Homoly – Pedley Perfect Envelope?
I was just asked again today… “What is the best way to build the envelope of a home?” I know the real answer is “it depends” and then we could launch into the hundreds of different techniques. I feel the green building community, however, needs to provide an answer that applies to 90% of all homes. This shouldn’t be that hard as 90% of all homes are built the same way now…stud walls, OSB sheathing, house wrap, some kind of siding, and batt/blown insulation.
I have a proposal I would like to run by my fellow green thinkers that basically just puts a lot of great existing ideas together in one package (most notably the Joe Lstiburek Perfect Wall/Roof concept). I am building this home right now as a test case. Here are the details…
Foundation: Traditional poured foundation (or whatever is most common in your area) with 2 inches rigid foam on the outside and termite shield at the top.
Walls: 2×6 walls, OSB (I’m even using Zip Wall for a belt and suspenders approach), 2″ rigid foam (the key here is to use 40psi foam to keep the foam from “sucking in” different amounts to give the siding a wavy look – this is a standard stock item), house wrap, firing strips, and then the siding (we are using Hardie). Open cell foam between the studs (although there are other options here – I like open cell to allow the wall to dry out to the inside WHEN water gets in).
Roof: OSB sheathing (again, I am using the Zip System for belt and suspenders), 2″ rigid foam (this foam can be the traditional 25 psi because the foam won’t squish unevenly as badly when an entire 4×8 sheet is being screwed down), firring strips, OSB sheathing, roofing underlayment, shingles with a ridge vent at the top.
Here is the big new idea that makes it an “envelope”. My framer Mike Pedley came up with this detail, so hence his name at the top.
Soffit and Facia: OSB sheathing, 2″ rigid foam, firring strips, soffit and facia material.
Note the “gap” goes all the way up the wall, around the soffit, around the facia, and up the roof to the ridge vent. Periodic vent strips could also be added in the soffit if desired. The entire home from foundation to wall to soffit to facia to roof is wrapped in an uninterrupted thermal envelope.
I believe this system accomplishes multiple goals (as outlined in detail throughout this website) and could be easily be mass produced since it utilizes all common building techniques and materials.
What do you think?
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Replies
Maybe it should be the Pedley-Homoly Envelope.....
Good design.... Not new however....
Andrew,
Many people have established the air barrier at the sheathing layer, as you propose, and have used exterior rigid foam on walls and roofs. That has been the standard way to build PERSIST homes for decades.
I think the air barrier details are improved if you follow the usual PERSIST recommendation of framing the house without any eave or rake overhangs, and running the rigid foam up the walls and over to the roof, with tape installed at the wall/roof intersection. Once this is done, rake and eave overhangs can be built separately and scabbed onto the building, over the foam. This technique is called "applied overhangs" in a JLC article that includes detail drawings showing how the overhangs are built: “High-Performance Homes on a Budget.”
In the article, the designers did not include exterior rigid foam. But they still found the "applied overhangs" technique useful because they were establishing the air barrier at the sheathing layer.
I think your proposal to bring the foam insulation around the soffit and fascia is fussy and likely to increase air leakage rates.
Below is a detail from an article I wrote about PERSIST homes. The article appeared in the July 2002 issue of Energy Design Update.
The “Perfect Envelope” will always be DEPENDS. There are many different climate and humidity zones, building materials, insulations, WRBs, claddings, techniques, likes and dislikes, material preferences and availabilities that most be analyzed on how they interact with each other and therefore the solutions are different.
Andrew,
Martins details above is the way my home was built in '91 by timberpeg. Good but not new.
As far as the 90% goes... I disagree (respectfully). Try doing the same project with no foam. Eliminating foam is important to some of us.
We are a very large and diverse country. We need more than one solution to a high quality envelope.
Have fun building. Your enthusiasm is great!
Andrew,
Your strategy contains WAY too much foam.
Build your above grade walls with plywood (or OSB if you must) and tape the exterior seams with 3M All Weather Flashing Tape as your air barrier, and use a conventional product like Tyvek, Typar, or #15 building paper (or combination depending on the exterior cladding) as your WRB. I don't like how the Zip system's weather barrier relies on tape.
Use a double wall, and pump the cavities with cellulose at the appropriate density.
For the roof, use a manufactured truss with a raised-heel to allow for FULL insulation, and soffit to ridge venting.
Foam is NOT green. Minimize it whenever possible.
Note- this is a cold climate approach. If you are building in the south, you may want to think about other strategies.
Nobody that owns fossil fueled vehicles should advocate to others to be so green as to not insulate with foam.
It's like being a vegetarian but still killing fish, ripping their skin off and consuming their muscles. Fish are just as alive as the cute bears you don't eat but take pics of eating the tasty beechnuts from the trees we cleared away atop your local VT ridge to make way for the 450' tall $20 million dollar windfarm of so called fossil free energy. (Nothing to date is manufactured fossil free and making fossil free energy that I know of by the way....)
Foam is legal and is up to whomever to choose when and how to use it IMO. Explain maybe how you feel about foam but then.... Let it go. Many people feel a product like foam is a much better use for dinasour flesh than putting it in your f250 and burning it to drive to a NASCAR race......
Just sayin....
I agree with many of AJ's points here. First, better that petroleum be used to insulate buildings than to drive around in circles at great speed. Second, fish are not vegetables. Third, wind farms require substantial investment of fossil energy.
I also agree with Brett and Albert that less-foamy buildings are greener than extra-foamy buildings. Thinking Green, better also to eat less meat (maybe some fish, no bear) and start backing away from fossil-fueled travel. May as well admit that with Seven Billion actors in this play, we should all be vegetarian bicyclists (using no foam or other petro products) to ensure health and happiness for our planet's inhabitants centuries into the future. Remember that everyone lived free of fossil energy for a thousand generations behind us, and life could continue just as far ahead of this present day.
Foam and vegetarians,
I do not like to use foam but I am also not advocating against it. As with everything we should ask ourselves where it comes from and what it took to make it. For a foam example why not use EPS over XPS all the time. Also close cell foam for all insulation could never be the answer in my opinion because of it`s nasty blowing agents. I did not agree so much on the vegetarian analogy. I call myself a vegetarian but if I (very rarely) catch my own fish "ripping their skin of and consuming their muscles" I do not feel bad at all because that is pretty sustainable. I also don`t feel bad if I use closed cell sprayfoam at an existing rim joist condition to ensure adequate insulation and airtightness.
Martin,
I like your detail on the "applied overhang", but in our area the industry standard is roof trusses that "cantilever" out to form the overhangs. With roof trusses, you would then have to cut the wall foam around every truss as it went up to the roof deck. Our technique allows the foam to be continuous and the air gap to be continuous. With roof trusses, do you think this would be a good technique? Do you see any downsides, maybe with the horizontal run across the soffit?
What about the concept of a universally accepted best practice (although I agree there would need to be one (with options) per region of the country)? It seems to me right now mainstream builders are too confused by all the choices and therefore default back to what they have always done. The green community is building some great product, but the big difference will come when everybody gets on board.
In regards to the foam discussion, I am focussing on the top "performance" that can be acheived. I don't think there is a good substitute for a product / technique like rigid exterior foam that can keep the outer structure of the building within the thermal envelope and above the due point (in addition to the other benefits that could be matched by other techniques). I think we will just need to keep continually searching for better products that will be better long term for the environment as well.
Why not just order trusses without cantilevered "rafter tails"?
This shouldn't be difficult for a truss manufacturer to do.
I think all of the foam and double layer of plywood on the roof is a lot of extra work and expense for achieving nothing, really. What is wrong with a typical vented attic? I am curious as to your reference to Joe L on his designs, yet seem to not want to do a typical vented attic, which he seems so fond of.