Affordable Single Family Passive House Construction Techniques
Hi all-
I’m designing a passive house single family home in northern Zone 4a, which is a mixed climate, and I need some feedback on cost effective assemblies please, specifically roof assemblies.
The house has a simple shed roof with a low slope, parallel cord mono truss aka sloping flat truss, and cathedral ceilings. There’s no functional attic.
The initial plan was to design an unvented roof with the appropriate ratio of blown-in rockwool beneath the OSB sheathing and continuous exterior board insulation above the OSB sheathing, and ventilation beneath the cladding. I’ve done this many times and I like the assembly, however what makes this house unique to me is that it’s technically an affordable house so I’m looking for efficiencies in places I hadn’t looked before.
A more cost effective roof assembly is to vent between the trusses and get all of my insulation in the truss cavities under the OSB roof sheathing, and vent the cladding. I’ve built this assembly many times as well however every time I built this assembly I didn’t care about passive house requirements, thermal bridging, or any green initiative. I’ve never put a thermal camera on this design and I wonder how bad the thermal bridges are.
Does a vented truss roof perform equal to a non-vented roof with a lot of insulation above the OSB roof deck? One certainly cost more than the other, but how does their performance compare?
It’s imperative that I build this house as cost effective as possible, but it still needs to meet passive house requirements, we are certifying. I’m looking for any tricks of the trade that people have done that help make a single family certified passive house affordable in a mixed climate. We are going with a panelized wood-framed wall assembly with OSB sheathing and continuous exterior insulation plus vented wood cladding, and an insulated slab foundation.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I consistently find that a truss roof with raised heels, vents at soffit and ridge and loose-blown insulation over an airtight ceiling gives the best bang for the buck, up to Passive House performance.
Thanks for the feedback. Is thermal bridging with this design a concern, considering the added insulation? The roof will be rated at R70, well above code in our area.
If the quality of the air barrier, in this case the latex painted drywall ceiling, becomes compromised, what’s the risk of condensation inside of the roof assembly?
There is some thermal bridging through the truss webs but it may or may not meet Passive House criteria for a "thermal-bridge free assembly," which allows for a tiny amount of thermal bridging.
It's possible to use painted drywall as the air control layer but in a Passive House it is usually a separate layer, as you will almost certainly have some penetrations and in a Passive House you essentially can't have any intentional penetrations. It's hard enough to meet the airtightness requirement with unintentional penetrations.
Are you the owner/client, designer or builder? Do you have a Passive House Consultant on the team?
Good feedback, thank you.
I’m the builder. I’ve built passive house before, but those projects were custom, larger, more robust projects. This one is different, and I’m trying to slim this one down.
We’re moving forward with the PHIUS prescriptive path. I’m a CPHB and I’m applying for certification directly, although my architect is a CPHC.
I usually rely on latex painted drywall as the air barrier, but I include an independent utility chase in front of the air barrier to keep it protected. Due to cost, I’m considering excluding the utility chase, but I have a concern that the occupants will compromise the drywall air barrier over time, so I’m looking for some feedback on the level of risk I’m taking by considering excluding an independent utility chase.
TimW,
Many builders now use the taped and sealed sheathing as their primary air-barrier, and the interior drywall as a secondary one. An interior variable-perm membrane is a good vapour-retarder in most climates.
Correction: the OSB sheathing outside of the framing is our air barrier for both the walls and roof. The interior latex painted drywall is the vapor barrier for both. My fault.
We usually protect the interior drywall vapor barrier by constructing an independent utility chase over it. If we remove the utility chase to save cost and expose the drywall vapor barrier, and the drywall becomes compromised over time, will condensation form inside of the wall or roof assembly?
TimW,
No. Unlike air-barriers, small holes in vapour-barriers are inconsequential.
That said, an interior air-barrier is always a good idea on a roof unless it has sufficient exterior insulation to prevent condensation.
For what it's worth, for compact ceiling assemblies like yours I usually run a variable permeance membrane and air control layer (Siga Majrex, Pro Clima Intello or similar), then strap with 2x material, then install drywall. That way wires and slim LED lighting can all be run inside the membrane. It costs a bit more but is more resilient than relying on the drywall to remain perfect forever.
MALCOLM TAYLOR,
Your feedback is very helpful, thank you.
Michael Maines,
Your feedback is worth a lot. I have experience with this method and I like it.
This method is best practice. I’m trying to understand the level of risk if I exclude.
Certification isn't really compatible with "as cost effective as possible," The only reason to certify is as a marketing aid in selling the house.
What I would recommend instead is doing in-depth energy modeling using BEOpt or something equivalent. That lets you try out different assemblies and see the real impact.
If you haven't designed a house before I suggest working from prepared plans. There's an adage: build your third house for yourself. Build your second house for a friend. Build your first house for an enemy.
Certification forces system commissioning, an important step often overlooked or eliminated in the interest of "saving money." It also means that someone knowledgeable has reviewed the details carefully. I agree that it's rarely worth the money, but the benefits go beyond marketing.
No reason you can't do the commissioning step without doing certification.
Absolutely. But it's often skipped when not required.
To my ear the title sounds like a string of oxymorons. Seems to me affordable and passive arsenical onsides and construction techniques implies a self-build would make it a rarity among passive houses.
Consider giving up on earning the “passive house” plaque you are free to meet their goals if you chouse but I see little value paying the gate keepers to get the plaque. Personally passive houses set of goals don’t really align very well with my personal goals. The way I read passive houses goals the core I come away with is Thou shall not consume more than X number of BTUs per square foot regardless of the cost regardless of your local climate.
Consider a PGH (Pretty Good House) no gatekeeps or plaques. The when I think of PGH goals the core I come away with is. Buy the amount of insulation and equipment likely to have the best return on your investment.
I think you should reconsider the idea if a cathedral/sloped ceilings I see them as a design flaw. To put it simply this idea forces you to fit too much stuff in to little space. If you want or need the feeling you get from high ceilings then make your walls taller and make your ceiling flat and cover it with a ton of cheep fluffy insulation.
I also think the idea of conditioned attic is another design flaw in that moving the insulation to the roofline increases the surface area of the conditioned space by more than 20% so now the building requires at least 20% more fuel.
Walta
Fair point. While I support the program, I have no authority to change it regardless.
I’ll take your other points under advisement. Thanks for your feedback.
Thanks for your feedback!