Is spray-foam worth the $23k extra cost?
Our new home is currently under construction in Toronto, Canada. I’m doing my best to take advantage of this opportunity to create energy efficiency where at all possible. We will have solar panels and hope to get a Tesla Powerwall when available.
However, in regards to insulation I’m unsure how to proceed:
The original quote from our builder includes batt insulation and spray foam in the so-called sensitive areas such as adjacent to the garage. I do not pretend to understand much of the technical discussion in regards to which areas should be used and what kind of foam etc. What I do understand is money. I asked for a quote on what it would take to spray foam the entire house and received a very detailed break-down for each floor. The net effect is it would cost about 23k $cdn with tax extra for the entire house of about 5600 sqf + basement.
The installer claims I should see about a 45% savings on my energy bills. I could probably assume a combined average hydro+gas bill of $400 per month without it. If 45% is an accurate number then we’re looking at 8+ years to recapture my investment. We do not intend to sell this home anytime soon and I don’t believe this will increase comfort levels (as we’re already doing the ‘sensitive’ areas).
What do you think? Do these numbers make sense? Anything else I should consider? Thanks!
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Replies
Bob,
I think that your builder's estimate of 45% savings is wildly off. If that number is accurate, it's an admission by the builder that he is one of the sloppiest builders in Canada. That's nothing to be proud of.
Manufacturers of batt insulation require that these batts be installed in an assembly with an air barrier. In other words, no matter what kind of insulation you specify, builders these days have to pay attention to airtightness.
Unfortunately, most builders don't pay attention to airtightness. That's what your builder is admitting when he says that when he insulates a house with fiberglass, the house requires 81% more energy than when he builds a house with spray foam. (If the fiberglass houses requires $100 of energy bills instead of $55 for the spray foam house -- that's your 45% savings -- then that means that the fiberglass-insulated house has energy bills totaling 181% of the spray-foam-insulated house.)
This is an extremely complicated discussion, but it should have happened before you chose your builder. If this were my house, I would sit down with my builder and discuss an airtightness target, verified by a blower door. You don't need spray foam to build a tight house.
Seriously! Spend $3K on making the shell air-tight, verified by blower door as Martin recommended, and spend the other $20K on more solar panel.
In a 2x6 framed house with studs 24" on center even if you put some magic-mouse-milk foam that's R100/inch between the studs you won't get more than about R27 for "whole-wall R" due to the thermal bridging of the framing, as well as interior & exterior sheathing, the siding and interior & exterior air films.
With 3" of rigid rock wool between the wall sheathing & siding and R21-R23 batts in air-tight wall cavities you can meet or beat that performance, at an all-in R28.
If you did 5.25" of R6.5/inch closed cell foam and no exterior continuous insulation you'd be at about R18, compared to 5.5" of R23 rock wool at about R16.
Is the additional R2 worth an additional 23 grand?
I don't think so. You can buy comparable performance out of the R23 rock wool wall by adding 10mm (3/8") XPS siding underlayment at a tiny fraction of that cost.
@ Martin. I'm guessing the builder is just repeating the "rule of thumb for spray foam" he heard from whoever he contracts it out to. I've heard the R value myth and whatnot from many spray foam people, and they are good at confusing air tightness with insulation in my experience, so in the "good" old days the increased tightness was a much larger benefit then it is from a properly built house today.
I would avoid fiberglass, its airtightness is practically nonexistent, at low temps it loses insulating value, and is very susceptible to wind washing, i would go for dense packed cellulose or roxul in the walls. Many people around here don't know how to dense pack (they loose fill) so for simplicity i would go with roxul in the walls and cellulose in the attic, personal preference i guess.
What is the current code in Toronto for under slab, basement, above grade and attic insulation R values?
High density fiberglass batts (R21s) is every bit as air retardent as rock wool, and are nowhere near as susceptible to wind-washing as (everybody's favorite straw man) crummy low density R19 batts.
At 1.8lbs density or higher blown fiberglass is somewhat more air retardent than 3.5lb dense packed cellulose.
Spray foam will not stop the thermal bridging. I would ask him what his targets are for air tightness and he he cant answer you are in trouble.
First set out to make the home very air tight and then add rigid insulation to the outside as well as the planned insulation.
@ Dana, my mistake, i am used to dealing with older houses, even in my house i had fiberglass that i didn't even know was there because of the massive air leakage on a windy day until i opened up one of the basement headers (balloon framing).
Is this high density fiberglass also used in modern attic fill?
whoops
Alan B: Some of the newer attic fill wools are chunky bits that look like shredded high density batt, others aren't. All have much improved from 1990s vintage stuff, but it's not clear that they can't be "fluffed" by less than scrupulous installers. Unlike cellulose that was blown with a higher-than-spec air/fiber ratio, fluffed fiberglass doesn't settle, and performs poorly. My personal preference is to go with cellulose for open-blown attics, but there is sometimes a case to be made for dense-packed fiberglass in walls or cathedralized ceilings.
All fiberglass insulation has issues with glass particulates suspended in air, which makes air sealing even more critical. When cutting batts rock wool particles don't seem to have nearly the same hang-time of fiberglass, but I don't have third party data to share- just my perception.
1990s is vintage, darn i feel old, all that bad techno music and rap, president Clinton, Fresh Prince of Belair :)
Central Texas weighing in to say the only residential design clients that are prepping for solar collection are retired Exxon and over the decades only one finally mounted some. This is preface to .... many other clients are being talked into Open (UGH) Cell foaming their attics, leaving the attic/Living floor/ceiling UNinsulated. EZ on builder, profits in pocket of installer. Imagine the sf difference between a tent surface & floor, then the added costs, out-gassing, wood rot potential, shorter roof shingle life (heat=time) (if not metal), etc.
So ... if 7-years is a short enough time frame for choosing solar or windows' payback for extra costs, then 8-years (if not disputing their own numbers, often "puffed" for sales) might be seducing if in a much harsher environment than here, av. temp 65F but humidity seems to same (65%rh).
I would spend more focus on things more obvious to an architect: solar gain orientation, perhaps breezes one month/year, WINDOW R-values!, more wall insulation (thicker walls or exterior insul.board), Attic floor DEEP insulation, top dog HVAC equipment and belt/suspenders protection of anything that can freeze.
Kudos for design-building!