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JackOfAllTrades 12-31-2011 11:44 PM

R-60 through R-100 - Is it possible?
 
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 01-01-2012 08:46 AM

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

joecaption 01-01-2012 09:56 AM

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 01-01-2012 09:01 PM

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 01-02-2012 12:43 AM

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

darlingm 01-02-2012 01:47 AM

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 01-02-2012 07:40 AM

+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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