Central PA Contractor Rec – Ext Rockwool Comfortboard
Chowtime
| Posted in Green Building Techniques on
Hi all. I’m based in Central Pennsylvania and looking for insulation contractor recommendations. I’m hoping to add 2.5-3 inches of Rockwool Comfortboard as exterior continuous insulation to my ~3600 square foot home. It has relatively new siding, so it would involve removing the siding, adding the insulation, and then (hopefully adding the siding back) with any necessary changes to accommodate windows, etc.
EDIT: Vinyl siding
I’ve not been able to find any insulators who have expertise in this space and would appreciate this forum’s help…thanks in advance!
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Replies
What type of siding?
Vinyl. Sorry about that. Edited post to clarify.
Are you expecting to re-use the siding?
I might consider a nailbase product rather than rockwool ...
https://www.greenbuildingadvisor.com/article/nailbase-panels-for-walls
I was really hoping to reuse the siding. It’s less than 5 years old.
As for why Rockwool, I have 2 inches of closed cell foam on the inside wall plus R-13 fiberglass carts, so I think I’m not easily able to add another vapor barrier.
Some people have reported that vinyl lap siding over vertical strapping can end up looking wavy, while others say they've had no problems. It might depend on the local temperature extremes, sun exposure, and the stiffness of the siding (vinyl siding with foam inserts will be stiffer). Confirm with the manufacturer what their recommendation is. Otherwise, a nailbase panel would be a safer choice.
Edited to add:
I missed your question about vapor retarders. A 2" or less thickness of foam would be a class II vapor retarder, not a vapor barrier. The sheathing would still be able to dry slowly in both directions.
re: "another vapor barrier"
Presumably you did a "flash & batt" type of insulation system ...
Did you do any type of vapor control to the inside of the FG batt or is the "flash" of closed cell foam acting as the vapor control?
If you didn't, by adding a zipR type of nailbase to the outside, it would be just adding to the vapor control layer you already have (the 2" closed cell spray foam).
I am missing something?
"Did you do any type of vapor control to the inside of the FG batt or is the "flash" of closed cell foam acting as the vapor control?"
Good question. The wall is as follows (from outside to inside): vinyl siding --> WRB (Kimberly Clark) --> plywood sheating --> 2" of closed cell foam (interior) --> R-13 fiberglass batts --> drywall.
I do not believe there is any other vapor retarder and as a result the closed cell foam is the only vapor control. The foam may be slightly over 2 inch in places (I didn't get to inspect much before the drywall went up).
As a result, I understand my best exterior insulation material is mineral wool so I avoid a double vapor barrier with plywood sheathing in the middle. I'd love to do foam board but I don't think that is advised. Let me know if I'm misunderstanding or if there is another approach that might work better?
so if you look at this from a first condensing surface perspective, the reason why the "flash & batt" system works is because the surface temperature of the closed cell foam never reaches the dew point.
by adding an additional 2" of polyiso foam in the nailbase to the outside of the plywood, this shouldn't change that (it should actually improve it)
are you concerned with bulk moisture or vapor from the outside wetting the plywood?
@FrankD " missed your question about vapor retarders. A 2" or less thickness of foam would be a class II vapor retarder, not a vapor barrier. The sheathing would still be able to dry slowly in both directions."
I understood that 2 inches (again may be slightly more) of closed cell foam is essentially zero perm so a vapor barrier. As a result, I should not consider foam for exterior insulation, which would include nailbase products (I think they are almost always foam-based). Let me know if this is faulty as I'd love to have more option than mineral wool!
Most if not all ccSPF at 2” is a class ii vapor retarder, meaning it’s less than 1 perm but over .1.
Here is a spec sheet for one from demilec
https://www.amddistribution.com/wp-content/uploads/2022/05/Heatlok-TDS-1.pdf
Chowtime, I wanted to be sure so I checked some sources including the ASHRAE Handbook of Fundamentals, John Straube's Building Science for Building Enclosures, and a couple manufacturer specs.
The lowest metric permeability I came across was 1.8 ng∕s·Pa·m.
Calculating the permeance of a 2" (0.05 m) foam thickness: 1.8 ∕ 0.05 = 36 ng∕s·Pa·m^2
Converting to U.S. perms: 36 / 57.2 = 0.63 perms → class II
Also note that the permeability of spray foam actually varies with the relative humidity. If the sheathing got wet, the foam would become more vapor open, closer to or even over 1 perm.
That said, it would be a good idea to check the specs of the foam you actually used.
Many thanks for this detail all.
I'm out of my depth at this point. In your experience, assuming my 2+ inches of closed cell foam is indeed a class ii retarder (I will confirm), would I then have more options for continuous exterior insulation vs mineral wool, wood fiber, cork etc? In other words, could I potentially use 2-3 inches of polyiso, EPS or XPS foam boards? Or a foam-based nailbase product?
Thanks again.
Look into siding contractors. Or just a general contractor. This is not insulation contractors territory, at least not in my region.
Thanks. I’m hoping to find someone who has some experience at least doing this kind of work with Rockwool. Not really an energy efficiency minded region as you say, but I’m hoping someone here has a rec!
Siding guys would do this type of work, or "regular" builders. Insulators don't typically work with continuous insulation in a project like this.
It's not difficult to do this type of insulation installation, it's essentially like hanging panel products like plywood. You don't really need special skills or tools here.
Bill
Thanks Bill. Are there any challenges with adding an extra 3 inches of insulation between the sheathing and the siding? I understand there may be some challenges with the windows but don’t know the details.
Using rockwool, you'll have somewhat squishy layer of insulation. You'll need some kind of strapping over the rockwool to hang the siding. Exactly what strapping and how much depends on what kind of siding you're using. Common materials to use for this are 1x4 and ripped strips of 3/4" plywood. I prefer using the plywood strips because they are less prone to splitting when driving in fasteners.
Over 2" is when most consider that you're into the "thick insulation" category here, which will complicate trim details. Ideally you want to find a builder that is familiar with using thicker layers of exterior insulation, since they'll be the ones that know how to handle all the details properly.
Bill
Many thanks Bill!
What are you looking to accomplish by adding exterior Rockwool to your home?
Decreasing HVAC load through a higher effective R-Value. House is well air sealed but thermal bridging of studs is quite high.
I would spend some time to do some energy modeling with a software like Beopt. My guess is you might save ~$200 a year in utility cost with the added insulation vs a ~$20k cost?
Here is a link to a similar discussion I posted about before.
https://www.greenbuildingadvisor.com/question/beopt-modeling-of-ci-energy-savings
Very interesting thanks Kyle. Will definitely dig into this. Did you do the triple pane windows? I have a bunch of double pane windows in the house and triple pane are also on the list. I had planned to do those later due to cost but maybe I need to rethink.
I personally have triple pane windows and appreciate the added comfort, as windows and doors are by far the weak point in a wall for r value.
Upgrading to triple pane windows over double pane is usually easier to justify because it might be, say a 15% adder. But that assumes you are replacing the window regardless. If you are in considering replacing perfectly fine double pane windows with triple pane windows just for r value improvement, the payback will never be there.
I wouldn’t consider exterior insulation or windows until you are at the point that you need new siding/windows and are going to replace anyways. Then you start thinking about envelope improvements.
In the meantime you can get a resnet certified energy auditor to do an audit with a blower door test and pick off the low hanging fruit. Air sealing improvements will be the best bang for your buck.
Agreeing with Kyle re payback, I none the less will exterior air seal and add continuous via Confortboard for breathability when the time comes for new windows. Having planned it out, yes to Bill's strapping for siding, but it seams the Comfortboard 80 is not so squishy. If concerned about contractor's commitment to getting the wall dead flat, there's always the 110 version, though perhaps less available / special order.
I've used Comfortboard 80 as exterior insulation, 1 1/2" - 2" thickness, on a number of projects and not had issues with it squishing under strapping.
I'm doing a small remodel on a portion of my own house (built 1880, CZ 6), and was planning to use Comfortboard 80 as the exterior insulation for exactly the reason the OP mentioned: it is vapor open, allowing the assembly to dry to the exterior (hodgepodge of insulation on the interior, including some spray foam). BUT, it's virtually impossible where I am to source an amount of Comfortboard 80 in a small enough quantity for my job, so I'm looking at a foam product instead. My workaround is to use a drainable self-adhered housewrap, probably HyrdoGap SA from Benjamin Obdyke. This will keep a tiny air space between foam and housewrap, so in theory the assembly still has drying potential to the exterior, albeit at a minor energy penalty. I can't find the original BSI article, but Malcolm references it here in #2:
https://www.greenbuildingadvisor.com/question/location-of-drainable-housewrap-in-wall
So, that could be an option, IF you were planning on replacing the WRB as well. That might get complicated around windows/doors, though.
Good additional option. Thank you! I do like Rockwool Comfortboard as an option, as it's a pretty high R value per inch and the fire barrier and sound deadening effects are nice side benefits as well.