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Stone wool versus blown-in cellulose insulation in double wall assembly

I am designing a wall assembly for a home in a very cold climate Zone 7. Considering labor and material costs, what is the most cost effective insulation providing me with the highest thermal resistance? I like the fire resistance of Roxul stone wool batts and the idea that it's more DIY. How about the air transference between the two products? Thanks.

Asked by Matthew Michaud
Posted Sun, 12/30/2012 - 08:12

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4 Answers

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I'm no expert but in regard to price, you'll want to get two quotes for comparison. One for roxul from the Home Depot bid room for roxul. They are cheaper than Lowes in the north east. I'm in NH. Orders over $2500 will get you a discount. I have a spreadsheet that breaks down the cubic inches and cubic feet of the different size batts, that way you can make decisions between batt sizes if you have options. I can email it to you. The second price will be for DP cellulose including labor because I assume you can't store and DP cellulose yourself. I didn't bother looking at the cellulose option because I like DIY of roxul. I assume roxul will have a higher perm but the experts here will have to confirm. Take both products and spray some water on them.. Then ask yourself which you'd rather have in your walls. Maybe it's better to have insulation that stores moisture but I preffered the way Roxul seems to shed water. But perhaps this puts more moisture on the studs. I'd also suggest treating your studs with boric acid if you have time.

Answered by stephen edge
Posted Sun, 12/30/2012 - 09:46

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High density rock wool batts (R4/inch) have comparable air retardency to low density cellulose (~R3.7/inch) , but are no match for 3.5-4lb density dense-packed cellulose (R3.5/inch- lower R but far more air retardent.) No fiber insulation is sufficiently air retardent that it can be considered part of an air-barrier system, but dense-packing fiberglass & cellulose DO reduce the rate of air-transport of moisture.

Damp sprayed cellulose is usually comparable per unit-R to high density rock wool, but it takes a very-slightly deeper wall cavity to match high density rock wool. At a typical 25% studwall framing fraction, the difference in "whole-wall" value with the thermal bridging factored in is miniscule- about R0.25 in a 2x4 studwall, or R0.4 for a 2x6 studwall. The inherent lack of voids in the cellulose installations compared to the unavoidable imperfections in batt installation evens up the score, or even tips the performance balance toward cellulose. But even 1/4" of foam for a thermal break on the stud edges would far outweigh any differences in center-cavity R value.

Spraying water on the insulation is a pointless test, (unless you plan on not flashing the windows & doors, mayhaps?) Yes, liquid water beads up and rolls off stone wool whereas cellulose absorbs it. But liquid water intrusions are rightly minimized by the siding/flashing/drain-plane design, and NO fiber insulation should be subjected to chronic liquid water. What happens to water vapor in the wall cavity and insulation is of greater interest. Whereas stone wool fibers adsorb but the tiniest hint of film of water on the exterior of the fiber when it's below the dew point of the entrained air body, it can quickly turn to liquid water or frost, and the framing/sheathing end up adsorbing the bulk of the moisture out of the air. By contrast cellulose fibers are hollow at the microscopic level and can adsorb on the order of 15% of it's mass in moisture without damage or diminishing it's insulating function, and results in lower levels of moisture accumulation in the susceptible wood.

In any zone 7 location taking back-ventilated "rainscreen" approach to the siding does a WORLD of good for minimizing bulk water intrusions from wind-blown rain making it past the siding, and dramatically improves the capacity of both the siding and sheathing to dry toward the exterior. But installing foam on the outside of the structural sheathing of sufficient R to be able to skip an interior side vapor retarder is being even kinder to the sheathing despite lowering the capacity for drying toward the exterior. Virgin stock foam isn't very cheap, (EPS and polyiso run about 10-cents per R per square foot) but if you can find a source for factory seconds, or reclaimed foam board from commercial re-roofing or demolition it can cost even less than low density fiberglass batts. (In New England there are several vendors of reclaimed roofing foam, but insulationdepot.com ships nationwide if the order is large enough to fill a decent sized truck.)

Answered by Dana Dorsett
Posted Mon, 12/31/2012 - 16:00

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Great details as always Dana. I knew roxul would put more moisture on the wood but didn't know why. Cellulose seems like a better moisture buffer.

You mention 1/4" min of foam to break a thermal bridge. Do you have any experience with this? Reason I ask is because I'm using 1" in zone 6 on the warm side and wasn't sure it it's enough.

Answered by stephen edge
Posted Tue, 01/01/2013 - 08:16

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Dana,
I Know from other conversations with Matt that he is considering a double stud wall. You stated "But even 1/4" of foam for a thermal break on the stud edges would far outweigh any differences in center-cavity R value." I can't see how 1/4" of foam on the stud edges can be at all significant when there is 5 1/2" + of insulation between the inner and outer wall.

With an assumed framing fraction of 20% the wall Matt & I have been discussing, and I'm planning on using, is r 48.3 with Roxul (using your value of r4/in , not what the us labels claim) and r45 with cellulose @ r3.5/inch. That means that it takes a 1" thicker cellulose filled wall to get the same thermal performance as the mineral wool wall. The plan is to have a vapor-air barrier on the outside of the inner wall and fiberboard sheathing on the outside covered with Tyvec, rain screen and siding. With proper attention to detailing the inner vapor-air barrier and taped Tyvec on the outside does the air retardency of the insulation matter?

Answered by Jerry Liebler
Posted Tue, 01/01/2013 - 18:31
Edited Tue, 01/01/2013 - 19:12.

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