Designing Superinsulated Walls
Once they’re filled with cellulose insulation, our 14-inch-thick double-stud walls will perform at R-53
[Editor's note: Roger and Lynn Normand are building a Passivhaus in Maine. This is the 12th article in a series that will follow their project from planning through construction.]
I’ve always enjoyed watching new homes being built. From the humble beginnings of a simple hole in the ground, a job site gradually changes as a succession of tradesmen arrive daily to craft concrete, lumber, roofing, windows, drywall, copper pipes into basic shelter, before giving way to a parade of cabinets, appliances and other finishing touches.
I like looking at the individual building components, wondering on what basis they were selected, how they are assembled, and how they come together to become someone’s home. The construction journey I observed in other people’s homes was always as interesting as the destination of their completed home.
It’s our turn to design a house
So here we are with our first, and most likely only, opportunity to design and build a home just the way we would like it. Most may yawn at the sight of looking at the cross-section or a wall; not I.
I love to understand the nuances in the structural design, the pros and the cons. I’m sure there’s a medical term for this mental condition. Lynn likes admiring the finished product; I also like understanding the bones, organs, and muscles beneath the skin.
So if you like cross-sections, read on. Otherwise, the details below might not be your cup of tea.
Here’s the wall section that our architect, Chris Briley, has drawn. Overall, we like it. Key aspects:
- Basement floor poured on 8 inches of rigid EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. (a type of rigid foam) subslab insulation (R-41).
- Insulated concrete forms (ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure.) basement walls with exterior Roxul insulation (R-40). No additional interior insulation in the basement; the drywall mounts on the ICF foam.
- Double-stud above-grade walls, with the interior wall (called a wythe) load-bearing. The entire 14-inch cavity between the exterior and interior wythes is filled with dense-packed cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. (R-53).
- Lower exterior wall clad in 1-inch-thick stone veneer, atop a backerboard with a pressure-treated 2X4 attached vertically to allow additional Roxul batts in the 1 1/2-inch cavity for insulation and drainage. Upper portion clad in fiber-cement siding.
- Main floor has a concrete slab floor for 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. , though we have backed off using radiant floor heating – we’ve become convinced that’s not cost-effective for such a tightly insulated home.
- Raised-heel roof trusses with the full 24 inch depth of blown-in cellulose extending over the top of the exterior wythe for R-91 – Yikes!
Call the drywall ceiling support that much insulation?
We discussed the details of this cross-section with Chris today. This design has not been tested in the energy model yet, so the R-values my fluctuate up or down.
Meeting the Passivhaus standard requires substantial levels of insulation. Nonetheless, this is more insulation than even I expected in the ceiling. Lynn thinks we could live at the North Pole in a house this tight! Chris said that he believes the required levels to be in this ballpark, and that he wanted to ensure that the engineering review consider the weight of the likely 20 in.+ insulation in detailing the roof trusses and support for the drywall ceiling.
Having the inner wythe be load-bearing provides an great way to provide a thermal break at the intersection of the wood I-joist floor and the exterior wythe. Because lots of solid wood that comes together here, this area is always a weak point in the thermal envelope of a typical home. I had not seen this detail in the many other double-wall cross sections I have seen in magazines. Kudos Chris!
I am leery of how the exterior wythe bears on the 2x12 bottom wall plate with no other support below. Note how the concrete core of the top ICF course flares to the inside to support the load bearing walls and floor I-joists above. Chris designed it this way to maximize insulation at this point, believing that most of the weight of the exterior wythe will transfer to the outer edge of the ICF cement core. Hmm. Though it is the inner wythe that bears the weight of the floors and roof, the exterior wythe will still bear substantial weight: the wall itself, the stone claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. at 14 lbs/sq, ft,, the cementitious backerboard, and the cementitious siding.
We agreed that this detail would be reviewed by both the ICF manufacturer and the engineer who will approve all the structural aspects of our home prior to construction.
Housewrap plus Zip System sheathing provide two layers of defense
Chris includes some 4 inches of high-density EPS foam at the bottom of the ICF core as a means of thermally isolating the concrete core of the ICF with the concrete footer. Interesting, but I’m not sure how much complexity (read material and installation cost) this adds compared to the benefit of a thermal break at this point in the basement.
We spoke about the possibility of the truss manufacturer ending the bottom web (which will be the ceiling of the main floor) at the outer portion of the exterior wythe, while continuing the upper web (which will support the roofing) to the eaves as shown. This would allow continuing the Adventech Zip sheathing without interruption to the bottom of the upper web, rather than fitting and sealing individual pieces of polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. (a type of rigid insulation). Tightly insulating the cavities where the roof meets the wall is just as challenging as where the floor meets the exterior wall. The upper web will serve as a venting channel for the attic, with intake at the eaves out air flow to a ridge vent.
I also asked Chris to include an air-barrier fabric like Tyvek outside the Adventech Zip sheathing. Although the Zip sheathing is rated as 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. when all seams are properly taped, I have read a number of articles expressing concerns about water intrusion if the taped joints fail over time. Dense-packed cellulose could wick any moisture into the cavity, thereby (possibly) causing unseen mold issues. Using the fabric would provide a second level of moisture defense, as well as facilitate sealing around the rough openings for exterior doors and windows.
All in all, we think this is a very well polished design.
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