How to Insulate a Shiplapped Roof Deck
I would like to convert my traditional, unconditioned attic to a conditioned one, without removing the roof’s shingles. There is no external insulation on the roof deck. Climate zone 2. I’m not certain, but best I can tell the only vents the roof has now are a couple of gable vents.
* They might not technically be shipplapped. They’re like 1×6 boards orthogonal to the rafters.
The problem is my roof deck is made of shiplapped boards, and they’re far from air or water tight. In some places I can see the roof underlayment.
With flash-and-batt / closed-cell, I’m concerned that the gaps between boards would end up being sealed pools in which water collects.
Originally, my thinking was that I would insulate with mineral wool and cover with drywall. But then I read https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling which kind of blows that idea out of the water.
So how does the presence of shiplapped boards affect the viability of the methods for insulating the roof of a conditioned attic, if at all? What should I do? Is there an option that doesn’t involve closed-cell, maybe with a vapor diffusion ridge?
(Whatever I do, I’ll need to deepen the rafter bays, as they are 2×4 20″ OC.)
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Replies
You'd have gaps between plywood or OSB panels in a roof with panel sheathing too. As long as your underlayment and shingles (or other roof covering) are in good shape, you shouldn't have any issues with insulating on the interior. I've never bought into the "I need to use something that can let leaks come through so that I can see them" argument, it's better to just maintain your roof and not have leaks in the first place! In terms of insulating, there really isn't any difference what the roof structure is made of (boards or panel products), as long as it's structurally sound.
I would go ahead and use closed cell spray foam here in the appropriate ratio for your climate zone. Don't worry about water leaks, but do do the usual roof maintenance to minimize the potential for leaks, which is something you really should be doing anyway.
Bill
I agree with Z7.
To answer your question about non-closed cell spray foam solutions, there are two issues.
First, we don't know what is legal under your governing building code. We can't answer this because we don't know where you live and we aren't your local inspector.
Second, there are plenty of examples where other insulation may work out fine in this application, for example there are some proponents of dense-pack cellulose in these kinds of roofs coupled with an impeccably installed vapor barrier below. There are also millions of old batt insulated cathedral roofs around the country. What we do know is that, generally, closed cell spray foam in cathedral ceilings is the safest approach because it helps avoid condensation issues against the roof sheathing while also providing additional protection against vapor migration.