Can framing bear it ?
We are planning a retrofit of our 1949 Cape at the VT-MA-NH intersection. Currently, there is no insulation in the roof. In walls, some stud bays have fg batts while others are empty. Among other things, the building scientist we hired is specifying 6 inch insulation outside roof sheathing and 4 inch outside wall sheathing. We were thinking to use Neopor GPS in both locations but at these thicknesses, very little vapor can pass through. Ideally, we would like drying potential both inward and outward. For this reason, we are wondering if it might make sense to use Roxul comfortboard 80 mineral wool (which has high drying potential) instead of Neopor GPS. A third option would be to use mostly roxul comfortboard outside roof/wall sheathing, with only the outermost inch or so being gps. Can our roof frame support the weight of 5-6 inches roxul comfortboard 80 outside existing roof deck? Can our studs support the weight of 3-4 inches roxul outside wall sheathing? Must we hire a structural engineer to make this determination? The only things our roof framing is currently supporting on the outside are: tng sheathing, underlayment, asphalt shingles (which we will swap out for standing seam metal). Much lighter per square foot than 5-6 inches comfortboard plus 3/4 inch new decking outside that, plus furrings, plus standing seam panels. As for the studs, the only thing they are holding on the outside currently are the orig barn plank sheathing, the orig wooden clapboards (which we will swap out for vinyl siding), and (in some spots) some very flimsy mechanically fastened membrane (looks like burlap grain sack). Again, a lot lighter per square foot than 3-4 inches roxul comfortboard. If we can avoid the expense of a structural engineer, great. But who else would know how much exterior weight our 1949 timber framing can handle ? Thank you
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Without knowing more specifics about the construction, I'd be hesitant to throw that much comfortboard up there in a region that's already prone to snow loads. However, I wouldn't use comfortboard in this location anyways. On the roof you want polyiso, nearly without exception. It's significantly lighter at 2.5 lb/ft^3 vs 8 lb/ft^3), less compressible (25 PSI vs moderate firmness), and has higher r value per inch (6 vs 4). Granted, it's not vapor open, but it doesn't matter. If you sheath it with OSB and seal the seams (as you ought to if you're already going through with the retrofit), that layer is going to be the major limiter in vapor permeability anyways.
Thank you, Kyle. We were thinking to tighten our belts and shell out the extra $ for plywood decking largely because of its higher vapor permeability (10 perms) as compared to osb (2 perms)
https://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials
Also, we walked away from considering polyiso when we learned that during our cold winters, the R value is equal or below that of other types of rigid foam. We also read that over time, its R value declines
While you're correct that the R value does degrade with colder temperatures, even at its worst rating, it is higher than rockwool and EPS, and nearly tied with XPS, all while being lighter. I'm not a polyiso apologist by any means. I've got exterior rockwool insulation on my walls for the reasons you describe, but used polyiso on the roof for the reasons I listed above. Here's a link to an article on this site that talks about it a little more.
https://www.greenbuildingadvisor.com/article/cold-weather-performance-of-polyisocyanurate
I'm also casting doubt on the 10 perms plywood claim (and for that matter 2 perms for OSB). Its highly dynamic based on the relative humidity, inside, and outside, as well as the temperature. It's true that plywood is more vapor permeable than OSB, but I'd put the realistic permeability at somewhere around 4-5 on average, at best.
https://foursevenfive.com/blog/why-the-vapor-curve-matters/
https://www.fpl.fs.fed.us/documnts/pdf2007/fpl_2007_glass001.pdf (see pages 5-8)
I didn't answer on the walls, but pretty much any vertical stud wall can handle any type of insulation and cladding, unless we're talking about hanging concrete block from the studs themselves.
Thank you, Kyle, for your time and the helpful info
Also don't forget that only the very outside of the insulation is near the outside temperature, and gets warmer the closer it is to the interior. So not all of the polyiso will be derated r-value.
Where did you hear about r-value reduction over time? I've seen that many times about XPS, and I don't remember seeing that comment on polyiso.
Very good point Trevor. The outer layer will keep the interior layers warmer. I tend to comment thinking about single layers, sometimes those kind of details slip past me.
It has been a widely documented that polyiso derates with time as the gases inside escape. I believe it’s part of the same reason that it shrinks slightly as it ages. Check the gba link I posted in one of my replies, I’m sure it is mentioned in there somewhere.
Good point, Trevor—thanks
Anonymous user,
The loading on structures is typically divided up into dead and live loads.
The dead loads are those that are constantly imposed by the weight of the house itself. So they are the things you mentioned for the roof, but the walls, along with the sheathing and siding, also pick up the weight of the whole structure (roof, floors, walls above, etc).
The live loads are those imposed by other conditions. Snow, occupants, wind, etc).
The load you are asking about - the weight of more insulation - is dwarfed by the existing live and dead loads you already Have. A more pressing concern is how you will attach that thickness of exterior insulation so that it is adequately supported. That's where you may need some engineering advice.
There is no point trying to get any drying to the outside with that much exterior insulation of any type. You are better off using foam, and designing the wall so the it has good drying to the interior.
Thank you, Malcolm—very helpful !