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Double stud wall -- mid-wall insulation = EPS or mineral wool?

I plan on a double stud wall with both walls framed 24" OC and aligned, and OSB sheathing on the outer face of the inner wall as the primary air barrier.

For essentially identical material cost I can use either mineral wool bats or EPS in the mid-wall cavity. The construction sequence would be slightly different as the EPS would be installed on the inside of the outer wall before the inner wall was erected while the mineral wool would be installed after the inner wall was up, through the openings in the outer wall.

The mineral wool offers a slight advantage in advertised R-value but it seems more labor to install would be needed. The EPS being in larger sheets and an effective air stop would seem to be an air sealing benefit.

Am I missing anything? What would you do?

Asked by Jerry Liebler
Posted Jul 17, 2014 5:05 PM ET
Edited Jul 18, 2014 6:07 AM ET


4 Answers

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EPS in the middle of your wall will be a class II vapour retarder (semi - impermeable), which may or may not work for your assembly.


In addition, to make EPS an effective air barrier, it would need to be taped at the seams, an added expense.

Finally, the sequence of construction sounds more complicated with EPS.

I would go with mineral wool for increased R value, high perm rating and ease of installation.


Answered by Jason Hyde, Peterborough 6A
Posted Jul 17, 2014 9:32 PM ET


I agree with Jason. There is a third option you may want to consider: dense-packed cellulose. The cellulose fills every nook and cranny, and does a good job of reducing the air leakage rate.

Answered by Martin Holladay
Posted Jul 18, 2014 6:05 AM ET


Martin: How would he install cellulose from the outside? Would he need to sheath the outer wall and drill holes to get the hoses in? If he sprays cellulose from outside before sheathing the outer wall, how does he keep the cellulose dry if it rains before the sheathing is installed and taped? Does insulweb suffice? Does it matter if the cellulose gets wet, so long as it can dry either to the inside or outside?

I suppose he could place the outer wall first, sheath it and then insulate it from the inside, and then place the inner wall, assuming the outer wall is carrying the load. That option in turn depends on the foundation detail.

Answered by stephen sheehy
Posted Jul 19, 2014 7:53 AM ET


Cellulose could be blown from the outside through insulweb on the outside, or possibly through the Tyvec which will be attached to the outer face of the outer wall's studs. The roof with overhangs will be present before the insulation so the wetting would be slight, or non existent if it was blown through Tyvec. BTW I plan on NO sheathing on the outside! Over the Tyvec will be Greenguard DC14, a 1/4" fan fold XPS drainage mat that adds r1 and Ambrico EZwall support system for thin brick.

While cellulose would possibly be a bit cheaper, the down side is the r value, which most seem to call 3.7/" meaning I'd need another 1" of wall thickness for the same r value (r30 in the outer cavity).

It seems the consensus is to NOT use the EPS option. However the choice between mineral wool bats and cellulose for the outer cavity is difficult!. Is the air sealing and moisture buffering of cellulose worth an inch of wall thickness?

Answered by Jerry Liebler
Posted Jul 19, 2014 11:44 AM ET

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