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.

Design - Whole Wall Insulation (Cont'd)

Super-Insulated Envelope

Our Annualized Passive Solar system, which figures in dozens of previous posts, qualifies

our project as passive solar even if winter solar gain is only adjunctive. Johnston and Gibson in "Toward a Zero Energy Home" have this to say about good passive solar design:  ".....thermal load of a building can be reduced by 90% primarily through super-insulation, an air-tight envelope, good windows, and heat recovery ventilation."  Further along, they state that "the National Renewable Energy Laboratory (NREL) advocates a simple formula when it comes to insulation:  30-40-50.  In colder climates, zero energy homes start with R-30 for floors, R-40 for walls and R-50 for ceilings/roofs.  Further north where it's really cold, green builders are using even higher figures." 

I am assuming that, despite global warming, our St Louis climate still fits what they call "colder climates".  Accordingly, our design should be right at R-48 for our truss walls with almost no thermal bridging.  We should achieve about the same R-rating for our cathedral ceilings with a thermal bridging factor that is unfortunately slightly higher than for the walls due to the need for structure to carry the weight of the roof. Essentially, the walls will be overkill and the ceiling right at the "super-insulated" threshold.  Our floors are already taken care of by the AGS system. Our windows will be high-end fiberglass and we will have energy recovery (instead of heat recovery) ventilation. 

Super-Insulate with What?

Consistent with my philosophy of sustainability, the last insulation I would want to use is, unfortunately, the most effective -- sprayed-in-place foam such as closed cell polyurethane at R-6 per inch.  Some brands are touted as being soy-based but competitors say that the claim is greenwashing in that the amount of soy in it is a pittance. In any case, spray foam is a turn-off for me because it contains fossil fuel and it is the most expensive product.  Hopefully, the manufacturers claims are legit when assuring that the toxic VOCs (that necessitate space suits for the installers) dissipate rather quickly. 

Cellulose has a lot going for it.  For walls, it is most often mixed with a little water and polymer then sprayed into wall cavities for a very dense configuration held together by the polymer until supported by drywall.  For attics, it is merely blown in like loose fiberglass.  For cathedral ceilings, it is installed with a process called dense-pack so that it is much more compacted than if it were sprayed.  And, since cellulose is ground-up newsprint and other post-consumer paper, it qualifies unequivocally as a green product.  It was our first choice for walls and cathedral ceilings after giving up on structural insulated panels and spray foam.  However, it still exceeded our budget, did not lend itself so well to DIYing and has some downsides that I will compare with rice hull insulation in a near future post.

Loose fiberglass should be blown to an R-50 in an attic.  And, according to the engineer who quoted our job, it can also be dense-packed into cathedral ceilings for a higher R-rating than dense-packed cellulose.  We did not discuss walls.  However, based on the quote for the ceilings, it still exceeded our budget.  (In my view, fiberglass batts are a joke and should not be mentioned in the same breath as super insulation.)

Rice Hulls

This subject is covered in detail in prior posts --  early thinking (best post for details) and exterior walls.  A couple of near-future posts will explore rice hulls in even more detail.  Suffice it to say that rice hulls are cheap and very effective, being highly resistant to fire, pests and mold and similar to cellulose with regard to conductive heat loss. Their R-value is slightly over 3 per inch allowing us easily and inexpensively to meet the NREL recommendations for walls and ceilings.  The only kicker is one of logistics -- how do we get them from the Mississippi delta to inside our walls and ceilings?  For this, watch future posts.  It is a challenge we look forward to meeting.

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Spoiler alert!  As explained in the updates in the posts on rice hill insulation referenced above, hulls should not be considered for insulation due to the uncontrollable rice weevil infestation.

Construction - Pre-Made Trusses for Exterior Walls

"Super-insulated" seems to be the green building catch-phrase for exterior walls that greatly surpass the recommended R-factor.  For our passive solar design that eliminates convention HVAC, super-insulation is not an option.  We need to turn away as much heat as possible in summer and retain as much heat as possible in winter. Consequently, we are using wall trusses in lieu of "two-by" studs in order to minimize thermal bridging and maximize the space for wall insulation. (Reminder: click on any photo to enlarge it for detail.)

The trusses are thick enough to hold 15" of rice hull insulation at slightly over R-3 per inch for a total of R-45+.   By contrast, the R-value for fiberglass batts in 2 x 4 walls is 15 and, for 2 x 6 walls, 19.

Truss jig; the aluminum angle "iron" provides rigidity
when crooked salvaged 2 x 4s are forced to fit the
jig in order to give absolutely straight trusses; notice
the pre-cut truss parts in the background

Another important design feature from an energy conservation standpoint is that the trusses eliminate through-the-wall penetration of two-by structural members, thus minimizing thermal bridging (see prior post, whole wall R-value).   For the sake of consistency and reproducibility, the trusses are built in a jig that is not our original idea (as explained in a previous post, stick-built exterior walls and in the original reference for the trusses).

Truss Configuration
A finished truss has 2 x 4 vertical members arranged flat-ways, i.e., rotated 90 degrees from the usual.  The tops and bottoms are joined by short 2 x 4s. Then there are six gussets, three to a side, made from 3/8 or 1/2" thick OSB or plywood to impart structural integrity while limiting through-the-wall thermal bridging.  All components come from recycled lumber so a certain amount of twisting and bowing is to be expected (but not unlike new lumber today, right?).  And recycled lumber comes with many nail holes on the 1 1/2" side, so exposing the 3 1/2" side for new nailing is another advantage for using trusses.  The second photo below is a prime example of the nail hole problem.

Fabrication
On rainy or cold days and other such house construction downtime, I have been making trusses in front of the

Framing nailer used to fasten 2 x 4
components together; the long 2 x 4s are
the bowed ones shown in the photo below;
notice how straight they are after the short
2 x 4s are wedged to placew

double garage workshop next door to the building site. At the time of this writing, most of those that will be needed have been assembled and stored.

The components are pre-cut to master patterns.  The long and short 2 x 4s are placed edgewise in the jig and fastened together with a framing nailer -- one nail at each corner. Then, with a trim nailer, three gussets are attached with 2" nails -- 11 nails in the end gussets and 8 in the middle gusset.  Since trim nails are virtually headless, they are driven at various off-angles to provide more holding power. The truss is pried out of the jig, turned over and forced back into the jig for identical nailing and gusseting of the second side.

When the long 2 x 4s are bowed, they are positioned in the jig with the convex side against the jig.  Then the short 2 x 4s are driven to place at each end to straighten them. Because the trusses are held jig-straight by the gussets,  the future sheathing and the drywall will lay perfectly smooth.  When bowed in the other direction, straightening is unnecessary.  A typical 2-by stud with only a 1 1/2" nailing surface has to be straight in order to catch enough of the sheathing and drywall panels for secure nailing.  By

Driving the short 2 x 4 to place at
the far end of a bowed board pulls
it away from the jig on nearby end.
Forcing a short 2 x 4 to place on the
 nearby end creates a straight truss
 that the gussets fixate 

contrast, each truss offers 3 1/2" of nailing surface so almost any amount of bowing left or right is acceptable.

Other Green Features
As with 2 x 6 construction, a wall will be plenty rigid with trusses on 24" centers tied together with side-by-side 2 x 6 mud sills and side-by-side 2 x 6 top plates.  The span between nailing surfaces, by virtue of the 90 degree rotation of the vertical members. will actually be 3" narrower than with 2 x 6s.  The double sills and plates are necessary because there are no 2x boards wide enough for a 15" walls -- which is a blessing. Having to tandemize the mud sills and top plates leaves a sizable gap that can be filled with insulation and thereby provide two more breaks (mud sill and top plate) against thermal bridging.  

Doors, Windows, Corners and T-Walls
Trusses that frame openings for doors and windows will have to be modified to carry headers and, for windows, sills.   The plan for headers and sills is to make regular trusses, minus the gussets on the side facing the opening, then let the headers and sills into the truss 2 x 4s to a depth of 1 1/2" to give the same amount of support as a jack stud aside a king stud.  I plan to tie a window-supporting truss to the closest regular truss with horizontal 2 x 4s in line with the header and sills to support the header/sill truss in one direction.  The opening itself  will be lined with OSB or plywood, not only to provide anchorage for the window or door, but to support the header/sill truss in the opposite direction.

When a "T- wall" intersects a truss wall between trusses, horizontal 2 x 4 blocking will join the two trusses and provide fastening for the T-wall, much like what is done with advanced framing techniques.

Gussets are fastened with 2" nails using a
nailer

Truss walls intersecting at corners will utilize three trusses arranged so that the entire corner is accessible to 15" of insulation and thermal bridging is held to a minimum just like the rest of the walls.

Fire-Blocking, Electric Cables and Plumbing Pipes
Fire-blocking is not possible with trusses but rice hulls are virtually impossible to ignite (paper on rice hulls as insulation, page 3) thereby rendering fire blocking in exterior walls moot.

Running electric cables and plumbing pipes inside of trusses will be a joy since holes in studs are not necessary as with stud walls.  The worst case scenario is that an occasional gusset would require drilling. What's more, plumbing supply pipes can be held to the interior side of the wall to isolate them from the exterior with more than enough insulation to prevent freezing.