Poly Iso behind Rockwool?
smgbayonne
| Posted in General Questions on
We are remodeling our 1900s-ish house. The exterior walls are built with 4″ dimensional lumber and we are adding insulation. We were planning on using rockwool but that is only 3 1/2″ (sized for normal 2x4s). We have an extra 1/2″ so I was thinking about adding 1/2″ poly iso board behind the rockwool to up the R value. Is there any problem in doing this? Is it likely worth it? It would up the R-value of the insulation from R15 to about R18 (realize actual R value will be less because of the studs).
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I wouldn't think it was worth it. It's on the interior side, so that air is essentially at ambient (indoor) temperature.
Another option you have, if you're worried about filling the cavity, is to use a typical R19 fiberglass batt, that is nominally 6.25" thick, or a R20/21 batt compressed into the 4" space. There's a chart that shows different combinations of nominal -> compressed r values, linked below. That indicates a value of R14 when a R19 batt is compressed into a 4" cavity, and R16 when an R20/21 batt is compressed into a 4" cavity.
https://insulationinstitute.org/wp-content/uploads/2016/08/Compressed_R_values.pdf
Thanks - I think I'm less worried about filling the cavity and more feeling like that is 1/2" of wasted space that could go to insulation.
The additional R-value would reduce your heat loss (or gain) through the walls by something on the order of 10%, once thermal bridging is accounted for. Walls are typically responsible for somewhere around 15% of a home's heat loss (though it varies greatly, of course.) 10% of 15% is 1.5%, so that's the reduction in the home's total heat load you can expect. It may or may not be worth the cost of installing the polyiso.
Another factor to consider is vapor control. Most polyiso has foil facers that block vapor movement. That can be a good thing, but walls are usually more resilient if they can dry to both the interior and exterior. If you can get fiber-faced foam it would be safer.
One more thing to consider is the impact on our environment. Polyiso and mineral wool are in the same ballpark when it comes to up-front carbon emissions but polyiso is a bit worse, so it would be better to find thicker mineral wool. Or choose an inherently low-carbon insulation such as cellulose.
When I was searching for that fiberglass chart, I did a quick scan of both thermafiber and comfortboard's available thicknesses. I was a little surprised that neither had a version in an easily available 4". There's an 8" thickness available that I suppose someone could in theory slice in half, but that's a practical nightmare without a non-negligible amount of work.
I've never tried to compress a 5.5" mineral wool batt into a 3.5" space., but I imagine it would bulge pretty bad in the center. On top of that, you'd be paying at least 4x for the price of the R19 fiberglass.
It's a shame there aren't more options is all I'm saying.
You can special-order mineral wool in any thickness but that's rarely practical. I agree that it's not very compressible.
The OP might consider using 1/2" wood fiberboard such as Homasote (http://www.homasote.com/) to fill the space. It won't provide as much R-value as foam but it would fill the space and provide a little R-value without the downsides of foam.
Or blow cellulose into the cavities. It will naturally fill the cavities and it's got a negative carbon cost.