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.