Wise to use two different insulation approaches in same space?
Hi all,
I’m renovating/retrofitting a cabin in the Catsklls area and have questions about the insulation method proposed by the architect. We are adding an extension to the cabin, among other things, whose floorplan will be open with the main rest of the space. The plan right now is to use a different insulation method in the new construction than what will be added to the existing cabin. The existing attic will have open-cell spray foam in the rafters while the new addition roof (vaulted ceiling) will have fiberglass between the joists and a few layers of polyiso rigid foam on top of them. The dubious “greenness” of spray foam aside, would it not be just as wise to use spray foam throughout as a means of really sealing air gaps, or is there wisdom in doing the more sophisticated method where we can in the new addition? If air gaps aren’t sealed well there, it seems like it would defeat the point of doing spray foam elsewhere, but perhaps I’m thinking about it all wrong.
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Replies
TAG,
There are no building science reasons not to use two different insulation strategies in one house. The question is whether they are the optimum ones for each roof. If you wanted to describe how the two are (or will be) built, you could get some feedback on that.
Sure. THe existing roof is shingled and framed by 2x4s. The spray foam will be on the underside of the roof. The new roof would be (from interior to exterior) sheet rock, 5.5" fiberglass batt within 2x6 framing, then sheathing, three layers of polyiso, ice/water barrier, shingles.
I would not do open cell spray foam in cold climate. You could perhaps do it and have no issues, but there are definitely case studies of failures using open cell alone in roof assemblies, as vapor has a tendency to find its way through to roof deck, but not back out again. You would be better off doing flash and batt with ccSPF if you decide to go that route.
I think your idea is fantastic to not spray.
Imagine in a few years if you had to replace the roof of the existing and the sheathing you can't see currently is toast. Now you are doing a roof and insulation.
It's hard for me to not think spray foam as the most awful product to use as the owner in the future. Everything has to work perfect, otherwise you're in for more work. And the bonus is it isn't green and it's expensive.
I agree with freyr; while open cell foam is code-compliant because it is airtight, it is vapor open so in a cold climate, moisture will work its way toward the sheathing. Moisture-related problems can take a decade or more to present themselves so don't rely on salespeoples' word that they haven't seen any issues.
From a building science perspective, continuous exterior insulation with a fluffy insulation such as fiberglass on the interior is ideal. Just be sure to cover the fiberglass with at least a class 3 vapor retarder, even in attic spaces.
And I agree with Ryan as well--spray foam is best avoided when possible, for many different reasons. There are situations where it's the only reasonable solution, and in those cases, I'm grateful that it's available. In most situations, however, there is a less-toxic, less-permanent, more-forgiving option.
I think it's important to note that none of this insulation is at all green; it's all basically plastic generated from fossil fuels and will all take centuries to fully break down. But points taken.
What about mice and fiberglass? Is that not a problem to throw into the mix? They are terrible up here.
Robert
You're on a green building site; we're going to talk about the relative "greenness" of different materials. Fiberglass is not plastic and it has much lower levels of embodied carbon emissions than foam. Cellulose or wood fiber would be even better, and more pest-resistant due to their borate content.
The argument that no insulation is perfect so why think about its environmental impact reminds me of an argument I once had where I was accusing someone of stealing and they said that I drove a few miles over the speed limit so I had no standing to say anything, since we both broke the law.
That absolutely wasn’t what I meant to imply. If anything the opposite. Fair point about being on a green building site, but “greener” is always a better/more accurate word than green to avoid unintentionally contributing to greenwashing. Preaching to the choir here but it’s important.