R-60 through R-100 - Is it possible?

JackOfAllTrades

Is it possible to achieve a R-60 or higher in an attic space? IBC and others are now requiring R-49+ for 2012. How difficult is it to achieve R-60 or higher in a new build?

joasis

First of all, the "R" value system is a rating that has a lot of flaws and assumptions, but, it is recognizable.

Try here, and see what the experts in the field say. http://www.buildingscience.com/index_html

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joecaption

Why not, let us know what you plan on using for insulation.
You can have to little but I see no way to have to much.

j kuzma

Volunteered at a Habitat for Humanity build last year that ended up being certified as LEED platinum. The attic was done with 7 1/2" Roxul batts for R28 and topped up with 12" blown in. Using the Roxul allowed us to install it before the drywall - much easier with volunteers. The total calculated was R62. The only issue was getting the baffles up high enough so they didn't get filled when blowing in.
A bigger issue was the air sealing - used gasketed boxes everywhere and foam at all penetrations and that probably made as much difference as the added insulation.
A better way may be to "butter" the attic floor (meaning attic side of drywall), penetrations, and ceiling joists with 2"+ of closed cell foam for the air sealing properties and then top up with 16+" of blown in. That should come out to R60.

Jim.

mae-ling

Yes, using spray foam, raised heel truss, or other ideas it is possible.

darlingm

You very well may already know this, but I'm not very familiar with the regular poster's names yet. The R-value scale can be quite misleading. The number has to do with a percentage of airflow that gets through, but it's not how most people would think. R40 isn't twice as efficient as R20. R5 means that 1/5 (20%) gets through. R20 means 1/20 (5%) gets through. R49 means 1/49 (2%) gets through. This means there's diminishing returns. I'm sure you could achieve R100, but that just means that 1/100 (1%) gets through. Not a real difference there, especially considering the cost.

Once you hit R49, you've taken 100% getting through down to 2%. Getting down to 1% from there isn't going to make a noticeable difference in heating cost or comfort of the home. (Technically you aren't starting with 100% getting through with no insulation, because sheetrock/drywall provides some R value... but the point remains)

Many people consider the R-value to be overblown, and believe that properly air sealing while maintaining proper attic ventilation is more important.

I can't say these next numbers are dead on, but I'm pretty sure you're better off having a properly air sealed R20 attic than an unsealed R49 attic, because air just finds a way around the insulation generally, if it's not air sealed.

Windows on Wash

+1

Air sealing is more critical than higher R-Value.

R-49 is usually more than sufficient. If you want to blow in a bit more (R-60) that is not difficult with additional loose fill.

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