To vent or not to vent the attic? (New construction)
I am building a new home that will have a conditioned crawl space in which the air-handling equipment and all ductwork will be located. I plan on tightly air-sealing my ceiling (likely with a 1-2″ of spray foam as a sealant). I’ll then insulate the ceiling with cellulose insulation which exceeds the 2012 IRC requirement for my climate zone (Zone 3, northeast Texas – R-38 required, likely will shoot for at least R-50).
The Texas summers here are hot, and regardless of whether I vent or not vent, the attic temperature will be high. That’s fine with me – I won’t be using the attic for storage or other purposes. Venting the attic would seem to accomplish little other than allowing humidity into the attic – an obvious undesirable result.
Am I correct in thinking that I could NOT vent the attic, seal and insulate the ceiling, and not condition the closed attic space? If I am not correct, what is the reasoning behind venting it?
I apologize for what appears to be some duplication with other posts, but I’m not sure that anyone has addressed this particular situation of not venting AND not conditioning the attic.
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How are you not conditioning the attic? I would think that if the insulation is at the roof, the attic will be conditioned by default.
Brian - I'm not conditioning the attic because there will be NO insulation at the roof. All the insulation will be at the ceiling.
This would be identical to the traditional attic where the ceiling held all the insulation, there was no insulation at the level of the roof, and the attic was vented. The difference is that my attic would NOT be vented.
My question is this: if I have sufficient (greater than IRC 2012 standards) insulation on the ceiling, AND the ceiling is very well air-sealed, is there a need to vent the attic?
Again, the attic will contain no mechanical equipment or ductwork - all of that will be in a conditioned crawl space. Thanks for any insights you can give.
I missed that, my bad! I’m trying to decide whether to insulate my ceiling or roof as well. I think I just read an article on here that talks about venting being unnecessary with the right air sealing. I’ll just wait for the experts to weigh in.
Is your plan code compliant? Does not sound likely to me, but I do not spend a lot of time reading the code books.
I would be concerned that bottom of the roof will be cold and wet all winter get cover in ice and on the first warm day wet your ceilings and then grow moldy.
Why would you want to take the risk? I see no advantage in not venting an empty attic.
Please take the time to watch this video by Dr. Joseph Lstiburek a building science god.
https://www.youtube.com/watch?v=Ld8pzIu45F8
Walta
Hi Walter,
In the building science field, there is now debate about whether or not it is necessary to vent an attic, assuming that the ceiling is tightly sealed such that there is NO communication between the conditioned living areas of the house and the attic. That's the purpose of my proposing that I spray the top of my ceiling with an 'air barrier' layer of foam (1-2" thick) and then, on top of that, put an insulative layer of cellulose.
In his article titled 'All About Attic Venting' (link: 'https://www.greenbuildingadvisor.com/article/all-about-attic-venting') , Martin Holladay debunks several common myths about why attic vents are needed. Martin goes on to make this statement: 'It’s safe to say that ventilating attics in a hot, humid climate is just plain stupid.'
Martin acknowledges in this article that Joe Lstiburek is a proponent of attic venting. So obviously, at least at the time the article was written, there was disagreement among the experts. Most codes (which tend to lag the latest technologies) still require venting.
So, there is apparently ongoing debate concerning whether to vent or not to vent, hence my asking for the community's current thoughts as to such. I'd prefer not to vent unless someone has a compelling reason for why I should. Then again, I don't want to be on the forefront of an unproven building method.
By the way, ice and snow is not typically a problem in the area of the country where we live - and thank God for that! And, as to your question whether my home is code-compliant, that's a little tricky to answer. I live in a rural area without codes. My house is being built (by myself) to be code-compliant in all areas EXCEPT areas such as energy conservation where there is a trend towards more futuristic technologies that code hasn't yet caught up with.
You'll need to vent your attic because water vapor (showers, laundry, people) will still migrate through the ceiling into the attic space. You need to some way of getting rid of it otherwise the attic will turn into a moldy mess.
If you opt to not vent that attic (insulation in the roof rather than on top of the ceiling) then you bring the attic into the conditioned space of the house and the HVAC system can be used to deal with winter time moisture in the now unvented attic space.
Hi John,
I appreciate your thoughts. See my reply to Walter above. Definitely, if I have an unvented attic my approach would be to isolate the attic from my living space so that vapors do NOT enter the attic from the house.
I did consider doing as you suggest - insulating the roof, putting a duct into the attic, and bringing the attic into the conditioned space of the house. In fact, that was my first thought. However, it's definitely a substantial expense to do so. I can't really find a compelling reason to spend the money to have a conditioned or semi-conditioned non-vented attic since I won't have mechanical equipment and ductwork in the attic and I will not be using it for storage.
Hence, my thoughts on sealing off the attic from the rest of the home and not venting it. Apparently this idea is being kicked around in the building science field, and I'm hoping to find out what the current thoughts are. If it ends up that I'm way out in left field, I'll just default to a standard vented attic.
Stephan,
The recommendations and code requirements to vent unconditioned attics assume you will isolate the house below from the attic through good air and vap0ur sealing. All ventilated attics are built that way, it isn't an extra step you are taking that will insulate you from the problems unvented attics experience.
I don't know enough about hot humid climates to comment on what the best approach in your area is. No doubt Martin and others will have some suggestions tomorrow.
I just re-read your post and I assumed you were talking open cell spray foam.. If you use 2" closed-cell spray foam to seal the ceiling then moisture migration isn't going to be an issue. Of course getting code approval seems to be the biggest issue along with warranty on your choice of roofing material.
Hi John - thanks for your additional comments. Code approval is not a big issue for me. I live in a rural location in a county without code enforcement. That said, I do of course want to do the job right. So, your input along with the others on this forum has been very helpful to me.
Stephan,
The main issue here concerns code requirements. As far as I understand, unconditioned attics in Texas still need ventilation, according to code requirements.
You're right that providing an airtight ceiling is far more important than your attic vent details. That said, soffit vents and a ridge vent won't cause any problems, so you might as well install them.
If you were interested in insulating along the sloped roofline -- and you're not -- you might consider using vapor diffusion ports. (For more information, see "Vapor Diffusion Ports.")
Thanks, Martin. Yours is one of the definitive wordsthat I was waiting for. I have nothing against ridge and soffit vents, but if not necessary, I was more than happy to omit them. And, in reading your article 'All About Attics' referenced above, I began to question the need for soffit and ridge vents in a properly sealed and insulated ceiling.
Now, with your current suggestion that they should be used, I'll definitely do so.
I originally did plan to insulate the sloped roofline. However, with no ducts or mechanical equipment in the attic, I began to question my original plans of having a conditioned attic. It seemed to be a lot of additional work and expense for no discernible value.
I'll look into the vapor diffusion ports.
Stephan, we live in a very different climate (zone 6 in NH) compared to you, so I'm not sure our experience will help, but it might. The barrier between our second floor and unconditioned attic is uninterrupted drywall (done before the second floor partition walls), finished/painted, with no penetrations except one plumbing vent and the solar conduit. It's very well air sealed with R70+ of cellulose above, so the attic is very isolated from the conditioned space.
We wanted a minimal amount of easy to install attic ventilation, so we blocked off the soffits and just have a vent at each gable wall, one is a hinged gable vent big enough to enter via ladder. We ordered the vents from: https://www.customgablevents.com
This has worked well and we have no moisture or other issues in the attic, which I visit a couple times a year. If you need to vent your unconditioned attic because of code, this might be the easiest method vs soffit to ridge venting. Some photos in our GBA posts:
https://www.greenbuildingadvisor.com/article/windows-housewrap-and-roofing-underlayment
https://www.greenbuildingadvisor.com/article/siding-and-interior-finish-work-in-new-hampshire
https://www.greenbuildingadvisor.com/article/finishing-touches-for-a-pretty-good-house-in-new-hampshire
Thanks, Brian. That makes perfect sense to me. I'll look into the hinged gable vents.
Stephan,
One more point: The biggest worry surrounding the introduction of hot, humid air into a vented attic in Florida or Texas concerns attic ductwork -- the humidity in the air can condense on galvanized register boots. In your case, there is no attic ductwork -- so there are fewer worries.
Thanks for the additional comment, Martin. So, if I air-seal the ceiling very well, use ample insulation on the ceiling (cellulose, likely), and ventilate with soffit and ridge vents, I should be okay both code and energy-wise?
Still, is this the preferable way to go? Or, should I go back to my original plan of putting an inch of rigid foam (above the sheathing) under an air gap and metal roofing , and then spraying between the rafters, with the end result being an unvented, unconditioned attic. Which of the two options is preferable in your opinion?
If I go with the rigid foam on sheathing and the metal roof on top of that, do I need an air gap between the two leading up to a ridge vent? Should my WRB be under the foam (adherent to the sheathing) or should it be on top of the foam (adherent to the foam). How do I vent the air gap at the edge of the roof so that it lets air in (which can then flow up under the metal to the ridge vent) without also letting in insects?
Thanks so much to you and the others who have contributed to this discussion.
Stephan,
Q. "If I air-seal the ceiling very well, use ample insulation on the ceiling (cellulose, likely), and ventilate with soffit and ridge vents, I should be okay both code and energy-wise?"
A. Yes.
Q. "Still, is this the preferable way to go?"
A. Yes.
Q. "Or, should I go back to my original plan of putting an inch of rigid foam (above the sheathing) under an air gap and metal roofing , and then spraying between the rafters, with the end result being an unvented, unconditioned attic?"
A. No.
Q. "Which of the two options is preferable in your opinion?"
A. Cellulose above a flat ceiling, with a vented attic above.
In zone 6 where I live Martin's answers are spot on.
The other reason to ventilate is to control heat. Venting your attic might drop its temp in the vicinity of 40 degrees or more. Making your house more energy efficient and comfortable. In the past when I add ventilation or insulation I notice it most in comfort.
Stephan,
You say that your ceiling insulation will include 1 to 2 inches of spray foam. Is your plan to have a contractor spray foam over the ceiling drywall? If so, does your design provide adequate access and clearance for this work to be performed?
I wonder how the cost of that work might compare with the cost to continue the wall sheathing up and over ceiling joists? Rather than spray foam, your air barrier would be taped sheathing. Two benefits I see from this approach:
1) You are left with a service cavity between the ceiling drywall and the air tight sheathing above the ceiling joists. A service cavity would be great when running wiring and installing overhead lighting, etc.
2) A framed and sheathed attic floor would provide a safe deck to walk/work on when framing the roof or setting roof trusses.
John Brooks, an architect in the Dallas area, proposed this idea on GBA back in 2011. See the first several comments following the GBA post of Joe Lstiburek’s ‘Rules for Venting Roofs’. https://www.greenbuildingadvisor.com/article/lstibureks-rules-for-venting-roofs
Carl Seville, the self proclaimed Green Building Curmudgeon, used this approach with Zip sheathing for his own build in Georgia. https://www.greenbuildingadvisor.com/article/topping-out
I really like this detail, and am thinking about including it in my own project. For me the material costs aren’t huge since the house I’m planning is small and simple in shape. I’ve mentioned this idea in other QA threads but havn’t gotten much feedback.
Hi Aaron -
I like the idea of having a walkable attic, however, in my case the ceiling joists are an integral part of the trusses that I will be using. I think because of the truss webs, my attic is not going to be useable for much - other than possible storage, which I don't need because I live on 57 acres with plenty of barns! If I were stick-building the roof, I'd definitely consider this idea.
My house is also small (1229 sf living space with 650 feet of screened porches) and is a 1-1/2 person job (my wife and I - I'm not saying who the 1/2 person is!). We opted for trusses rather than stick-building the roof because they go up quickly AND they minimize the time that I'm up in the air ... I hate heights.
We're still kicking around ideas about how best to insulate and vent the top of the house. We're planning a metal roof, and my initial thought was to put 1" of rigid foam under the metal and then spray foam between the rafters and have no attic venting. I still like that idea, but haven't been able to find details on how to provide and vent an air gap between the metal and the 1" of rigid foam on the top of the roof (or comments as to whether such an air gap is even necessary). This would give us an unvented attic, but since there are no air ducts in the attic (they are all in a conditioned crawl space), I'm unsure if this would be acceptable per code as it would give us an unvented, unconditioned attic.
If you go this way, your attic will be conditioned because your ceiling will not be airtight. You have to understand that heat can be transferred from one place to another by three different methods : conduction, convection and radiation. When you put insulation, you are only slowing conduction transfers but you are not blocking air movement (convection). Warm air will move right through your ceiling insulation into the attic unless you put up an air barrier. Drywall can act as an air barrier but you'll have to seal every penetrations. So in this case, since this is a conditioned attic, there is no reason to vent it but you will pay to heat it even if you don't use it unless you air seal it.
As for a having a gap between the metal roof and the foam, you just to need to add furring. Most vertical seams metal roofs can be directly installed on furring strips from what I've read. You want to do this for two reasons. One : you are in Texas, therefore your roof will get very hot which can damage the foam or the underlayment under it. A gap behind the roof will provide some ventilation and therefore cool it a little bit. Two: if your roof has a leak, the gap will drain the water out of the roof or at least allow it to dry quickly.
Why not just add a window or skylight to allow natural light into the space and the option of opening the window if ventilation is needed or solar powered vent.
Tom,
Q. "Why not just add a window or skylight to allow natural light into the space and the option of opening the window if ventilation is needed?"
A. Because this is an attic, not finished space. No one is ever going up there.
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Tom,
Martin knows that no one will be going up there because I told him that would be the case. I have absolutely no need for additional storage space (I live on a farm with plenty of barns) and attics in Texas are miserable places to be, even for minutes at a time. I'll have no mechanical equipment or ductwork in the attic - it will all be in a conditioned crawl space. Other than during the build, blowing in insulation, I have no plans to ever go up there other than an perhaps a quick annual inspection - during the winter!
I have been in the roofing industry for over 45 years. My experience tells me that not venting an attic will have a negative effect on roof covering. Asphalt shingles will have a decreased life span and wood shakes or shingles we'll also last half as long as expected. Kynar coated metal roof systems will peel before their 20-year warranty expires. If a roof system is installed that lays roof panels on batten's the warranty may still hold up. The cost of installing attic vents properly will be recovered very quickly by extending the life of a roof system. Also a completely sealed Attic in that neck of the woods will definitely have increased attic temperatures and could affect trusses and sheathing! Vent the Attic!