[Greenbuilding] attaching 2" rigid foam

Reuben Deumling 9watts at gmail.com
Thu Dec 1 12:29:39 CST 2016


On Thu, Dec 1, 2016 at 10:15 AM, Topher Belknap <topher at greenfret.com>
wrote:

> Greetings,
>
> Firstly, be sure you have the moisture issue figured out.  Polyiso has
> low permeability even without foil.  And so, it should be taped, and
> considered a vapor barrier.  No wall should have 2 vapor barriers, of
> course.
>

Around here we always have the paper/barrier/whatever on the inside of the
batts. And I seem to recall that latex paint on sheet rock is also assumed
by some to count as a vapor barrier. I know there is more to know about
this, but at this level of approximation having the foam on the inside
seems reasonable.


>
> Thirdly, R-10 will probably double the insulation value of a 2x4 wall,
> perhaps more.
>

I recall well conversations you started ages ago here about the real-world
R-value of whole wall assemblies as opposed to the rating printed on a
batt. Those were so helpful. In my case what I was comparing were
(a) two adjacent 2x4 walls with a half inch gap, filled either with dense
pack cellulose or with fiberglass batts, and
(b) one 2x4 wall with fiberglass batts in the cavities with a 1.8" rigid
foam panel affixed to the inside.
If we assume that the somewhat isolated double stud wall manifests
something closer to the R-13 x 2 = R26, the foam panel--in the best of
circumstances--would be R-13+ R10.3 = R23. If you (or anyone) would care to
critique any of those assumptions I would be most obliged.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20161201/e1129dec/attachment.html>


More information about the Greenbuilding mailing list