I have a basement project I'm working on that has Superior Walls with 2.5" of Dow Styrofoam 15psi XPS. I need to add another 1.5". Would it be OK to add faced polyiso (more R-value) or add 1.5" of XPS? Are there any issues with mixing? Thanks.

Either will work. The foil facers of the polyiso are true vapor barriers that could potentially trap moisture in the XPS, which would reduces it's performance somewhat, but if you have at least a foot of exposed above grade foundation wall and decent roof overhangs so that the foundtion isn't constantly rain-wetted it won't be a problem

Polyiso is far less damaging to the environment, and would be preferable on those grounds.

If you are concerned about the ground moisture issue 1.5lb density "Type-II" unfaced EPS would be better than XPS or polyiso, since it has about 3x the drying capacity of XPS. Type-I EPS (1lb density) is even more vapor open, but is more easily damaged in handling.

Both EPS and polyiso are blown with pentane, which has a global warming potential about 7x that of CO2. XPS is blown with a mixture of HFCs, the predominant component of which is HFC134a, which has a GWP about 1400x CO2. As it outgases over the next 50 years it's R-value will drop to that of EPS of similar density anyway. EPS sheet goods outgas the pentane within weeks, and it's labeled R is the fully depleted performance, which remains stable over time.

Where are you located? Given this is for a basement, If your deep earth temperature is 45 deg F or less, 1.5" of 2# density EPS foam will out perform 1.5" of polyiso due to polyiso's characteristic drop off in R value as its temperature drops below ~60 degrees F. And the EPS foam should cost less assuming you are purchasing new material. And the EPS is essentially unaffected by ground moisture and has a stable R value over its lifetime (polyiso and XPS drop off in R value as they age).

Posted By arkie6 on 06 Nov 2014 05:47 PM
Where are you located? Given this is for a basement, If your deep earth temperature is 45 deg F or less, 1.5" of 2# density EPS foam will out perform 1.5" of polyiso due to polyiso's characteristic drop off in R value as its temperature drops below ~60 degrees F. And the EPS foam should cost less assuming you are purchasing new material. And the EPS is essentially unaffected by ground moisture and has a stable R value over its lifetime (polyiso and XPS drop off in R value as they age).

That's a gross overstatement of the performance roll-off of polyiso. Given that it would be on the INTERIOR side it would be warmer and at a temperature where it may even outperform it's labeled R-value.

Only when the MIDDLE of the polyiso or EPS is at 45F would EPS outperform it. (That scale is "mean temperture" of the foam not the exterior or interior side temperature.)  If it was all polyiso and the room was 65F, a mean temp of 45F would require a ground temp of 25F (permafrost!) for EPS to be at par with polyiso.

With ground temp of 45F and a room temp of 65F, the polyiso would in fact beat it's rated R, since at a 55F mid-foam temp it is near it's absolute peak performance. (Which occurs at at mid-foam temp of ~60F.)


But even the cold side of the polyiso is warmer than 45F.  With an inch or two of XPS between the polyiso & ground the polyiso will perform at it's labeled R (or better) in any location in the lower 48 states.

Where polyiso derating is an issue is when it's on the cold side of the assembly in a very cold climate, with insulation between the polyiso & interior.

At any labeled R value EPS and polyiso are usually comparable, but XPS commands a 20-30% premimum.

Posted By Dana1 on 06 Nov 2014 05:58 PM

Posted By arkie6 on 06 Nov 2014 05:47 PM
Where are you located? Given this is for a basement, If your deep earth temperature is 45 deg F or less, 1.5" of 2# density EPS foam will out perform 1.5" of polyiso due to polyiso's characteristic drop off in R value as its temperature drops below ~60 degrees F. And the EPS foam should cost less assuming you are purchasing new material. And the EPS is essentially unaffected by ground moisture and has a stable R value over its lifetime (polyiso and XPS drop off in R value as they age).

That's a gross overstatement of the performance roll-off of polyiso. Given that it would be on the INTERIOR side it would be warmer and at a temperature where it may even outperform it's labeled R-value.

I wasn't familiar with Superior Walls and assumed the foam was on the OUTSIDE of the walls.  I just googled it and now see that it is on the inside.  If you've only got foam on one side of the wall, it just seems bassackwards to put all of the foam on the inside from a thermal performance standpoint.

If he hasn't backfilled yet, maybe he can put the EPS on the outside of the wall?

Indeed he could put EPS on the exterior!

To do that he would have to figure out the flashing & drain-plane issues, and put some sort of finish on the EPS. (All soluble problems, to be sure.)

Putting polyiso on the exterior below grade would be a waste of good polyiso, since polyiso is hygroscopic and would be guaranteed to take on significant water over time, losing performance when wet.  EPS has interstitial spaces that take on moisture, but unless it's pond-soaked and fully submerged the performance hit is subtle, not of any concert. (If the ground is that wet he'll have problems with that foundation system too.)

This is just for an extra layer on the interior over the existing 2.5" Dow board that's built into the wall. The exterior is back filled, but the Superior wall is about 40% above grade so adding to the outside would not be easy from an aesthetics view. Because the wall has a large above grade exposure, I figured adding more to the wall would be a benefit. It will be finished , conditioned space. I should get R-21 which is close to the R-26 for the stick framed wall on the first floor.

The building is in Central Maryland zone 4.