Is There Such a Thing as a Perfect Building Envelope?

Is there such a thing as a perfect building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.? One that could be mass-produced from readily available materials, and be appropriate for 90% of all new homes?

Andrew Homoly thinks he’s found one, as he explains in this Q&A post at GreenBuildingAdvisor.

Here are the basic elements:

• A conventional foundation insulated with 2 in. of rigid foam on the outside.
• 2x6 walls sheathed with Zip System OSB with 2 in. of rigid foam on the outside and open-cell spray polyurethane foam in the stud bays.
• A roof sheathed with OSB, insulated on the exterior with 2 in. of rigid foam capped with another later of OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. over furring strips.
• Soffit and fascia made from OSB sheathing, insulated on the exterior with 2 in. of rigid foam, and topped with furring strips and finish material.

It is this last detail, the insulated and vented soffit and fascia, that has Homoly thinking he’s nailed it. He calls his approach the Homoly-Pedley Perfect Envelope, sharing the credit with his framer, Mike Pedley, who came up with the soffit/fascia detail.

“Note the ‘gap’ goes all the way up the wall, around the soffit, around the facia, and up the roof to the ridge vent,” Homoly writes. “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 ... and could easily be mass-produced since it utilizes all common building techniques and materials.

“What do you think?”

Sorry, but it’s not especially new

Homoly’s plan hinges on establishing an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. at the sheathing level. But as GBAGreenBuildingAdvisor.com senior editor Martin Holladay points out, that’s not a new idea. In fact, he says, it’s been the standard way of building 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,” Holladay adds. “Once this is done, rake and eave overhangs can be built separately and scabbed onto the building, over the foam.”

He adds that bringing foam insulation around the soffits and fascia is “fussy and likely to increase air leakage rates,” and recommends a Journal of Light Construction article that explains how “applied overhangs” can be built.

Although Homoly likes the suggestion, he’s not sure it would work in his area because roof trusses that cantilever past the wall line are typical. Rigid foam on the walls would have to be cut around each truss.

And as to the notion that one building envelope could be used in 90% of new residential construction, don’t bet on it.

“The ‘Perfect Envelope’ will always be ‘it depends,’ ” writes Armando Cobo. “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.”

“We are a very large and diverse country,” adds Albert Rooks. “We need more than one solution to a high quality envelope.”

Plus, there’s too much foam

Homoly’s plan relies heavily on foam — both sheets of rigid insulation on the roof and walls and open-cell spray foam in the stud bays. Although foam has excellent properties as a thermal insulator, it has several attributes that make it worrisome to green builders, including the blowing agents used to make it — some of which have a high global warming potential — and its petrochemical content.

“Your strategy contains WAY too much foam,” writes Brett Moyer. “... Foam is NOT green. Minimize its use whenever possible.”

Rooks also raises the same point: “Try doing the same project with no foam,” he writes. “Eliminating foam is important to some of us.”

But to AJ Builder, the argument doesn’t make much sense. “Nobody that owns fossil-fueled vehicles should advocate to others to be so green as to not insulate with foam,” he says. “...Foam is legal, and it is up to whomever to choose when and how to use it, in my opinion. 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 dinosaur flesh than putting it in your F250 and burning it to drive to a NASCAR race.”

TJ Elder agrees that insulating is a better end-use for petroleum than motor vehicle fuel used for non essential purposes such as driving “around in circles at great speeds.” But he also sees the point that Rooks and Moyer are making.

“I also agree with Brett and Albert that less-foamy buildings are greener than extra-foamy buildings,” he writes. “Thinking green, it is 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.”

Our expert’s opinion

Here's what Peter Yost, GBA's technical director, had to say:

It just would not be right to discuss this without citing Joe Lstiburek’s “The Perfect Wall, Roof, and Slab.” A concrete (or concrete block) wall with all of the insulation to the exterior, pulling the structure pretty much into the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. , is elegant. The R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. or thickness of the insulation is climate-driven, but the configuration of the assembly does not change. Depending on the insulation and finished surfaces or claddings, this assembly can dry to the interior, the exterior, or both.

The thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. becomes an interior flywheel, buffering changes in both relative humidity and temperature. And the structure does not “care” if it gets wet; it won’t rust or rot.

Jim Sargent designed a house using Durisol blocks, exterior insulation, a variety of finishes on the exterior, and a clay plaster on the interior that was pretty much the perfect wall in my book. From both a building science and an environmental footprint perspective, the fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info-laden concrete cores and blocks made of softwood and Portland cement are 500-year materials and walls.

And interestingly, on the first floor of my 100-year old home, we retrofitted the hollow concrete block (interior plaster and lath) with 3 in. of high-density spray foam, foam that serves as my continuous thermal barrier, air barrier, and drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope..

I just today took off several pieces of the clapboards to check on the insulation/air barrier/drainage plane. (I intentionally fastened much of this wall with stainless-steel trim screws so that I could easily remove the cladding to inspect the assembly over time). The spray foam and assembly is in perfect condition, still completely secured to both the block and furring to which the spray foam was applied. No deterioration, no shrinkage, no loss of adhesion. The spray foam was applied in the fall of 2002.

Do I wish that the blowing agent for this spray foam job was not 245fa, with a really high global warming potential (GWP)? Absolutely, but at 3 in. and R-20, I still feel the payback is reasonable and you can be sure that chemical companies worldwide are working on replacement blowing agents with much lower GWP.

There are also rigid insulations other than XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. to consider for the “perfect envelope.” Rigid mineral wool from Roxul, for example, and the more expensive Foamglas should be considered.

The key thing is that exterior insulation makes continuous air and thermal barriers possible and “warming” any building assembly reduces or eliminates moisture management issues in any climate.

And for complete information on all types of insulation, BuildingGreen has just released a special report on insulation. Scroll down a bit.