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My question is regarding exterior mineral wool insulation and if there is a minimum thickness (R-value) one should install

Hi. My name is Mike Precious for nelson BC Canada. My question is regarding exterior mineral wool insulation and if there is a minimum thickness (r value) one should install. Should i follow the same guidelines as for foam insulation to keep the sheathing safe?

Thanks for the info and for this great and informative site.

Asked by michael precious
Posted Feb 4, 2018 8:08 PM ET
Edited Feb 5, 2018 5:48 AM ET


18 Answers

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Because, unlike foam, mineral wool doesn't impede drying to the outside, you can use any thickness you'd like. Determining how much you want to use will depend on how much R value you want to add to your walls, remembering that as the thickness increases so does the difficulty of flashing and siding the house.

Answered by Malcolm Taylor
Posted Feb 4, 2018 9:06 PM ET


Here is a link to an article you may want to read: How to Design a Wall.

In that article, I wrote, "Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)"

Answered by Martin Holladay
Posted Feb 5, 2018 5:53 AM ET
Edited Feb 5, 2018 5:54 AM ET.


While I agree with you guys that sheathing can still dry to the outside regardless of mineral wool thickness, I think that if you're going to the trouble of installing it, you might as well try to get enough R-value to keep the sheathing dry from accumulating moisture.

Answered by Michael Maines
Posted Feb 5, 2018 9:53 AM ET


I agree - thicker better resists heat flow AND reduces condensation on the sheathing. If your wall has the wrong interior/exterior wetting/drying rates, you do need to worry about minimum R value.

Answered by Jon R
Posted Feb 5, 2018 12:50 PM ET


I'm not sure that the discussion needs to stray into the whole wet sheathing debate. If the wall sheathing isn't risky without exterior insulation, then adding mineral wool won't cause a problem the way adding inadequate foam might.

Answered by Malcolm Taylor
Posted Feb 5, 2018 4:31 PM ET


Malcolm, since Michael (the OP) didn't mention his plan for siding, sheathing material, or cavity insulation, and just asked about keeping his sheathing safe, I think it is worth bringing up the wet sheathing issue. I recently consulted on a project with wet OSB, conventional housewrap and fiber cement siding. Including a thin layer of mineral wool would not have helped the sheathing--in fact, it would still accumulate moisture, while raising the dew point--the mineral wool may have even exacerbated fungal growth.

Using mineral wool on the exterior doesn't mean everything is automatically problem-free, it just means that the exterior insulation will not block water vapor flow to the exterior.

Answered by Michael Maines
Posted Feb 5, 2018 4:54 PM ET


Michael, I didn't mean to be dismissive, and you are right - we don't know what other measures the OP was including in the wall. My suggestion that we avoid the whole wet sheathing debate comes from this:

For some time builders like Dan Kolbert have suggested that the wet sheathing problem might be apocryphal. The theory (and some test walls) show that thicker walls might be risky, but as time passes we aren't seeing the failures the building science suggests we should. Added to that, several jurisdictions both in Canada and the US have recently mandated exterior foam thicknesses well below what should be safe, and thousands of houses have been built to that standard, again without apparent failures.

From my perspective this is one of those periods when if we don't know quite what is going on, we should just wait until we do. Prudence suggests following the minimum foam thicknesses GBA has promoted as good practice, but I'm not sure it makes sense to increase mineral wool exterior insulation thickness just to counteract a problem we don't know exists.

I may be off base in my analysis. i'd be interested in your's and other's take on this.

Answered by Malcolm Taylor
Posted Feb 5, 2018 9:42 PM ET


Hey guys
Thanks for the feedback. I'm a builder working for a contractor here in nelson BC. Zone 6.
I know it was kind of a general question without any other wall details, just exploring more affordable options for exterior insulation then foam. We would need around 3". I agree that thicker (higher r value) is better. However as I'm sure your all aware building is very expensive and green building even more so. Not easy to convince a client that they need to install 3 inches of expensive foam (even if we all agree they should). While 1 1\2" of mineral wool isn't as good as 3" of foam it is better then no exterior insulation and at least provides a thermal break. And at half the price for materials a little easier sell.

Answered by michael precious
Posted Feb 5, 2018 10:35 PM ET


I agree with you both (Malcolm and Michael) that some mineral wool is better than none, in most cases anyway. If you have a vented rain screen cladding and dense-packed cellulose insulation, you are probably all set. If you use CDX sheathing (or board sheathing) instead of OSB (including Zip), even better. Finish it off with a variable permeance interior membrane and it's a rock-solid assembly, and probably overkill--don't let the perfect be the enemy of the good, etc.

I understand keenly how the cost of energy efficient details can add up quickly, and that buildings seem to be more resilient than we give them credit for. I just think it's still worth being cautious until we understand all of the mechanisms and how newer products react--issues can take ten years or more to show up. A rain screen gap and cellulose insulation seem to solve a lot of problems, so I try to include those on every project.

Answered by Michael Maines
Posted Feb 5, 2018 10:55 PM ET


> If you have a vented rain screen cladding and dense-packed cellulose insulation, you are probably all set.

I suggest that ~8 perms on the interior side, < 1 perm on the exterior side (eg Zip sheathing), 1" exterior mineral wool and no interior side air barrier (eg just taped Zip) would be quite risky in a cold climate, even with rain-screen and cellulose.

Use CDX, <= 1 interior side dry perms and an interior side air barrier and I agree with you.

More data is needed, but until then, we should be careful with comments like "don't have to worry" and "probably all set".

Answered by Jon R
Posted Feb 6, 2018 12:42 AM ET


I take his question to be "if I add to the default wall used here in Nelson, (which is a 2"x6" one with an interior poly vapour-barrier), is there a problem with how much mineral wool I choose to use?" Surely the answer is no, especially with a rain-screen.

Answered by Malcolm Taylor
Posted Feb 6, 2018 12:54 AM ET


No disagreement once you clarified in #5.

I suggest that with that assumption (and good poly air sealing), non-faced rigid foams (of any thickness) aren't a problem either.

Answered by Jon R
Posted Feb 6, 2018 1:17 AM ET
Edited Feb 6, 2018 1:24 AM ET.


The wall assembly I have in mind ( based on what would be a typical wall in nelson), is as follows - siding, rain screen gap, 1.5 to 2 inches mineral wool, WRB, CDX, 2x6 framing with either dense pack cellulose or mineral wool bats, interior poly vapour barrier.

Also any thoughts on an insulated 1.5 or 2 inch wiring chase inboard of poly vapour barrier? I knoiw I was complaining about costs earlier but just something I'm interested in personally.

Answered by michael precious
Posted Feb 6, 2018 2:22 AM ET


I think the insistence on interior polyethylene by Canadian building inspectors is misguided. If there is one thing I would change in your proposed wall assembly, it would be to install MemBrain (a smart vapor retarder) instead of polyethylene.

A wiring chase won't affect the moisture performance of the wall assembly either way. That said, most builders who've looked at wiring chases conclude that the cost and the disadvantages (including loss of interior space) outweigh any possible advantages.

Answered by Martin Holladay
Posted Feb 6, 2018 5:16 AM ET


I definitely like the idea a smart vapour retarded instead of poly. I have never been entirely comfortable with poly.

Answered by michael precious
Posted Feb 6, 2018 12:26 PM ET



You know a lot about pressure differentials - you will like this one.

I'm doing an addition to a restaurant in a very leaky, uninsulated building from the 1950s. No insulation at all, and leaky as in you could probably read a book with the light that comes through the ceiling into the attic. About 25 years ago when the kitchen went in, a commercial range hood was installed over the line, with matching supply-air nearby. The owner felt that the supply-air was cooling the food, and had it blocked off. So during business hours, the whole building operates under negative pressure strong enough to open doors and force strong air currents through electrical boxes and any other opening it can find. Then at night the whole process reverses, and the warm interior-air flees the structure at a similarly alarming rate.

Trying to immunize the addition from all this has been a challenge. It's like someone is doing a blower door test every day. Perhaps unsurprisingly, the existing structure is in very good shape.

Answered by Malcolm Taylor
Posted Feb 6, 2018 11:13 PM ET


That's one way to keep the framing and sheathing dry!

Answered by Martin Holladay
Posted Feb 7, 2018 5:22 AM ET



Luckily it's not my money doing so!

Answered by Malcolm Taylor
Posted Feb 7, 2018 12:18 PM ET

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