Design - Whole Wall Insulation

Whole Wall R-Factor

The R-factor for the whole wall is less than the R-factor attributed to the insulation itself due to conductive heat loss or gain through the structural members (thermal bridging). The conventional R-factor also disregards convective heat loss/gain through the wall (air infiltration), which has even more potential than thermal bridging for diminished whole wall performance. Furthermore, it disregards heat loss/gain through windows and doors. Whole wall R-factor reconciles all three.

The subject of air infiltration pops up in many prior posts and will keep popping up in future posts.  It is hard to discuss green design and construction of exterior walls, cathedral ceilings, windows and doors without proper attention to air-sealing.

Our present discussion, though, is mostly about conductive heat loss through a wall or cathedral ceiling whether through the insulation or through the structural members. For a more complete discussion of convective vs. conductive heat loss, link to another post, Odds 'N Ends - Whole Wall R-value.

Modern Walls

Two modern methods for super-insulated wall construction are structural insulated panels and insulated concrete forms.

Structural Insulated Panels (SIPs)

SIPs are basically a sandwich with OSB board for the bread and solid foam plastic for the meat.  They typically are fabricated off-site and "flown" to place at the job-site with a crane.  When the joints between panels are caulked and foam sprayed, air infiltration is virtually eliminated.  Their R-value per inch is more like the solid foam board found on the rack at the home center -- much higher than fiberglass or cellulose.  And the foam core is available up to nearly a foot thick for a variety of R-values.

With regard to sustainability, SIPS rank high.  OSB is an engineered wood that comes from sustainable tree plantations and contains no VOCs while the core, expanded polystyrene, no longer requires ozone-depleting manufacturing processes. They are so strong that they do not need traditional framing for support which saves finite resources (and costs).  Offsite fabrication is more sustainable than on-site stick-building.  And on-site labor costs are less because they go together so fast.

The major downside to SIPs is initial expense (the cost of the crane alone for the time it takes to assemble a house is substantial).  

Our first choice was SIPs but they were ruled out early on the basis of cost.  My labor is free so there was no sense paying someone else to build walls.  Also, I have a substantial stash of (free) recycled lumber for wall construction that shouldn't go to waste.

Insulated Concrete Forms (ICFs)

An ICF is another sandwich.  The bread is 2.5" of solid foam insulation and the meat is reinforced concrete of varying thicknesses.  The whole wall R-value is +/- R-22 for the brand with which I am most familiar.  The forms are stacked and braced then the concrete is poured inside much like pouring basement walls with metal forms.

 Our ICF frost protected shallow foundation

The downside to ICFs is that the foam comes in only one thickness so their R-value is what it is.  Another is that concrete walls complicate wiring and plumbing and are hard to remodel later.  Also, unlike SIPs, they are not suitable for roofs.  Their upside is that their R-value exceeds the recommended of R-18 for our climate zone, are relatively easy and inexpensive to construct and are gang-busters in hurricane- and tornado-prone areas.  R-22, good as it is, does not qualify as "super-insulated" so I felt that there would be considerable risk in using ICF construction in conjunction with our passive solar Annualized GeoSolar system (in lieu of conventional HVAC) whereby conservation of every BTU counts.

However, we did use ICFs for our frost proof shallow foundation under our truss walls.The cost was little more than for a conventional concrete wall with foam board DIY-bonded to both sides.  And, at least for a short foundation like we needed under the stick-built walls, their R-factor is acceptable and they were fast and very DIF-friendly.

A more recent approach to whole wall insulation than SIPs and ICFs is the "super-insulated envelope" that I discuss in the next post.