What’s the most cost effective way to build an R60 wall using XPS and Fiberglass?
brcait
| Posted in Green Building Techniques on
Hi All,
We need to build a super insulated wall (R60) using only fiberglass and XPS. What would you recommend? PS: I have already looked it up in the forum but couldn’t find anything that would only use these 2 products.
Thanks,
Brian
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Replies
Brian,
The more you flesh out your question the more useful the responses you will get.
- What climate zone are you building in?
- Does the wall have to include both materials, and does the proportion matter?
- Why those two materials?
- Why R-60?
Is that R60 "whole wall", factoring in all thermal bridging, the R-values of sheathing & air film layers, etc, or R60 center-cavity?
The only "green" XPS is reclaimed/reused XPS. All XPS in North America is manufactured using combinations of HFC blowing agents, all powerful greenhouse gases, the main component being HFC134a, which punches well above it's weight, roughly 1400x CO2 @ 100 years. New virgin stock XPS is VERY expensive, both financially and environmentally. Used/reclaimed XPS doesn't have that issue, since re-use piles on to the benefit side of the cost/benefit balance, against an environmental hit already taken.
A ~16" deep double-studwall filled with 1.0 lbs per cubic foot density blown fiberglass runs about R60 (without using any foam), and is about the cheapest way to get there using all virgin-stock goods.
To some small extent, buying reclaimed XPS encourages some others to keep using new XPS (because it lowers their eventual net cost). So I'd assign some environmental impact to using reclaimed XPS.
To my ear your question makes little sense.
Why limit your choices of fluffy insulation to fiberglass if cost matters? Generally cellulose insulation costs less for the same R value and is far less likely to be compromised by poor workmanship.
If cost is a factor how did you select R60? Unless fuel cost in your location is much higher than normal or you happen to be inside the Arctic Circle, an R60 wall is unlikely to ever save enough fuel to offset its cost.
Walta
Brian,
Here is a real r-60 wall that fits your criteria:
Build a double stud wall- each wall being a 2x4 wall. Space the studs 11 inches apart. Sheath the outer wall with plywood or gypsum board (dense glass or equivilant). Fill the 18" wall with dense back fiberglass. Add drywall as normal.
No need for XPS.
Actually 16" would suffice as Dana stated. I was using a full, 20% framing fraction- far too high for a wall like this.
>"Sheath the outer wall with plywood or gypsum board (dense glass or equivilant). Fill the 18" wall with dense back fiberglass. Add drywall as normal."
Dense packing fiberglass is considerably more expensive than 1lb fiberglass- it's nearly twice the material and more than twice the labor. It's cheaper to just make the wall thicker when looking at "...the most cost effective.." criterion.
Reclaimed/used foam board can often be cheaper than fiberglass (blown or batt).
>"If cost is a factor how did you select R60? Unless fuel cost in your location is much higher than normal or you happen to be inside the Arctic Circle, an R60 wall is unlikely to ever save enough fuel to offset its cost."
Depends on the cost.
According to Table 2, p1o of BA-1005 an R50 whole-wall would be the middle point of cost effectiveness in climate zone 8, which isn't too far from the cost of R60 when going with the MOST cost-effective methods.
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Dana- yes, I was thinking a 1.8 to 2.2# density.
I am not even familiar with 1 pound density fiberglass option. Is this a blown-in product (loose fill?)? Or a batt?
Could we get some more background on the purpose of the exercise? It may change our answer.