Vented vs. Unvented Attic
Ventilated vs unventilated attics – Which would you do?
All else being equal, if you were building yourself a new house would you go for a ventilated or unventilated attic? And what are your reasons?
Yes, this is a very broadly worded question, because I’m hoping to get a broad array of answers.
Risinger and Baczek from the Build Show, and Lstiburek all are big advocates of the unventilated approach. Pros are that it makes it easy to bring HVAC into the conditioned space, and also that it makes for a clean space that can be used for storage or even living space. Cons are that it is typically going to be more expensive, at least up front.
Ventilated attics, on the other hand, can be the simplest, least risky, and cheapest way to get a well insulated ceiling. Simply build yourself a rectangular shell, drop some prefab raised-heel trusses on it and lay down a nice thick layer of cellulose. Easy. On the downside, the attic space itself is basically lost to usefulness. Yes, it’s possible, by adding complications to the insulation scheme, to make conditioned space to run ducts but then you lose some of the simplicity.
So which route would you take?
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Replies
Vague questions will always get "it depends" as an answer ;)
Site specific considerations play a role (e.g. are you on a small lot where the building footprint needs to be small and you have strict height limitations and you want to maximize your usable volume?)
Climate zone and other local climate considerations play a huge role as well.
Roof type also factors into things and the roof type depends on factors like whether you are in an earthquake zone.
You might want to throw in a few extra conditions to get more useful insight.
I think it's dependant on the house very much so. I'm in Germany at the moment, and the idea of an unconditioned attic is basically unheard of. Most homes here have 12/12 or steeper roofs and both the 2nd floor and attic/3rd floor are under the roof. I think with a steep roof, it's a lot of wasted space to leave it ventilated. I would much rather have the third floor under a 12/12 roof as a future expansion space than a basement.
I'd go with vented whenever possible, since it's easy, cheap, and robust. I'd go with unvented in those areas where there are outside risk factors involved, such as fire-prone areas. It's just so much more involved to make a reliable unvented assembly, especially with crews unfamiliar with how to do it properly.
Bill
Attic ventilation requirements are a holdover from the era of lots of movement of air through a structure. I have owned 3 houses, each of which had much less ventilation in the attic than as required by code. Diligent air sealing cured the problems 2 of the houses had with ice dams and icicles. When I built the third house, I was once again diligent with air sealing, and added some ventilation, but not nearly as much as required by code. It's perfomed perfectly since then.
seabornman,
I guess the question is whether we want to rely on the premise that houses with good air-sealing will stay that way over time, or build in a bit of redundancy which moves the assemblies further away from being risky?
Around here where space is at a premium attics don't exist, unfortunately lot of it is done with spray foam.
No attic doesn't have to mean you need to go unvented. Vented compact roofs with batts are easy and quick to build if the details are sorted out as part of the initial design.
Anything unvented with exterior rigid is too far from normal construction unless you are talking about a low slope roof. Most low slope roof contractors know how to install exterior rigid and the cost delta for it is pretty small.
My thanks to all for your responses.
I find the title of your question misleading. Vented vs conditioned would be more accurate.
Way too many people think they can get a free lunch and simply by simply not venting and not condition the attic it is "encapsulated" and will take care of itself. Some do win at this game while other end up with a moldy mess.
If the plan is to build a half story building with living space in the attic, then you are not really building a conditioned attic at all.
I think it is a silly idea to put you HVAC equipment in your attic but where I live almost every home has a basement. We all put are HVAC and storage in our basements. I might differently if digging was almost impossible because it is solid rock.
Putting the insulation at the roof line almost always requires some type of foam the most expensive way to buy an R of insulation and the most ungreen choice possible.
Walta
Thanks for your reply. I had no intention to "mislead" and I don't believe I did mislead. Using the word "unvented" does not preclude conditioning the space. In fact I guess I unconsciously assumed that the people responding to my question would assume that that an unvented attic I was referring to would not only be properly constructed but also conditioned, as it should be.
I'm not looking for a "free lunch". I simply wanted people's views on the pros and cons of vented vs conditioned attics, in broad terms. I think that is a reasonable thing to ask.
In your last sentence you've provided one good reason for going the vented route. Thanks.
You've also brought up the interesting issue of basements. That's worth a question in its own right. But I think I'll pass on it this time.
I think the phrases “unvented attic”, “unvented crawlspace” and “encapsulated ___” are best avoided as the free lunch crowed is always looking for an opening and using them leaves the door open. Also, they push my buttons. LOL
Walta
coincidentally, i was just ruminating on this same topic wrt current project.
before i got into the hvac planning aspect, i was very happy with the vented-attic scenario exactly as u say:
pre-fab-trusses with energy-heels, 16-inch blown-in > r-50, sounds good, but in this case, ducted two-story, now figure out how to get ducts to the second-floor rooms.
three options (retaining vented/unconditioned attic) on short-list:
(1) open-web-floor-trusses, in my case, plan currently at 12" floors, so kinda tight (eg max duct size for trim-joist is 8"x16"), so unclear if that will work without going back to the drawing board for thicker floors.
(2) run ducts in drop-ceiling-soffits, but second-floor ceiling only 8", so that introduces challenges with head-room and design
(3) run ducts into attic-space, but want to keep ducts in envelope, so would require fancy inverted-soffit strategy which introduces challenges with framing, insulation and envelope continuity
all the things i think jgsg was alluding to in terms of trade-offs...
I have recently been ruminating over this topic myself. I am curious about an assembly that looks like a traditional vented attic, but is unvented and vapor open with an air gap below the roofing. If you had an interior air / variable vapor barrier and an external vapor open membrane you could prevent your attic insulation from getting washed with cold air. The air gap below the roofing would keep the roofing cold to prevent ice dams. I need to ruminate on the physics of this a bit more...
Matt,
That sounds a lot like this roof assembly - although the gap under the roofing is vented.
https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
There are code compliant assemblies that only use permeable insulation and just rely on being vapor-open rather than venting, by using a vapor-diffusion port near the peak. But they only work in climate zones 1, 2, and 3.
Hi Matt,
Interesting idea. If I understand correctly you'd have your typical "attic" space above the ceiling insulation. And that space would be unvented. It's upper boundary would be the roof sheathing, typically OSB or plywood. The issue then would be the permeability of that sheathing. Would it be adequate to be the "external vapor open membrane" you mentioned.
I understand that plywood is quite a bit more permeable than OSB but I don't know whether it would be open enough. It's beyond my expertise.
Malcolm, thanks for the video link. It's a very interesting approach which doesn't stray too far from current common practice. I like that because it doesn't scare away too many of the contractors who aren't familiar with the most up to date best practices.
How practical is it to use something like Salinger’s approach in a retrofit? I know the usually recommended approach is to put X% of the insulation on the exterior of the roof and the rest on the inside. But if you’re replacing the roof and cost is a factor what about furring it out as a vented over roof and then dense packing below the original roof sheathing. (I’m also assuming old tounge and groove boards because it’s what I have.)
iaianb,
The roof you are suggesting would rely on moisture diffusing through the board sheathing, much as it would though the membrane in Josh Salinger's one.
Practically, I don't see why that wouldn't work, if you had a good interior air-barrier. However, as I posted in #13 above, the codes I'm familiar with don't allow venting above the sheathing with permeable insulation below, except in very specific circumstances. It would definitely be something to run by a building inspector before proceeding with.
I would use neither. Another method I saw in an issue of Fine Homebuilding magazine is to build your house (for example, a one story house) with the attic floor built as a second floor. You put a layer of OSB or plywood sheathing ON TOP of the attic floor joists and seal the exterior sheathing edges to the tops of the exterior wall top plates. Then you put your attic trusses (or stick built rafters) on top of the "attic floor".
The method allows you to completely separate the attic space from the conditioned main floor living space. All your ductwork can run in the "attic floor" system since it is within the conditioned environment. Plus, your attic truss installer has a nice flat platform to walk around on while they do the install, and all your insulation sits on top of the attic floor sheathing (pile it as high as you want).
As a bonus, you don't have to worry about installing pot lights in your ceiling since the pot light cans are also totally within the interior environment. Now you can have a vented attic, which Joe Lstiburek says is the simplest, easiest attic to build and not screw up, totally separate from your indoor space.
rockies63,
I have no experience with the assembly (I think Mike Maines has used it). When I've thought about it I've have a few reservations, which may or may not be worth worrying about.
- The assembly adds an entire, largely redundant floor system to the cost of a house, including deep joists needed to clear span the structure.
- It does help air-seal the ceiling, but introduces another set of rim-joists to the building, which are difficult to insulate and air-seal.
- A service cavity consisting of floor joists only allows ducts to move on one direction. It still probably needs some dropped bulkheads to get the ducts to each room.
- You build a complete floor system, but don't end up with a usable attic, as the trusses still sit above the sub-floor.
- While it may appeal to DIYers, having a floor to set your trusses from doesn't make sense for most builders. It's a job that takes maybe two days at most.
On a simple roof shape, the cost to do all this seems like it would be comparable to the cost in doing the foam free sarking membrane.
Tim_O,
No, it would be considerably more labour and materials. That's the point I've been making. It represents a lot more stuff. The question is whether the benefits make it worthwhile?
Malcolm:
1. You're going to build a "ceiling joist system" above your living space anyway so that you have something to attach the drywall to. The method I propose simply moves the air/moisture barrier from the drywall to the OSB or plywood sheathing. The joists don't have to be too deep - it's not like anyone's going to be walking around up there, other than the truss installers.
2. You're going to have to insulate the rim joist between the first floor and the basement (probably with spray foam) so what's difficult about doing a second rim joist?
3. You can use open web trusses or I joists for the attic floor if you have large ducts.
4. Other than for living space, why do you need a "usable attic" - all most people stick up there is HVAC equipment and Christmas decorations.
5. The primary benefit is air and moisture sealing, plus keeping ducts and light boxes within the conditioned space.
6. Bonus: All the insulation on the flat, open attic floor can be easily installed - you don't have to fit some in between attic floor joists.
7. Bonus: If you ever do need to go into the attic, you don't have to worry about your foot missing the top of a joist and plunging through the drywall ceiling :)
These seem like a lot of advantage just from adding a layer of OSB sheathing to the attic floor.
rockies,
I remain unconvinced.
1. If there are trusses above, the "ceiling joist system ", both joists and sub-floor are entirely redundant. Drywall gets attached to the bottom chord of the trusses.
2. Many one storey houses have no rim-joists. Rim-joists are time consuming and difficult to insulate and air seal. The assembly introduces another layer of them that with just trusses doesn't exist.
3. Open web trusses or I-joists add about 18" to the height of the whole structure - and both are expensive.
4. You don't need a usable attic - but you just built a floor system for one, then covered it with trusses.
5. It makes sense to provide space for ducts within the conditioned space. There are a variety of ways of doing that which don't necessitate another complete floor system. Light boxes and wiring simply aren't very difficult to seal effectively.
6. What attic floor joists? Maybe we have been talking at cross purposes? I thought the assembly you have suggested is essentially a service cavity below a trussed roof (like the FHB article). If you are talking about building an attic with floor joists and a stick framed roof above, then sure it makes sense to sheath the floor.
This type of assembly only makes sense if you are stick framing the roof with the goal of finishing the space down the road.
I really think it is a solution in search of a problem.
A ceiling air barrier membrane is easy to do and easy to air seal. Many ways of doing ducts that don't require any service cavity or bulkheads.
Adding a couple of grand in materials plus the day or two to frame up the attic floor seems like a waste.
Akos and Malcolm, I disagree. The system I suggest is similar to the "Monopoly" system that Matt Risinger used on his own house, although he created his "air-sealed box" to include his attic space so it was inside the conditioned envelope. What I suggest moves the air-tight layer from the roof sheathing to the top of the attic floor.
As to using drywall to air seal a ceiling from the attic, I've read many articles about how difficult it is to do it properly. Plus, with your ceiling drywall attached to the bottom chords of the attic trusses there will always be truss uplift and that can potentially result in air and moisture leakage into the attic.
I'd rather build an air-tight box and stick a roof on top of it - trusses or stick built. Do it once and very quickly you'll forget about any additional expense.
rockies63,
This isn't one of those pure building science questions where there is a right answer. It's a perfectly valid approach, that there can be differing opinions on. It's just not one that for me seems worth incorporating into the way I build.
Rockies,
If you are going down the road with this type of assembly, do the future a solid and spec scissor/parallel chord trusses for the roof. If you tell the truss supplier that bottom is tied together with a floor system, they won't have to be anything too beefy and cost about the same as a regular truss.
This will keep the attic floor open to walk around during the build, plus if you or future owners ever want to finish the space or even use it for storage, it will be a breeze.