Conditioned Attics for Airtightness
Are conditioned attics the only _real_ answer when airtightness is a priority?
Hello: I’ll provide backstory, but first, the thrust of my question: if I want an extremely airtight home, is my only _real_ option to condition the attic?
Why I’m asking:
I’m a future homeowner building a 2 story home in Climate Zone 5, which means I am fortunate enough to have a basement.
That said, even with the basement, it is a struggle to keep ductwork out of the attics – we will have a dedicated ventilation system, and dehumidification… etc… it is not impossible, but it is challenging to run all of this in walls / floors.
I’ve read every article on here and on building science.com, and every page of “a house needs to breathe?”… I’m very thankful for these resources, and I’m very familiar with our theoretical options.
That said, even if we keep the ducts out of the attic somehow, still, the sheer volume of penetrations through the attic floor have me nervous. Can lights, speakers, attic hatches, wires, wall tops, etc…
It honestly feels like the decision the community avoids accepting is either:
1. Spend a tremendous sum of money to put 7.5” of CCSF under the roof*, or…
2. Spend a lot of time and energy and slightly less money to finagle ducts into weird locations and settle for “we’ll do our best!” air sealing of the attic floor.
My architect and builder are awesome, so no concerns there… they will do a good job, and they are hesitant to see me spend a ton of money on spray foam.
I’m just trying to be realistic with what can really be achieved if I don’t bite the bullet.
* – yes, I know this varies by climate zone, and also that a hybrid assembly would theoretically be an option in mine… but that’s a topic for another question, since I can’t seem to find a real way to get a hybrid assembly to work here unless I just use open cell foam over closed cell foam, which saves almost no money. Exterior roof deck insulation is a non-starter around here… no one does it, so I’m swimming upstream.
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Replies
User ...443,
Let's unpick this a bit.
There are viable alternatives to running the ducts below the attic when there isn't a good path for them in the basement. Using these singly, or in combination do that:
- Plenum trusses
- Dropped bulkheads
- Thickened walls
- A ceiling service cavity
There are two types of penetrations into the attic:
-Those that go through the top plates of walls - pipes and wires - are very simple to seal.
- The ones in drywall can be limited to standard electrical boxes, which again are easy to make airtight.
That leaves the interior partitions:
- Drywall the ceiling prior to framing the interior walls
- Seal from the top side once the drywall is up
- Run strips of poly over the interior partitions prior to installing the trusses, and seal these to either the drywall or the ceiling membrane if there is one.
It's also worth remembering, that while you don't of course want air-leakage for the energy penalty it brings, unconditioned attics with good ventilation are relatively unaffected by incidental air leaks, the way enclosed assemblies like walls and cathedral ceilings typically aren't.
All this is a long-winded way of saying I'm not at all convinced conditioned attics are the only choice in a Pretty Good House.
Malcolm, replying here because I ran out of thread space below: thank you for clarifying your location and for your last response below.
I suspect that the fact that Canada habitually builds better homes is a big factor. You likely have a more realistic shot of taking some of the good steps you describe here, and also a realistic shot of even non-custom homes being able to take some of these steps without needing CCSF.
I'm going to push as hard as I can to make some of these things happen. Wish me luck!
MikeB555,
I am shocked by some of the construction I see from south of the border. Some of those no sheathing except foam houses in Nevada seem insane.
I should clarify too: I'm not against using conditioned attics, I just don't think they are the only assembly that works - although depending on what builder's regular practice is where you are, they may well be the only viable alternative.
Good luck! Building a house is a difficult and complex process without having to worry about the deficiencies of the trades in your area.
When you build a house people will propose lots of stupid ideas to you because it will make their life easier. As the home owner it is your job to say no to stupid it will not always be ease to say no. be strong and lean to say no.
Say no to half story second floors.
Say no to cathedral ceiling.
Say no to spray foam.
Say no HVAC in the attic and the conditioned attic.
Say no to skylights.
Say no to recessed lighting.
Say no to plumbing in exterior walls.
Say no to the wood stoves /fireplaces.
The conditioned attic is stupid because the roof line of you home will have 30-60% more surface area than the flat attic floor. If by some miracle you can afford an R60 at the roof line you will always loos 30-60% more energy than the flat ceiling. No one builds an R60 conditioned attic because it is almost impossible and would be total unaffordable. Anyone can dump R60 of cheap fluffy insulation on an attic floor at a very low cost. When you try putting insulation at the roof line you are all but forced to use sheet foam or spray foam the riskiest most ungreen and expensive way to buy an R of insulation possible.
Conditioned attic almost never meet the code min R value and side step that common sense rule thru a dubious loophole. In short the conditioned attic cost more and underperforms!
Walta
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Just two cents from someone going through the design process now - putting foam on the exterior of your roof might not be /that/ hard if your builder is willing to put in time for a bit of education. Ours is doing it for the first time with us, and he has had conversations with his material suppliers on their recommendations, he's read articles from BSC on the topic, and we have details in the plans for the roof eaves, the roof ridge, roof-to-sidewall transition, the roof headwall transition....
It's not as cheap as putting loose fill cellulose on the floor, but it's also not going to be tens of thousands of dollars. I can't counter the argument that it's not green, though - foam isn't as green as cellulose.
There is more to buildings than energy efficiency.
But if you're designing from scratch, where do you have to sacrifice energy efficiency to get what you want?
When you don’t want a flat ceiling, or you are designing to size constraints that require the entire use of the shell, or want to bring more light into a space that can’t be achieved with windows. Or perhaps you just want your house to look and feel a specific way that isn’t necessarily an airtight box.
But the OP is asking if the conditioned attic is the only viable path *from an energy efficiency standpoint*. They didn't say they wanted a cathedral ceiling and ask if that is energy efficient.
I was not responding to the OP, I was responding to Walta list of things to say no to. The OP also didn't suggest a wood stove.....
Apologies, I didn't notice where it was nested.
Thanks Walta... dumb question, what is a half-story second floor? I have a feeling I've already said yes to this (2,500 sq ft. main level, 1,000 sq ft second floor)... so I may be on the naughty list there.
Other than that and recessed lights, I'm leaning towards following your guidelines and saying no to all the others. :)
Half story is when the walls only go half way to the ceiling, then the slope of the roof connects the wall to the ceiling.
Whoa... staying away from that concept altogether.
MikeB531,
Half storys are only a problem when they include knee-walls enclosing a space between the outer ones and the rooms. Otherwise they perform just like any other cathedral ceiling. I have them in my own house.
If you set you heart on a new construction half story home Yes it can be built in a tight energy efficient way but it will be an up-hill battle.
Generally, the air barrier is not drawn in the plans. At the point where the second story floor end and the attic begins this connection becomes a huge void in the air barrier if left to the carpenter’s discretion. If you take the time to think it thru and draw detailed air tight plans it becomes a budget item.
The next challenge is when the knee wall ends and the sloped ceiling begins the sloped ceiling is a cathedral ceiling and there is not enough room for all the things that need to fit in the tiny space you have especially when you must have recessed lighting and of course you will have recessed lighting. The recessed lighting displaces the paltry amount you can fit in the tiny space to make matters worse the “air tight” fixtures leak like a sieve and let warm moist air get to the cold roof deck allowing it to get wet grow moldy and rot away.
Pick one of the cathedral ceilings in this article.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
They now have surface mount LED lights that are less than a half inch thick, functionally equivalent to recessed lighting.
I'm in the land of basements and except for retrofit of older homes, never seen ducts in the attic. Definitely no ducts in the attic for any new build.
The reason for this is simple. Cost. In cold climate, you want registers at the floor level, so the 2nd floor ducting runs through the floor joist. Running up to the attic and then to the rooms is actually more complicated, more material and more labor.
As Malcom has said, ducts can be hidden in plenum floor trusses if you want to minimize bulkheads. Bulkheads are also not a big deal, think of a bulkhead above kitchen cabinets, nobody would notice that it is there. Same for a dropped ceiling in hallway. You can also build thicker interior walls and run the plenum through that if needed. A service chase from the basement can be hidden behind a pantry or behind built in shelving. If you want it neat, clean and unobtrusive, it does require a bit of design up front though.
About the only duct that you should run through the attic is the bathroom exhaust fan.
In new construction you can design in space between the second floor ceiling and the attic floor for ductwork, all your mechanicals, and a lot of insulation. You can make that space as thick as you want. Either use open trusses, or separate joists for the ceiling and floor.
I like this idea... the problem I'm encountering is that no one seems to do it around here. Open trusses seem to be effectively out of play.
The tightest project I've designed had a ventilated attic and a full basement. It tested at 0.12 ACH50.
Daaang... I'm hoping to get to .5 ACH50, so you've got me beat. Did you end up keeping all ducts out of the attics?
If so... single story, or multiple? If multiple, how did you address this concern?
I was the designer, not the builder, and I had only spec'd 1.0 ACH50 as I think that's as tight as buildings need to be. But I have no problem with builders who want to try to get tighter. The only duct in the attic was for makeup air. Single story on a full basement, about 2200 sq.ft. Ductless mini-splits for heating and cooling and open web joists for running the ERV ductwork in the first floor framing, with wall stacks.
I am home owner of a 2 bedroom house in zone 6a with ductwork in the attic, but this was done in 1986. To mitigate the issue, we sprayed 2 inches of ccSPF on the ceiling. I installed a false floor above the ducts to keep them from being crushed for the occasional maintenance. We then have 16 inches of fluffy stuff on the floor.
The attic is 32F when the outdoor temp is -22F, so we should have installed 4 inches of ccSPF instead of 2 inches.
During the summer, the attic stays below 100F, so things are comfortable. I do monitor the RH of the attic to make sure it remains below 80%.
So yes, it can be done, but I would instead install a floor truss instead of rafters and focus on air sealing.
Is it kosher to do the air sealing insulation at the roofline, along with supplemental fibrous insulation on the attic floor?
I've had folks advise me to do that, but haven't been able to find it blessed formally anywhere.
MikeB You are proposing an unvented unconditioned attic.
I call it the free lunch attic because you are unwilling to pay to install and operate the equipment necessary to control the temp and humidity of the air in the attic. What will happen is totally unpredictable and likely a code violation.
The air in the attic tends to be very humid making the dew point of the air in the attic very close to the air temp. From time to time the exterior surfaces happen to get colder than the dew point of the air. When that happens, things get wet. If things get wet often enough and stay wet long enough mold starts growing and things can begin to rot.
Some people win this lottery and get the free lunch while other get mold and rot.
Do you feel lucky?
Walta
Thank you sir... I'm more clarifying the idea since I've seen it multiple times - no proposal on my end.
What I REALLY wish I could do is a hybrid roof assembly with CCSF (thick enough to eliminate condensation risk) + BIBS. I see those described often theoretically, but can't quite figure out how to actually make one happen without magic.
This "add some insulation to the floor" is interesting because it's almost the same idea... but being as conservative as I am I would ventilate near the peak, and that would semi-defeat the split insulation approach anyway.
**Editing here since I can't reply to the comments below:
For clarity - I'm really not proposing this split insulation thing.
On the parallel chord trusses - I've requested that option and can't seem to get any support, i.e., they are rare around here. Do I live in the dark ages? Are these common in other areas?
MikeB531,
According to comment #2, it appears to contravene the IRC.
https://www.greenbuildingadvisor.com/question/insulation-19
Hybrid roof with ccSPF and dense pack (fiberglass or cellulose doesn't matter) is pretty simple with dimensional lumber, I-joists or parallel cord trusses for rafters. About the only challenge with trusses is you need to partition the cavities to be able to dense pack.
About the only light magic needed is if you are using batts, you need to hold them in place with something until the ceiling goes up. Duct straps or insulation wires work.
Maybe sketch what you are looking and and we can provide some ideas on how best to insulate it.
P.S. Besides being against code, split insulation is a bad idea as the in-between attic space is neither inside or outside. The temperature is much lower than the house but the air moisture content is about the same so RH can be very high in the winter.
I don't see a problem with it, it just seems like unnecessary trouble and expense. Normally you either insulate the floor of the attic and ventilate the space, or if you're not going to ventilate you use impermeable insulation against the sheathing so there's no surface cold enough for condensation to form. Usually the only reason not to ventilate is if you want to use the attic space as conditioned space.
Foaming the underside of the roof and not using the space as conditioned space is kind of the worst of all worlds.
Many people say not to split the insulation like that. But at the least remember that because your impermeable insulation covers more surface area, it's effective R value in terms of condensation control is lower than the fluffy stuff on the floor, which covers a smaller area.
And +1 on the "why?". I see little benefit and some potential downsides.
Another day, another GBA post with Walta, Akos, and DC making me rethink everything mid-build. I've been all in on the Matt Risinger conditioned attic boat and have planned on coughing up to do 2-3" of closed cell in my entire build, walls and roof deck. Now you three are making me think that I'm wasting huge money for no benefit. To be honest I've already been shaky now that the trusses are in and i can see just how big the roof deck is. But now I feel like it is too late. I'm also doing a second floor in part of it and being slab on grade, I'm struggling to figure out where to put air vents without dropping the ceiling
If you have no basement... you should feel pretty good about conditioning the attic. I've been digging into this for months (years?), and it's hard to get comfortable with all the hacky work-arounds needed to avoid conditioning the attic...
...and that's even WITH a basement.
MikeB531,
When vented attic roof assemblies are the overwhelming majority of those used in North America, and experienced builders are telling you they routinely use them successfully, it might be worth reevaluating that blanket statement.
Malcolm, with respect - to start to respond, I have to call out that your comment is misleading and avoids the topic at hand.
Regarding "when vented attic roof assemblies are the overwhelming majority of those used in North America": the overwhelming majority of homes built in North America... let's call it 98%... do not even grapple with the focus of GBA or with building science in general beyond threadbare code compliance. They have HVAC equipment and ducts in the attic, or run a single, huge, poorly-designed zone from downstairs, or take any other number of garbage approaches that result in uncomfortable homes.
We're clearly talking about the remaining 2%, so I ask that you not cite the irrelevant 98% to support a position.
Within the remaining 2%, my opinion is that we are still sorely lacking for an approach that scales. I think you undervalue the skill and thoughtfulness that you and other builders must bring to the table with several of the approaches described above. Putting aside bulkheads (ugly) - utility chases, careful truss design, attentive air sealing at every penetration - these require a level of thoughtfulness that is uncommon in the industry and unevenly available across the country at best. Believe me, I've pushed and pushed and pushed, and some of these approaches simply will not gain traction in my area.
On the flip side, conditioning an attic with CCSF is Geico caveman-simple, and it gives HVAC designers so much flexibility they could cry with happiness.
It is impressive and encouraging that some folks, including you, have been able to avoid the need for CCSF (or other options) with the techniques you describe above. If the industry could do this reliably and at scale, conditioned attics would never need to exist.
Until then, I'm hoping a very smart person invents cheap CCSF that doesn't burn and doesn't impact the environment. It could happen, right?
MikeB555,
I'm not trying to be argumentative. Let me explain my perspective. Here in Canada we don't see the same stark distinction between how houses are built. The industry here already puts out "code minimum" houses that perform quite well. They and many of the more efficient houses that get built often use unconditioned attics without any apparent issues. So it came as a surprise when you presented the premise that there was a problem at all - and one that could only be solved by using a conditioned attic.
Surely if we are only talking about a rarefied few high performance houses, they are by definition custom, with custom building assemblies, and custom duct layouts that demand custom solutions to dealing with the service spaces, rather than something that scales? Dealing with ducts in unconditioned attics is an obviously bad idea, but my point is that finding other places for them simply isn't difficult or complex enough to warrant dismissing a perfectly good assembly.
I come up against all sort of problem in my practice I wish there were better solutions for, but this just isn't one that taxes me. Better spray foam isn't something that would affect the way I design or build.
Eric, Now to are going to make me do type out my spray foam rant.
When you I large amounts of spray foam in a set of new construction it is a huge red flag for lazy design work done by someone who has zero regard for your budget.
Spray foam is the most expensive and least green way to buy an R of insulation.
Say NO and demand better alternative solutions.
Walta
I'm going to try to find time tonight or tomorrow for a full post seeking recommendations but the reason I was going to spray foam is that I didn't build a standard stick built house with exterior insulation and zip sheathing, I did a barndominium which wasn't going to have sheathing at all, though I did have the builder add 7/16ths osb and some 3M tape hoping it would be better than nothing. Spray foam is how I was going to make up the air tightness, though with a 5500sqft roof deck the price is pretty sizeable
DC: Your suggestion in comment #6 seems very similar to my suggestion (to another person's question about separating the attic from the conditioned living space) to place the air barrier (which is sheathing) on top of the attic floor joists and using the attic floor cavity for ductwork, pot lights, electrical wiring, plumbing pipes, etc. However, in my suggestion to him, the attic insulation would be on top of the attic floor sheathing, not in the cavity - and the attic would therefore be vented.
However, when I suggested this, several people said that building the attic floor system this way resulted in a lot of unnecessary framing expense. I think it's a perfect way of separating conditioned living space space from unconditioned attic space while keeping mechanicals, plumbing and electrical within the conditioned building envelope. Also, with all the mechanicals within the conditioned space you greatly reduce the number of penetrations from the conditioned space into the unconditioned attic to perhaps just a plumbing vent.
In your comment #20 you speak of foaming the underside of the roof sheathing and using the attic as conditioned space. In this article by Dr. Joe Lstiburek, he discusses problems with spray foaming roof sheathing. It can be done well, but probably not often.
https://buildingscience.com/documents/building-science-insights-high-r-value-roofs/bsi-143-snap-crackle-and-pop
rockies63,
A common problem with service cavities - and you see this especially with ones on exterior walls - is that very little ends up in them. If the intention is to provide a space for ducts, it makes more sense to me to build those service cavities where they are needed than to cover the entire ceiling with one.
Rather than a house with 8 ft ceilings and a framed service cavity above, why not frame the walls at 9 or 10ft and drop the ceilings in hallways or closets where required?
Malcolm, the main benefit behind my suggestion is moving the air/moisture barrier from the ceiling surface (in the form of drywall) to the top of the attic floor joists (in the form of sealed OSB sheathing). Everything below that sheathing in within conditioned space so you can run HVAC and place pot lights anywhere without punching a hole through into the vented attic. The design of the attic floor system is the same as if it was a "conventional" attic floor with attic insulation on top of the drywall. I don't see why adding a layer of sheathing is perceived by some as an "unnecessary framing expense" - I would think it greatly simplifies mechanicals and attic insulation.
rockies63,
I think we exhausted this discussion in the other thread.