building-scienceheader image
10 Helpful?

Lstiburek’s Rules for Venting Roofs

You need an airtight ceiling, lots of air flow, plenty of soffit vents, and deep insulation at the attic perimeter

Posted on Jul 24 2011 by Joe Lstiburek

Building Science Fundamentals: Roof, Part 1: Ventilation

By Dr. Joseph Lstiburek

Dr. Joseph Lstiburek talks about the not-so-controversial ways to maximize the efficiency and airflow of your roof and attic.

Video Transcript:
There’s been so much stuff said about roofs that you sometimes lose perspective. I’m going to start off by saying what might seem controversial but really shouldn’t be.

This is a vented attic, and it’s probably one of the most unappreciated building assemblies we have in the history of building science. It’s beautiful. It’s hard to screw this up.

For 20% of the effort, it gets us to 80% of optimal performance, and it works in hot climates, in mixed climates, the Arctic, the Antarctic, the Amazonian rain forest — it works absolutely everywhere. The value proposition of a vented attic, meaning the money that you invest in building one of them — it’s hard to argue with the benefits. But for all kinds of reasons, we manage to screw it up.

The single most important thing you have to remember about a vented attic is that the ceiling plane — the gypsum board layer, the drywall layer — needs to be airtight.

1) The ceiling plane MUST be airtight
Absolutely airtight. Above the airtight ceiling plane, the only thing that should be seen is insulation and air, nothing else. Not last year’s Christmas decorations, not your high school prom dress, not the tuxedo you were married in and can no longer fit in. Nothing but lots of insulation and air. Just an airtight ceiling and nothing else.

2) If you’re going to vent the roof, then VENT THE ROOF
If you’re actually going to vent the roof, let’s be serious about venting the roof. Wash the underside of the roof deck with air. That means the entire perimeter of the roof needs to have air inlets, meaning continuous soffit ventilation. It’s dumb to have baffles every third or fourth bay, the entire underside of the roof deck should be washed. Where the air leaves isn’t as important — whether it’s a ridge vent, or mushroom caps, or gables. What’s important is that you have continuous air entry at the perimeter of the roof down low.

3) Put more vents down low than up high
This is where the code tends to have it wrong. You want more entry points at the perimeter than exit points at the top.

People say you want to balance the lower down ventilation with the upper ventilation, and a lot of people interpret the codes to say that if you get it unbalanced you want more ventilation up high. That is absolutely wrong; you don’t want more places for the air to get out than to get in. The reason is, if you construct a house with a leaky attic ceiling and you have lots of ridge vents or you have lots of vents up high, the makeup air is going to be pulled from the house rather than being pulled from the outside. That scenario is a disaster.

Attics should be ventilated with air from the outside, not the inside. That’s why I hate these whirligig turbine vents — because they depressurize the attic, and if your attic ceiling isn’t perfectly airtight, you suck air conditioned air or heated air out of the house.

It’s even crazier when the powered attic fans can actually suck on the roof and they’re controlled by a thermostat. How stupid is that? Of course the attic is going to be hot. You turn them on and they suck all the air conditioned air out.

No powered attic ventilation; more vents down low than up high; wash the entire underside of the roof deck. But all of that is secondary to having the ceiling plane airtight.

This last tip is more important in cold climates than anywhere else. But where the ceiling insulation hits the perimeter wall, you don’t want the amount of ceiling insulation on the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. to ever be less than the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. in the wall itself.

4) Put more insulation on top of the wall than inside it.
In other words, if you have an R-20 wall, you want at least R-20 on your top plate. A higher R-value is better, but a lower R-value is not. If you have an R-30 wall you want at least R-30 on top of your top plate.

A reasonable rule of thumb is: Thou shalt never, according to Joe’s Rule of Thumb, have less R-value on the top of your top plate than in the wall. It would be nice to have even more, but not less.

Notice: nowhere in this discussion did the term “vapor barrier” come up. If you really want to have a vapor barrier in the ceiling, limit it to climate zone 6 or higher, but that’s really not important compared to the airtightness of that ceiling plane.

The building code calls for a vapor retarder in climate zone 6 or higher. It’s okay to put one in, but if you don’t, take a Valium and relax. You don’t want to go through a lot of brain damage in a renovated house to try and add a vapor barrier underneath insulation in an attic. What you really want to do is make that ceiling plane airtight, make it airtight, declare victory and be done. Don’t mess around with permeability’s and calculations and whatever.

To recap, airtightness on the ceiling; washing the underside of the roof deck; unbalanced ventilation should be in favor of the lower vents because you don’t want to depressurize the attic; no to powered attic ventilation or the whirligigs; and you don’t want to squeeze the insulation at the perimeter so it’s less than the R-value of the wall.

That’s it. You can build that everywhere in the world and life is good.


Tags: , ,

53.
Aug 1, 2011 7:44 AM ET

More David McNeely
by Joe Lstiburek

If the ceiling is airtight and the assembly has a high thermal resistance the valley rafter problem is not a problem with the detail I recommend. How high an R-value? Depends on the climate zone. Work done by Tobiassion with the Army Corps of Engineers in the 70's pretty much established these R-values. In the ice dam article I referenced earlier I talk about those R-values. About R-40 in climate zone 5, R-50 in climate zone 6. When you do this the snow melt does not happen. The stream river analogy does not apply. Valleys and hips are only a problem with cathedralized assemblies - they are obviously not a problem in the example I used as a basis for my presentation. In complex cathedralized assemblies the unbalanced ventilation approach I recommend for simple attics almost never occurs and is typically not possible. You do the best you can and rely on your airtighness strategy.

I have no sympathy for the difficulty in establishing airtightness. You either provide it under the roof deck with the gypsum board ceiling with a vented space below the roof deck or you provide it at the roof deck with a vented space above it (an "over-roof") in a high snow load area to control ice damming. The airtightness is necessary regardless.

When we do complex roofs in high snow load areas like ski resorts we typically have the roof deck act as the air control layer and provide a high level of thermal resistance at the roof deck (either rigid insulation directly above the roof deck or high density spray foam directly under the roof deck - and then we provide an over roof creating a vented space over the top of the entire assembly. In essence we created a vented unvented roof hybrid. An unvented primary roof assembly with a vented over roof.


54.
Aug 2, 2011 10:04 PM ET

What about better insulation over top plate?
by John Walker

All the diagrams I see show a squiggly line suggesting fiberglass or other loose fill type insulation. My first thought is to upgrade the insulation over the top plate to something with a higher R value per inch thickness like XPS. Two or three layers of 2" XPS over the top plate would boost the R value substaintially.

*Is there a good reason not to do this? Is there a wall dew point/condensation issue?

Thanks

John


55.
Aug 4, 2011 9:56 AM ET

to John Walker
by Joe Lstiburek

There is no good reason beyond convenience or laziness not to do this. There is no hygro thermal reason not to do so (note the $100 dollar hyphenated word replacing "dew point/condensation" - chicks dig this - ok so that is not true - dew point calculations are a horrible way to do analysis and so "hygro thermal" is the "politically correct"
phrase - and I am all about political correctness.....).


56.
Aug 4, 2011 2:05 PM ET

I first went to a Joe seminar
by aj builder, Upstate NY Zone 6a

I first went to a Joe seminar sponsored by NYSERDA I think at West Point decades ago if that is possible.

Joe, thanks for all your years helping us all out with the common sense of dealing with moisture and high levels of insulation.

aj


57.
Aug 17, 2011 10:28 AM ET

Venting a vintage house
by jeanne kruchowski

My 1914 house needs to be re-roofed this summer but I am at a loss re ventilation. Roof is 9/12. Floor area is 1200 sq ft. The attic space contains no mechanicals and is not used for storage. The ridge is about 40 feet long and there is currently a ridge vent but no other roof vents, no soffit vents, and no gable vents. The roofers who have given me bids have all recommended either cutting holes in the soffit to install [ugly] pressed metal vents, adding a bunch of vent boxes/mushrooms both low and high on the slope, and/or installing whirly-bird things to ‘promote airflow’. What is the best solution for the lack of air intake? The soffit is the original painted wood tongue & groove - do I really have to cut into my TG soffits? Are any of the deck-mounted products out there, such as Coravent’s In-Vent beneficial? I cannot install gable vents because the house is in an historic preservation area and such a modification would not get approval from the historical commission. House is located in Minnesota – hot summers / cold winters / lots of snow. The attic floor insulation will eventually be beefed up but [thank you, Mr L.] now I know that I must seal the ceiling plane first.


58.
Aug 18, 2011 3:57 AM ET

Response to Jeanne Kruchowski
by Martin Holladay

Jeanne,
Assuming that there are no existing problems, just relax. You are overthinking this.

Proceed with your plan to air-seal the ceiling and beef up the insulation on the attic floor. I don't think you need to do anything to change the ventilation situation in that attic. Nothing is broken.


59.
Aug 18, 2011 12:48 PM ET

Martin, thanks - so I will
by jeanne kruchowski

Martin,

thanks - so I will really be OK without any intake vents? It that is the case, that's wonderful. This is going to be an expensive enough project already, so I while I want to make sure I'm getting everything right up-front, I of course don't want to wrech my TG or spend on adding vents if I don't have to.

thanks.
JK


60.
Aug 27, 2011 8:28 AM ET

Workshop Venting
by Troy Farwell

I built a workshop last year (700 s.f.) with a vaulted ceiling. I decided to spend the time venting every joist bay of the ceiling at the side walls and a ridge vent across the top. I put 3/4" airflow channels integrated into the skylight trim as well, so every bay would have air flow. I installed OSB as the ceiling with battens covering the seams, caulking gaps. I also rain-screened the wall with the soffit vents doing dual duty as the outlet (good idea, bad - not sure - it's an experiment...). So I basically have a full-building skin, then airflow, then insulation, then air-tight (theoretically) interior plane.

So far, this system is working great. I have no cooling system other than two ceiling fans. On 90+ deg days, the shop remains comfortable to work in (mid-upper 70's) - opening the windows heats it up. I guess I met the ultimate goal of comfort without additional systems / cost. I am also hoping this causes the roof material (30 yr premium comp) to last a real long time. The extra material cost was basically nothing - it was attention to design and detailing.


61.
Oct 3, 2011 1:29 PM ET

Edited Oct 4, 2011 11:05 AM ET.

Hybrid above and below deck insulation
by George Champlin

I am rebuilding a cathedral ceiling in Portland Oregon I'm wondering if a hybrid approach is advisable. I am replacing a rotted plank roof deck with 2x6 rafters and sheathing. I'm planning on putting R-19 fiberglass between the rafters and 2" R-13 foil-faced polyiso or XPS on top of the deck, as illustrated in the recent Fine Homebuilding article on roof venting. I'm a little nervous about an unvented design and wonder if there are any advantages/disadvantages to adding a vent channel to each rafter bay. Does this defeat some of the advantages of the above-deck foam layer? Also does this affect any advice on vapor barriers?

Roof-above-below.jpg


62.
Oct 3, 2011 1:32 PM ET

Hybrid above and below deck insulation II
by George Champlin

It looks like the picture didn't get uploaded, I'll try again.


63.
Oct 3, 2011 1:48 PM ET

Edited Oct 3, 2011 1:49 PM ET.

Response to George Champlin
by Martin Holladay

George,
It looks like you are in DOE climate zone 4C.

According to the 2009 IRC (Section R806.4), it’s possible to build an unvented roof assembly with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation (like fiberglass) in the rafter bays. If you build this type of roof, the code requires that “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” The table calls for a minimum of R-10 for Climate Zone 4C.

So, your plan to install R-13 polyiso above the roof deck will work -- although the total R-value (19+13=32) is pretty low. The code calls for at least R-38 in your climate zone, so if you can make the polyiso thicker, your plan would be improved.

Whatever you do, don't install any venting between the roof sheathing and the fiberglass -- venting would introduce exterior air between the two layers of insulation, and greatly reduce the thermal performance of your roof.


64.
Oct 23, 2011 9:24 PM ET

vent or not with closed cell foam
by Erik Nelson

Joe, thanks for the great article. I'm building a house which has a steel roof over a frost barrier (i.e. no permeability). The frost barrier is laid directly onto the plywood decking. The roof is nearly flat (1.2/12). I've been strongly considering using a closed cell insulation product (2" thick) combined with a blown in fiberglass batting in the 2x12 rafters(10"). I live in Seattle which, as I understand is zone 4C.
Question: My house is set up for venting but some of the literature speaks to the fact that a closed cell foam material can be sprayed directly onto the roof decking without venting. Given this situation would it be your recommendation to vent or is that not necessary?


65.
Oct 24, 2011 4:30 AM ET

Response to Erik
by Martin Holladay

Erik,
Your plan will work, as long as the total R-value of your proposed roof assembly meets the minimum R-value requirements in your local building code.

The 2009 IRC allows the use a combination of air-impermeable and air-permeable insulation in unvented rafter bays, as long as the minimum R-value of the air-impermeable insulation that is "applied in direct contact with the underside of the structural roof sheathing" meets the requirements for condensation control shown in Table R806.4. Table R806.4 calls for a minimum of R-10 for Climate Zone 4C (that would be for the spray foam only).

The minimum R-value for ceiling insulation (the entire roof assembly -- in your case, spray foam plus fiberglass) in your climate zone is R-38.


66.
Oct 25, 2011 12:23 AM ET

Martin, thanks for the help.
by Erik Nelson

Martin, thanks for the help.

As we're planning on at least 2" of closed cell foam applied directly to the structural roof sheathing which I believe has an R value = 6.5/inch the minimum R value of 10 is met.

One other question... 2009 IRC R806.4 also states...

"No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly."

Does this refer to the sheetrock material or the paint primer?

Again thanks for the comments and helpful insights.


67.
Oct 25, 2011 4:43 AM ET

Response to Erik
by Martin Holladay

Erik,
The sentence in the code you quoted -- "No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly" -- is poorly written and confusing, and I have already had discussions with Joe Lstiburek and code officials about it. Hopefully the sentence will not appear in future code editions.

The intent appears to be to warn builders about the installation of polyethylene -- yet even that warning seems unnecessary. Clearly, as written, it appears to forbid builders from painting their ceilings.

Anyway, ignore it. As long as you don't have poly in your ceiling, no code official is going to have a problem with your ceiling.


68.
Oct 26, 2011 9:56 PM ET

can lighting
by Erik Nelson

Martin,
One other concern I've got is that I'm going to have a number of can lights (Juno) installed into the ceiling. As I've mentioned, I'm planning on 2 inches of closed cell spray in foam. Do I need to be worried about the heat generated from the cans being close to the foam? I wouldn't want to burn my house down because the can lights could be hot and close to the foam.

Thanks,


69.
Oct 27, 2011 4:41 AM ET

Can lights in an insulated ceiling?
by Martin Holladay

Can lights in an insulated ceiling? What were you thinking?

Call up your electrician now and pull them all out. Substitute surface-mounted fixtures. Yes, the substitution might be expensive. But it will never be cheaper to do it than now. After you finish your house, you'll be kicking yourself for years if you don't take this advice.


70.
Oct 27, 2011 7:13 PM ET

Sorry, I'm a bit of a novice
by Erik Nelson

Sorry, I'm a bit of a novice here. Are you suggesting that I remove them because its a fire hazard or because I've penetrated the ceiling? Or some other reason? Not sure why I would regret this for years to come?

- confused.


71.
Oct 27, 2011 8:02 PM ET

Response to Erik Nelson
by Martin Holladay

Erik,
I apologize for not explaining better; my fault.

An insulated ceiling is ideally airtight. That means you need an effective air barrier.

Such a ceiling should also be well insulated. These days, that means that it should have an R-value of R-30 in Florida, or R-38 to R-60 in a cold climate.

Installing a recessed can light in an insulated ceiling interrupts the air barrier, allowing escaping interior air to enter the roof cavity. Escaping air often leads to condensation, mold, and rot.

Recessed can lights also makes it difficult or impossible to achieve R-38 to R-60 above the can light, leading to energy waste and ice damming problems. That's why energy experts advise that no recessed can lights should ever be installed in an insulated ceiling.


72.
Nov 22, 2011 9:49 PM ET

Insulated ducting within the atic
by gerald pluard

Thanks for the great video. Is is practical to assume you can seal up an attic when you have insulated HVAC running through the unheated attic in a Zone 3 climate? Am I better off going with a hot roof or is the risk of moisture /condensation from the insulated ducts to high?


73.
Nov 26, 2011 6:16 AM ET

Response to Gerald Pluard
by Martin Holladay

Gerald,
Q. "Is is practical to assume you can seal up an attic when you have insulated HVAC running through the unheated attic in a Zone 3 climate?"

A. If you have ductwork in your ventilated unconditioned attic, you have several problems. Sealing air leaks in your ceiling is just one problem. That problem can be solved, although the duct penetrations certainly complicate things.

The more difficult problem is to seal all of the leaks in the duct seams, as well as the leaks between the ducts and the register boots. Then there is the problem of installing enough insulation around all of your ducts to keep your energy bills reasonable.

The best approach is to keep all of your ducts inside your home's conditioned envelope. If your ducts are already located in your attic, the best solution is (usually) to create an unvented conditioned attic.

For more information, see Keeping Ducts Indoors and Creating a Conditioned Attic.


74.
Feb 24, 2012 10:46 AM ET

Attic/Roof Venting
by Gary Steinfeld

In the Northeast (Long Island, NY) can i use 1" foil faced foam (Dow Thermax) to create 2" air channels in the attic rafter bays (2x12) then add fiberglass batt insulation in the bays, sheetrock and latex paint? Will the low perm of the thermax cause any problems at this location? Also, if desired can I add 1" rigid foam (dow blue styrofoam) between the rafters and sheetrock to help with thermal bridging?


75.
Feb 24, 2012 10:58 AM ET

Response to Gary Steinfeld
by Martin Holladay

Gary,
Yes, you can use rigid foam (for example, Thermax) to create a ventilation channel.

If you have 2x12s with a 2-inch ventilation channel, 1 inch of Thermax will give you an R-value of R-6.5, and the remaining 8 1/4 inches of fiberglass will give you about R-30.5. So you end up with R-37. I don't know if that is enough to meet minimum code requirements; it depends on your climate zone and the local code.

Yes, you can cut down on thermal bridging through the studs by adding a continuous layer of rigid foam insulation under the rafters. I don't think this type of assembly will have moisture problems.


76.
Oct 11, 2012 12:52 AM ET

Baffles in combo with closed-cell spray foam?
by Phil Lanier

This thread is a little bit old now, but still very helpful!

You talk about doing an over roof in areas with high snow loads, Effectively creating a vented unvented roof hybrid. Is is possible to effectively do the same things (although perhaps to a lesser degree) simply by installing baffles from eve to ridge before spraying the foam?


77.
Oct 11, 2012 4:37 AM ET

Edited Oct 11, 2012 4:59 AM ET.

Response to Phil Lanier
by Martin Holladay

Phil,
The roof assembly you describe can work well, as long as the following conditions hold:

1. You provide at least 1.5 inches -- 2 inches is better -- of depth to your vent space, with the vent space extending the full width of each rafter bay. This means that you need to install site-built baffles, not commercially purchased ones.

2. Your roof should be a simple gable or shed roof, without any valleys, hips, dormers, or skylights. If your roof includes valleys, hips, dormers, or skylights, it's not a good candidate for this approach.

3. You must be able to install enough R-value to at least meet minimum code requirements.

For more information on this topic, see How to Build an Insulated Cathedral Ceiling.


79.
Jan 30, 2013 10:04 AM ET

To Vent or Not to Vent
by Ronald Crouch, AIA

Dear Joe,

Here is a resend with an attachment drawing. pardon previous post.

On the topic of a Perfect Roof, please picture this unique commercial application. Large existing middle school. ASHRAE Zone 4. Roof retrofit. Steep sloped roof planes to be finished with conventional asphalt shingles. The attic will be ventilated to code in this retrofit. Cold formed steel framing truss “girders” are at 3’-0” OC create the slope. A galvanized steel deck 1.5” thick spans between the truss girders. The air barrier is the gypsum on the bottom of the trusses and insulation is between the truss bottom chords. A vapor barrier is integrated on the warm in winter side of the insulation so as to take the attic ventilation requirements to 1/300 vs. 1/150 using IBC rules.

There are two camps on how to do address the work above the steel deck:

Camp 1: Plywood sheathing placed over 2’-0” OC wood furring. The furring is placed over the steel deck perpendicular to the flutes. The gaps in between the furring could promote some air flow from eave to ridge. This is a second zone of ventilation in addition to the entire ventilated attic. Both ventilation zones would co-mingle at the top and bottom and have the same fresh air source low and same vent source high. We do need minimum thickness for this furring -- meaning thick enough only for roofing nails to avoid contact with the steel deck. The existing copper stepped flashing at the abutting walls beyond could be negatively impacted if furring is too thick. Somewhere between 3/4” to 1 ½” we would hope to stay. Under review.

Camp 2: Two layers of staggered plywood of sufficient thickness so that the roofing nails do not strike the steel deck. No furring - so no ventilation between the steel deck and plywood. The thought here is that the venting of the entire attic space below the steel deck is sufficient.

One variable to both trains of thought is uncertainty as to how well the existing air barrier at the bottom of the trusses was installed. More moisture vapor may be carried into the attic via air flow than would be desired if the Contractor did not seal it well.

Any thoughts are appreciated. Discussions with major shingle manufacturers got mixed comments. One initially jumped into Camp 1 then backed off later. The other seeming to not care either way. No real help there.

Thanks,

Ron Crouch

AttachmentSize
To Vent or Not to Vent.pdf 365.59 KB


80.
Jan 30, 2013 10:33 AM ET

Response to Ron Crouch
by Martin Holladay

Ron,
I've forwarded your question to Joe by e-mail; I don't know if he'll have time to respond.

Here's my take: I agree with the shingle reps who say, either way will work.

As a former roofer, I'd prefer the approach using the 2x4s above the steel, because it would allow the use of longer roofing nails.


81.
Feb 1, 2013 3:09 PM ET

Second response to Ron Crouch
by Martin Holladay

Ron,
I had a chance to talk to Joe Lstiburek on the phone today, and he agreed with me that either approach will work.

However, he had some advice for you if you choose to install two layers of plywood without the furring strips: since the plywood moves hygrically -- in response to changes in moisture -- while the metal moves thermally -- in response to changes in temperature -- you can get stresses that lead to problems.

The solution is to install the plywood in 4'x4' squares, not 4'x8' sheets, and to include gaps between adjacent sheets (gaps wide enough to insert a nail). The second layer of plywood should also be installed in 4'x4' squares, and should be similarly gapped. The two layers should be staggered (offset 2 feet in both directions). Following these recommendations, you should be fine.


82.
Mar 31, 2013 7:03 PM ET

Attic ventilation
by Paul Nigro

Great article on this topic. This is my situation. Please let me know your thoughts. I have a split level with three separate unattached attics. Each attic has vented soffits, gable vents and power fans. I am having the house resided. Should I keep the gable vents or close them up?


83.
Apr 1, 2013 4:19 AM ET

Edited Apr 1, 2013 4:20 AM ET.

Response to Paul Nigro
by Martin Holladay

Paul,
The most important thing you can do is to get rid of the fans. More information here: Fans in the Attic.

Second, most experts prefer ridge vents to gable vents. Removing the gable vents is probably a good idea -- but if you do so, have some ridge vents installed.

Finally, attic venting is relatively unimportant compared to making sure that your ceiling is as airtight as possible and that your insulation layer is deep. If you can make sure of these two factors, the presence or absence of attic vents hardly matters.


84.
Apr 1, 2013 4:53 PM ET

Martin
by Paul Nigro

Thanks for the info. For now I am stuck with the attic fans as I can't afford to redo the roof. The good part of my situation is that I have a very tight ceiling. No can lights or cracks and the hatches are sealed. With the gable vents and soffit vents do you really think I will depressurization the attic?


85.
Apr 2, 2013 5:08 AM ET

Response to Paul Nigro
by Martin Holladay

Paul,
Q. "For now I am stuck with the attic fans as I can't afford to redo the roof."

A. No, you are not "stuck with the attic fans." Unplug them! Or disconnect the circuit breakers for a few minutes and cut the electrical cords.

Q. "With the gable vents and soffit vents do you really think I will depressurize the attic?"

A. No -- as long as you unplug the attic fans, or cut their cords.


86.
May 20, 2015 3:01 PM ET

1912 craftsman in Los Angeles has gable vents, but gets hot...
by Dan Kegel

Our house is lovely, but has a serious heat problem upstairs in the summer. It has big gable vents but no other venting (other than leakage) and no insulation at all. We're reroofing with energy star shingles, air sealing the attic floor, then insulating the attic floor. Our contractor surprised me by also proposing powered attic vents. I'll push back on that, but it brings up the question: since we're going to have scaffolding up anyway, and will be restoring the raftertails, would it pay to also drill soffit vents?


87.
May 20, 2015 3:13 PM ET

Response to Dan Kegel
by Martin Holladay

Dan,
Your plan to push back against your contractor's suggestion to install powered attic ventilators is a good one. If you need more ammunition to resist the suggestion, you can find it here: Fans in the Attic: Do They Help or Do They Hurt?

Your plan to perform air sealing work and to add insulation to the attic floor -- I hope that you're planning to install at least the minimum code requirements for attic insulation -- should adequately address your "upstairs heat" problem. I don't think that you'll see much benefit from adding soffit vents (especially if the attic has existing gable vents). For more information on this issue, see All About Attic Venting.


88.
Jan 7, 2016 6:36 PM ET

Leaf Blower to clear ridge vent?
by Joel Heller

I live in a small colonial with ridge and soffit vents in Massachusetts and get ice dam damage every few winters whenever we get 36" of snow in a 2 week period like last year 02/2015. So this is when the ridge vents are covered with snow. Can I direct my leaf blower toward the ridge vent from inside the attic to clear the ridge vent without damaging anything? Thanks for your consideration, Joel


89.
Jan 7, 2016 8:17 PM ET

Response to Joel Heller
by Martin Holladay

Joel,
Give it a try and let us know what happens.

You should know that operating an internal combustion engine indoors is dangerous (due to the exhaust fumes in your attic) -- so I hope that your leaf blower is electric.


90.
Feb 2, 2016 12:15 PM ET

I know this is an old post...
by Steve Smith

I'm in a 1 1/2 storey post-war home, which means I can only access 2/3rds of the roof. I can access the lower third of the roof through crawl spaces on either side of the house. I can put in baffles to let air in. I can also access the top third of the roof through the attic and make sure air channels exist in there as well. However I cannot access the middle portion of the roof, since it's right above the sloped ceiling of the upstairs bedrooms (basically roof deck on one side of the rafters, drywall on the other side). Will I be doing anything useful by venting the lower and upper portion of the roof, knowing that cold air will go through the insulation in the middle portion before getting sucked up into the attic?


91.
Feb 2, 2016 1:22 PM ET

Not really (response to Steve Smith)
by Dana Dorsett

Yours is a common construction issue. Usually the rafter bays are blocked a the kneewalls, and there is no free-flow of air even if there WERE a sufficient air gap or sufficient R-value. The best solution is usually to go with an unvented roof assembly insulating between the rafters, then add rigid insulation above the roof deck when it's time to re-roof. The details of how to do this in a moisture-safe way prior to adding the above-deck R varies with climate zone.


92.
Feb 8, 2016 10:29 AM ET

Gable Vents only?
by Clay Whitenack

Am I reading this correctly that I don't have to have vents in the actual roof plane? It is acceptable to use only the gable end vents with the soffit vents? I'd prefer to make as few holes in my roof as possible from both a water intrusion issue as well as the looks of it.


93.
Feb 8, 2016 12:07 PM ET

Response to Clay Whitenack
by Martin Holladay

Clay,
Ordinary gable roofs don't have vents in the roof plane. The usual intakes are located in the soffit; these are the soffit vents. The usual exit points are at the ridge; these are the ridge vents.

Joe Lstiburek notes that you can use gable vents instead of a ridge vent if you want -- but you still need soffit vents as air intake locations.


94.
Feb 9, 2016 6:24 AM ET

Does the type of gable vent matter?
by Clay Whitenack

Right. Still need soffit vents. Does the type of gable vent matter? I assume the triangle vents that sit at the very top of the gable allow for the best removal of hot air collecting in the top of the attic, but what about the rectangle ones that are placed a little further down from the top?


95.
Feb 9, 2016 7:12 AM ET

Response to Clay Whitenack
by Martin Holladay

Clay,
I'm not sure what Joe Lstiburek thinks. But here is what I think: the obsession with attic ventilation details is misplaced. Most attics stay dry, as long as the ceiling has a good air barrier.

If the ceiling doesn't have a good air barrier, you can get into trouble quickly. But if you understand air barriers, these questions about attic ventilation details fade into insignificance.

Here is a link to an article that explains more on the topic: All About Attic Venting.


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!