roofing material.After doing that we put up some insulation in the beams.

After just a while we realized that the insulation is falling off on it's own due to excessive humidity and the boards are getting completely black.

Now there is a very thin insulation between the ceiling and attic floor(the heat access the attic anyway right now) but we were also planning to utilize the attic as a living space so it would be heated.We were told we need solar power gable fan to properly vent the roof

I am not sure of the insulation and actual heating in the attic,so it doesn't condensate and create more damage.Also the north side of the roof is excessively wet(I assume condensating,not leaking because the nails have droplets of water on them and then they freeze).

So my question is how to properly ventilate and insulate heated attic so we can get rid off the mold and not create more damage?Any advice would be appreciated.Thank you!

Reply:

In the ARTICLE INDEX you'll see a series of articles that offer detailed suggestions, beginning at

ROOF VENTILATION SPECIFICATIONS - home

and

ATTIC VENTILATION

On 2013-01-29 by RGH

I have an attic condesation problem in my 14 year old cape. I've tried everything, packed an additional 10 inches of insulation in the attic flloor, put a turbine vent in to circulate the air. The house had a ridge vent and soffits

. I recently had the ridge vent close and the soffits in the front packed and installed an electric fan.

The moisture is worse, ice all the way to the top of the roof now.

I had an insulation company come and they recommended a hot roof. I'm hesitant with this becuase I think the the bays in the front need to be unpacked and soffits added with insulation, the ridge vent open up to get the air moving. But this doesn't guarantee it will fix the condensation issue. Please help!

On 2013-01-18 by Theresa Hunt

We got a new roof put on our house this summer with the ridge vent installed, now we are having problems with the bathroom fans leaking water due to what our roofer said is water icing up in the pipe leading to the vent and then melting once it heats up, I cannot image every house with the ridge vent has this problem, what is going on?

On 2012-10-12 by Virginian

I am attempting to figure out how to best ventilate a building on a college campus. The attic space is 10000 sq. feet and I already have 7 8 ft. by 4 ft. gables spread out throughout. I plan on adding attic fans (probably between 6-8).

I read that you recommend closing off gable vents if ridge fans are being added, but I'm attempting to combine aesthetics with utility and cost. We want to avoid doing soffit slit venting as that would necessity excessive scaffolding but we also want to permanently solve our venting problems.

So, should we add soffits? If so what kind? Is there no way to keep using the gables along with the soffits?

On 2012-09-28 by (mod) - get better under-roof ventilation and thus to fight glaciation (ice dams),

Alaskan,

Is "Glaciation" the term you use for ice dams at roof eaves?

Adding a 2nd roof as you describe is an understandable thought one might have to try to get better under-roof ventilation and thus to fight glaciation (ice dams), but I'm not sure it's quite the best approach.

First, you may be violating local fire codes by layering roof structures - making a house fire harder to fight and thus risking increases losses or an insurance issue should a house fire occur.

Best to

- reduce heat losses into the attic or roof cavity (better insulation, find and fix air bypass leaks)

- reduce moisture escape into the roof cavity or attic (find and fix water entry, leask & above)\

- assert positive venilation from eaves to ridge by adequate roof eaves intake openings (screened of course) and ridge outlet opening.

What I am less sure about is the effect on roof ventilation in Alaska in areas of deep snow cover. If the ridge vent is buried in snow, the exit venting will not work well and the whole system will be subverted. What has been your experience?

On 2012-09-28 by alaskan

I have been fighting glaciation for years. House is built in 1962 with a 4/12 pitch roof with 2x4 scissor trusses...inadequate space for insulation, especially at the truss/external wall junction(2x4).

So, I've decided to add a second roof over the existing sheathing....using 2x4's on edge along (on top of)the existing trusses/sheathing, then another layer of sheathing on those new 2x4s, and roof material.

This will provide a very clear ventilation channel between the old sheathing and new sheathing (ridge and soffit venting), as all the insulation will be below the existing old roof sheathing. Expensive solution,perhaps overkill, but should be effective. Any thoughts??? This is cold climate Anchorage AK.

On 2012-09-06 by (mod) -

Simone, it sounds to me as if your experts aren't. Installing a ridge vent is highly unlikely to be a cause of an insect problem, and ridge vents are normally screened. But you live in a hot humid climate where leaks invite a serious mold problem.

If there is access to see inside the attic your experts (or a better one) ought to be able to see the condition of under-roof ventilation as well as where water comes from. Water flows downhill. So while finding the original point of leak in a roof can indeed be difficult, at least someone ought to be able to tell you that the leak is above the point where water is first observed in the attic.

That data combined with a careful on-roof inspection for damage or flashing errors is how I'd start.

If water is running behind the siding, that's a second problem to investigate. If not, I'm not sure why we're talking about ripping off siding to diagnose a roof problem.

On 2012-09-04 by Simonne

I have gables and a ridge vent over the main house and soffits along the addition. I now have a horrible insect problem in my attic since I installed the ridge vent. I live in New Orleans and I am not as concerned about heat loss as I am about adequate airflow in the attic.

Additionally, I have 1970's vinyl siding over clap board. Gutters were installed in 2004.

There is evidence of roof leaks and I smell musty odors on humid days. And, no experts can agree on the cause without removing siding or roofing materials. Please advise.

Any suggestions?

On 2012-07-01 by Lewis

I am concerned I do not have proper ventilation in my attic. Hopefully you can help. I have 1200 sq ft of attic space. There is 5 roof vent measuring 9 inched by 9 inches. There's 6 soffet/eve vents total measuring 6 inches by 16 inches with 3 rows of small openings on each vents.

Two questions. Do I have enough total eve vents? All the eve vents are on the backside of the house---is there a reason none are in the front of the house?

On 2012-06-29 by (mod) -

Need help:

Re: "The joists below are running perpendicular to the slope of the roof, not allowing for a stack effect air flow, from bottom to top."

I think you are describing a design that precludes a continuous under-roof ventilation system - fi that's the case, "side to side" ventilation at the gable ends of the building won't do a thing unless the wind is blowing in the right direction, which surely is not always the case.

You'll need to focus on

- a perfect vapor barrier on the warm side of the ceiling
- no air leaks around ceiling penetrations
- find and fix any abnormal sources of indoor moisture

On 2012-06-29 by (mod) -

Craig perhaps I'm missing something - why not a soffit and ridge vent ?

On 2012-06-29 by Craig Scott

Whats the best way to vent a 1 1/2 story house.

On 2012-02-09 by Help, we need advice on our vent

Hi, This is our first winter in a new home.

We have a split shed metal roof with a standing seam. One roof is about 42'W x 40'L (1.5:12 pitch) and the other is 32'W x 25'L (2:12 pitch). The joists below are running perpendicular to the slope of the roof, not allowing for a stack effect air flow, from bottom to top. There are 11-7/8" TJI's every 24" OC which have notched bracing around the perimeter, allowing for potential side-to-side ventilation since the insulation has been held down 2" in each TJI bay.

There is a continuous soffit vent on the low and high soffits but again that vents just the one TJI bay that it is in, since the sheathing is sitting on the TJI's. They have recently added six randomly placed 6" dia vents, along each sloped side. From the inside out we have cathedral ceilings: drywall, vapor barrier, R-38 fiberglass batting (10"), approx 2" air space, plywood sheathing, roof paper, metal roof with field crimped seam.

We are getting condensation and water is dripping through the sheetrock in various places.

We opened a soffit vent and found ice build-up under the sheathing, mold, and damp insulation.

The weather has been very cold and humid. We have been trying to minimize interior relative humidity (keeping it at 35-40%) by raising the inside air temp, not taking showers upstairs (even though we have high cfm fans - they are exhausted under the eave and may be contributing?), and opening windows periodically.

We have no can lights except in the master bath which has a dropped ceiling in the steam shower. How do we get better roof ventilation?

The architect is proposing we turn it into a non-vented roof by blowing expandable foam into the ceiling from below. The builder is proposing we add fascia vents (like a honeycomb cor-a-vent) along the sides of the roof, hoping for enough draft to ventilate under the roof expanse.

We are also considering raising the roof in the spring and framing in a two inch air flow space under the sheathing, but would that even be effective on such a low pitch roof? What is the best solution, for short term and long term? How can I test the solution is working?

On 2011-08-11 by (mod) -

Tom it does not sound as if whoever is answering the mail at A.V. has the same picture of your building as you've given me.

There is no reason that an angled eaves covering will block airflow compared with a horizontal soffit eaves covering. Just take a look at photos of typical soffit vent strips and vinyl or aluminum perforated panels used for soffit intake vent/covering.

Given the same materials used horizontally or on a sloped eave overhang, and given all other factors are equal such as absence of blockage of airflow in the roof cavity, what's left: perforated openings on a horizontal or perforated openings on a slant.

The same area of intake opening will vent the same in still air. In cases of wind blowing towards one of these roof eaves, the angled overhang will perhaps catch and deliver more air to the roof cavity by its shape.

On 2011-08-11 by Tom K

DanJoe,

I responded to the initial email from Air Vent this way:

"Thank you for responding to my email. However, I think you may have
misunderstood my description of the placement of the vents.

I plan to place the continuous vents along both eaves near the bottom of the
roof. On both sides they will run the length of the building. In this sense
they will be placed as would be normal. The only difference is that the
soffits will not be level (horizontal), but will slope with the roof at 45
degrees. This means that across the width of the vents they will be angled
up at a 45 degree angle. Along the length of the vents they will be
horizontal. The vents will not run up the rake soffit. They will only run
horizontally along the eaves.

I am only wondering if the vents being angled up at 45 degrees across the
width will affect the action of the louvers on air flow, snow and moisture."

This should have made clear that the vents were not going on the rake. But the response again was:

"Yes, we do not suggest that you install those at an angle, they could short
circuit the vent system."

Short circuit? How? They don't explain. I think they are wrong and I won't buy their product because of that. They could have stated that it is not the optimal placement, but that it will still be functional. Instead they make it sound like it will not work at all.

Their response to my inquiry has really confused me and concerned me. I probably shouldn't have contacted them in the first place.

On 2011-08-11 by (mod)

OK I agree with Air Vent on this. By "rake" they are referring to the gable ends of the building, not the lower roof edges or eaves. I agree that we do NOT want intake venting there - I have added details of our discussion above.

And I agree that if you are using louvered rather than simple flat perforated vent openings at a soffit, the louvers will pick up more air if their openings face outwards, away from the house walls - presuming there is a breeze blowing in that direction. No breeze, no difference.

On 2011-08-11 by Tom K

DanJoe thanks for your input. It is helpful. Obviously, the "Anonymous" comment below was mine.

On 2011-08-11 by Anonymous

This is the response I got back from Air Vent Inc.

"We have no testing on installing soffit vents up the rake, but it has been
our long standing opinion that this would or could short circuit the ridge
vent or whatever form of exhaust at the ridge. You may want to consider an
alternate form of intake, like the Edge vent at the bottom of your roof.
There is no difference performance wise from the plastic or metal soffit
vent."

Because they stated, "up the rake", I thought they might have misunderstood that I meant to put the vents along the eave, just with the eave sloped up.

But when I questioned them again, they reconfirmed their initial response. Their response caused me to make multiple inquiries.

This is what the National Roofing Contractors Association had to say:

"Soffit vents are designed with louvers that should have the fins facing out away from the structure when installed. If installed on an angle they will not trap the air flow as well as on the horizontal, but they will function. Louvered vent are the most efficient, but there are many nonlouvered open hole soffit panels that are part of a metal panel soffit system.

Louvered or open hole continuous panels should function in an angled installation."

Here is what another manufacturer had to say:

"As long as the intake air flow is entering the soffit/attic space and directed towards the ridge/peak of the roof, with no other “escape routes” it should serve as effective intake ventilation and it should balance out with the exhaust ventilation of the ridge vent."

My sense is that installing louvered metal continuous eave vents will be functional and will work. It just won't have the louvers angled in the optimum direction. Since I have multiple layers of 6 mil plastic vapor barrier over the ceiling boards and will have 2 inches of free airspace between the roof sheathing and cross members (2x4s) between the 2x6 rafters (2 1/2 inches between sheathing and insulation), I think the angled vents will be adequate.

On 2011-08-11 15:51:10.747980 by (mod) -

Tom I'd sure like to hear some specific reasoning from the manufacturer you spoke with.

Either I don't understand the question and mfg's comment, or else what you report sounds like arm-waving and speculation.

Your angled roof overhang is a very common design.

As long as the air path up under the roof is not blocked by insulation, perforated soffit panels on the under-roof side of that overhang will work just fine.

In more than 40 years of inspecting the exterior and interior of buildings for under-roof ventilation and related problems I have seen (nor read about) no evidence whatsoever that there is a measured difference in the effectiveness of roof ventilation between a horizontal soffit intake and an angled soffit air intake.

Incidentally in both designs, when wind is blowing towards the walls of the house where there is roof overhang, airflow increases inwards at that soffit. I speculate that an angled soffit of the same square-feet in area will catch and move more air up under the roof space than a horizontal soffit of the same square-foot area, for the same wind conditions.

Just be sure you provide adequate air flow space below the entire roof, from soffit to ridge, or the venting will be ineffective.

Finally, I have not found any soffit intake vent product that differentiates between horizontal soffit intakes and angled soffit intakes. I prefer to cover the entire soffit with perforated panels to maximize the air inflow, thoug vent strips can be adequate in many cases.

On 2011-08-09 by Tom K

I am currently constructing the roof on a small timber frame cabin. The roof has a 1:1 pitch and has a cathedral type ceiling. I plan to have a continuous ridge vent installed at the top. I will also install continuous soffit vents on the bottom sides of both eaves, running the length of the building.

The eaves are a boxed cornice type with the soffits attached to the bottom of the extended rafters. Therefore the soffits are sloped up at a 45 degree angle. I asked a prominent manufacturer of continuous soffit vents if having the soffit vents angled up at a 45 degree angle made any difference. They said they would not recommend that because they thought it might compromise the flow of air to the ridge vent.

I am wondering how that can be. Maybe louvers in the vent will be angled in the wrong direction and thus impede air from flowing in.

Most continuous soffit vents have directional louvers in them. Does this mean that one should only construct a soffit which is horizontal so that the louvers in the vent will be angled the right way? Certainly there must be continuous soffit vents which are functional when placed in sloped soffits? Can you suggest any such product to me?

On 2011-05-26 by Brandt

I employed a featured roofer from Angie's list at a high price to include all new covered gutters.

I relied on the representative from the company to look at this 1926 roof and the addition (1957) and determine all of the needs to bring it up to 2009 standards. It is a small house and the charge was $4000 which was generous for what was to be done. He did not mention to me the need to replace some rotten supports -- found by another countractor.

He did not mention that attic had no ventilation recommended here or any other ventilation by a sealed window. He said the composition roof was o.k. So he replaced the slate roof and never had it inspected or was here when non-English speaking day workers did the job.

The acutally manufactured the gutters here so I thought they were covered. Another contractor was here, took photos and no covers! I have the largest American Beech in this County which caused damage in the past so I emphasized the need for covered.

The people he sent only replaced Gutters on the part of the house that was re-roofed which is idiotic since the gutters in the worst shape were towards the back nearest the tree.

They were to resurface two flat roofs and instead, put black tar around the downspouts -- looks bad. The chimney needed flashing and the job is some sort of white material that is obviously not what is used today.

Angie's list was asked to help me and never even responded after calls and letters to the roofer. The representative came here, saw the rotten beams but he came at night so that complete inspection was not possible. Never came back. Angie's list said he had medical problems. They also removed $7+ dollars from my account for months when I only signed up for 1 month. Never use these people!

On 2013-01-29 by RGH

I have an attic condesation problem in my 14 year old cape. I've tried everything, packed an additional 10 inches of insulation in the attic flloor, put a turbine vent in to circulate the air.

The house had a ridge vent and soffits. I recently had the ridge vent close and the soffits in the front packed and installed an electric fan.

The moisture is worse, ice all the way to the top of the roof now. I had an insulation company come and they recommended a hot roof. I'm hesitant with this becuase I think the the bays in the front need to be unpacked and soffits added with insulation, the ridge vent open up to get the air moving.

But this doesn't guarantee it will fix the condensation issue. Please help!

On 2013-01-18 by Theresa Hunt

We got a new roof put on our house this summer with the ridge vent installed, now we are having problems with the bathroom fans leaking water due to what our roofer said is water icing up in the pipe leading to the vent and then melting once it heats up, I cannot image every house with the ridge vent has this problem, what is going on?

On 2012-10-12 by Virginian

I am attempting to figure out how to best ventilate a building on a college campus.

The attic space is 10000 sq. feet and I already have 7 8 ft. by 4 ft. gables spread out throughout. I plan on adding attic fans (probably between 6-8). I read that you recommend closing off gable vents if ridge fans are being added, but I'm attempting to combine aesthetics with utility and cost.

We want to avoid doing soffit slit venting as that would necessity excessive scaffolding but we also want to permanently solve our venting problems. So, should we add soffits? If so what kind? Is there no way to keep using the gables along with the soffits?

On 2012-09-28 by (mod) -

Alaskan,

Is "Glaciation" the term you use for ice dams at roof eaves?

Adding a 2nd roof as you describe is an understandable thought one might have to try to get better under-roof ventilation and thus to fight glaciation (ice dams), but I'm not sure it's quite the best approach.

First, you may be violating local fire codes by layering roof structures - making a house fire harder to fight and thus risking increases losses or an insurance issue should a house fire occur.

Best to

- reduce heat losses into the attic or roof cavity (better insulation, find and fix air bypass leaks)

- reduce moisture escape into the roof cavity or attic (find and fix water entry, leask & above)\

- assert positive venilation from eaves to ridge by adequate roof eaves intake openings (screened of course) and ridge outlet opening.

What I am less sure about is the effect on roof ventilation in Alaska in areas of deep snow cover. If the ridge vent is buried in snow, the exit venting will not work well and the whole system will be subverted. What has been your experience?

On 2012-09-28 by alaskan

I have been fighting glaciation for years. House is built in 1962 with a 4/12 pitch roof with 2x4 scissor trusses...inadequate space for insulation, especially at the truss/external wall junction(2x4). So, I've decided to add a second roof over the existing sheathing....

using 2x4's on edge along (on top of)the existing trusses/sheathing, then another layer of sheathing on those new 2x4s, and roof material.

This will provide a very clear ventilation channel between the old sheathing and new sheathing (ridge and soffit venting), as all the insulation will be below the existing old roof sheathing. Expensive solution,perhaps overkill, but should be effective. Any thoughts?

This is cold climate Anchorage AK.

On 2012-09-06 by (mod) -

Simone, it sounds to me as if your experts aren't. Installing a ridge vent is highly unlikely to be a cause of an insect problem, and ridge vents are normally screened. But you live in a hot humid climate where leaks invite a serious mold problem.

If there is access to see inside the attic your experts (or a better one) ought to be able to see the condition of under-roof ventilation as well as where water comes from.

Water flows downhill. So while finding the original point of leak in a roof can indeed be difficult, at least someone ought to be able to tell you that the leak is above the point where water is first observed in the attic.

That data combined with a careful on-roof inspection for damage or flashing errors is how I'd start.

If water is running behind the siding, that's a second problem to investigate. If not, I'm not sure why we're talking about ripping off siding to diagnose a roof problem.

On 2012-09-04 by Simonne

I have gables and a ridge vent over the main house and soffits along the addition. I now have a horrible insect problem in my attic since I installed the ridge vent. I live in New Orleans and I am not as concerned about heat loss as I am about adequate airflow in the attic.

Additionally, I have 1970's vinyl siding over clap board. Gutters were installed in 2004.

There is evidence of roof leaks and I smell musty odors on humid days. And, no experts can agree on the cause without removing siding or roofing materials. Please advise.

Any suggestions?

On 2012-07-01 by Lewis - Do I have enough total eve vents?

I am concerned I do not have proper ventilation in my attic. Hopefully you can help. I have 1200 sq ft of attic space. There is 5 roof vent measuring 9 inched by 9 inches. There's 6 soffet/eve vents total measuring 6 inches by 16 inches with 3 rows of small openings on each vents. Two questions.

Do I have enough total eve vents? All the eve vents are on the backside of the house---is there a reason none are in the front of the house?

On 2012-06-29 by (mod) -

Need help:

Re: "The joists below are running perpendicular to the slope of the roof, not allowing for a stack effect air flow, from bottom to top."

I think you are describing a design that precludes a continuous under-roof ventilation system - fi that's the case, "side to side" ventilation at the gable ends of the building won't do a thing unless the wind is blowing in the right direction, which surely is not always the case.

You'll need to focus on

- a perfect vapor barrier on the warm side of the ceiling

- no air leaks around ceiling penetrations

- find and fix any abnormal sources of indoor moisture

On 2012-06-29 by (mod) -

Craig perhaps I'm missing something - why not a soffit and ridge vent ?

On 2012-06-29 by Craig Scott

Whats the best way to vent a 1 1/2 story house.

On 2012-02-09 by Help, we need advice on our vent

Hi, This is our first winter in a new home. We have a split shed metal roof with a standing seam. One roof is about 42'W x 40'L (1.5:12 pitch) and the other is 32'W x 25'L (2:12 pitch). The joists below are running perpendicular to the slope of the roof, not allowing for a stack effect air flow, from bottom to top.

There are 11-7/8" TJI's every 24" OC which have notched bracing around the perimeter, allowing for potential side-to-side ventilation since the insulation has been held down 2" in each TJI bay

. There is a continuous soffit vent on the low and high soffits but again that vents just the one TJI bay that it is in, since the sheathing is sitting on the TJI's. They have recently added six randomly placed 6" dia vents, along each sloped side.

From the inside out we have cathedral ceilings: drywall, vapor barrier, R-38 fiberglass batting (10"), approx 2" air space, plywood sheathing, roof paper, metal roof with field crimped seam. We are getting condensation and water is dripping through the sheetrock in various places.

We opened a soffit vent and found ice build-up under the sheathing, mold, and damp insulation.

The weather has been very cold and humid. We have been trying to minimize interior relative humidity (keeping it at 35-40%) by raising the inside air temp, not taking showers upstairs (even though we have high cfm fans - they are exhausted under the eave and may be contributing?), and opening windows periodically.

We have no can lights except in the master bath which has a dropped ceiling in the steam shower. How do we get better roof ventilation? The architect is proposing we turn it into a non-vented roof by blowing expandable foam into the ceiling from below.

The builder is proposing we add fascia vents (like a honeycomb cor-a-vent) along the sides of the roof, hoping for enough draft to ventilate under the roof expanse. We are also considering raising the roof in the spring and framing in a two inch air flow space under the sheathing, but would that even be effective on such a low pitch roof? What is the best solution, for short term and long term? How can I test the solution is working?

On 2011-08-11 by (mod) - where do the roof vents belong?

Tom it does not sound as if whoever is answering the mail at A.V. has the same picture of your building as you've given me.

There is no reason that an angled eaves covering will block airflow compared with a horizontal soffit eaves covering. Just take a look at photos of typical soffit vent strips and vinyl or aluminum perforated panels used for soffit intake vent/covering.

Given the same materials used horizontally or on a sloped eave overhang, and given all other factors are equal such as absence of blockage of airflow in the roof cavity, what's left: perforated openings on a horizontal or perforated openings on a slant. The same area of intake opening will vent the same in still air.

In cases of wind blowing towards one of these roof eaves, the angled overhang will perhaps catch and deliver more air to the roof cavity by its shape.

On 2011-08-11 by Tom K

DanJoe,

I responded to the initial email from Air Vent this way:

"Thank you for responding to my email. However, I think you may have misunderstood my description of the placement of the vents.

I plan to place the continuous vents along both eaves near the bottom of the roof

. On both sides they will run the length of the building. In this sense they will be placed as would be normal. The nly difference is that the soffits will not be level (horizontal), but will slope with the roof at 45 degrees.

This means that across the width of the vents they will be angled up at a 45 degree angle. Along the length of the vents they will be horizontal. The vents will not run up the rake soffit. They will only run horizontally along the eaves.

I am only wondering if the vents being angled up at 45 degrees across the width will affect the action of the louvers on air flow, snow and moisture."

This should have made clear that the vents were not going on the rake. But the response again was:

"Yes, we do not suggest that you install those at an angle, they could short circuit the vent system."

Short circuit? How? They don't explain. I think they are wrong and I won't buy their product because of that.

They could have stated that it is not the optimal placement, but that it will still be functional. Instead they make it sound like it will not work at all. Their response to my inquiry has really confused me and concerned me. I probably shouldn't have contacted them in the first place.

On 2011-08-11 by (mod) -

OK I agree with Air Vent on this. By "rake" they are referring to the gable ends of the building, not the lower roof edges or eaves. I agree that we do NOT want intake venting there - I have added details of our discussion above.

And I agree that if you are using louvered rather than simple flat perforated vent openings at a soffit, the louvers will pick up more air if their openings face outwards, away from the house walls - presuming there is a breeze blowing in that direction. No breeze, no difference.

On 2011-08-11 y Tom K

DanJoe thanks for your input. It is helpful. Obviously, the "Anonymous" comment below was mine.

On 2011-08-11 y Anonymous

This is the response I got back from Air Vent Inc.

"We have no testing on installing soffit vents up the rake, but it has been
our long standing opinion that this would or could short circuit the ridge
vent or whatever form of exhaust at the ridge. You may want to consider an
alternate form of intake, like the Edge vent at the bottom of your roof.
There is no difference performance wise from the plastic or metal soffit
vent."

Because they stated, "up the rake", I thought they might have misunderstood that I meant to put the vents along the eave, just with the eave sloped up. But when I questioned them again, they reconfirmed their initial response. Their response caused me to make multiple inquiries.

This is what the National Roofing Contractors Association had to say:

"Soffit vents are designed with louvers that should have the fins facing out away from the structure when installed. If installed on an angle they will not trap the air flow as well as on the horizontal, but they will function. Louvered vent are the most efficient, but there are many nonlouvered open hole soffit panels that are part of a metal panel soffit system. Louvered or open hole continuous panels should function in an angled installation."

Here is what another manufacturer had to say:

"As long as the intake air flow is entering the soffit/attic space and directed towards the ridge/peak of the roof, with no other “escape routes” it should serve as effective intake ventilation and it should balance out with the exhaust ventilation of the ridge vent."

My sense is that installing louvered metal continuous eave vents will be functional and will work. It just won't have the louvers angled in the optimum direction. Since I have multiple layers of 6 mil plastic vapor barrier over the ceiling boards and will have 2 inches of free airspace between the roof sheathing and cross members (2x4s) between the 2x6 rafters (2 1/2 inches between sheathing and insulation), I think the angled vents will be adequate.

On 2011-08-11 by (mod) -

Tom I'd sure like to hear some specific reasoning from the manufacturer you spoke with. Either I don't understand the question and mfg's comment, or else what you report sounds like arm-waving and speculation.

Your angled roof overhang is a very common design. As long as the air path up under the roof is not blocked by insulation, perforated soffit panels on the under-roof side of that overhang will work just fine. In more than 30 years of inspecting the exterior and interior of buildings for under-roof ventilation and related problems I have seen (nor read about) no evidence whatsoever that there is a measured difference in the effectiveness of roof ventilation between a horizontal soffit intake and an angled soffit air intake.

Incidentally in both designs, when wind is blowing towards the walls of the house where there is roof overhang, airflow increases inwards at that soffit. I speculate that an angled soffit of the same square-feet in area will catch and move more air up under the roof space than a horizontal soffit of the same square-foot area, for the same wind conditions.

Just be sure you provide adequate air flow space below the entire roof, from soffit to ridge, or the venting will be ineffective.

Finally, I have not found any soffit intake vent product that differentiates between horizontal soffit intakes and angled soffit intakes. I prefer to cover the entire soffit with perforated panels to maximize the air inflow, thoug vent strips can be adequate in many cases.

On 2011-08-09 by Tom K

I am currently constructing the roof on a small timber frame cabin. The roof has a 1:1 pitch and has a cathedral type ceiling. I plan to have a continuous ridge vent installed at the top. I will also install continuous soffit vents on the bottom sides of both eaves, running the length of the building.

The eaves are a boxed cornice type with the soffits attached to the bottom of the extended rafters. Therefore the soffits are sloped up at a 45 degree angle. I asked a prominent manufacturer of continuous soffit vents if having the soffit vents angled up at a 45 degree angle made any difference. They said they would not recommend that because they thought it might compromise the flow of air to the ridge vent.

I am wondering how that can be. Maybe louvers in the vent will be angled in the wrong direction and thus impede air from flowing in.

Most continuous soffit vents have directional louvers in them. Does this mean that one should only construct a soffit which is horizontal so that the louvers in the vent will be angled the right way? Certainly there must be continuous soffit vents which are functional when placed in sloped soffits? Can you suggest any such product to me?

 

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