Insulate shop with polyiso between rafters?
Hi Folks …
[I am new here, but have read through enough posts to see that what I’m asking about seems to be within the normal range of discussion, but not answered directly by anything I could find.]
I am in MA (zone 5). I have a 12′ x 20′ prefab shed that is used as a workshop. It was insulated with fiberglass batts between the 2×4 (yes, 2×4) rafters. There was 3/8″ plywood below these covering the rafters, but not all the way up to the peak — top foot or so on each side was just the open insulation face.
I re-roofed it this summer due to shingle deterioration (it is 20+ years old) and found some wet insulation and some water damage. We removed all the old insulation and all the 3/8″ plywood, and replaced one 4′ x 4′ section of water-damaged roof sheathing. It’s not clear to me if the water issues with the insulation and that one sheathing piece were due to leaks, or condensation / lack of ventilation, but I’d guess it was a combination of the two.
There was no ventilation above the insulation, but when we did the roof we added a ridge vent and I have also installed continuous soffit vents.
At the moment it is uninsulated (and heat is off!) but I need to install something. My inclination was to put attic vent channels between the rafters, 2″ foil-faced polyiso under them, also between the rafters, then some kind of ceiling — perhaps plywood again, or Reflectix, on the bottom surface of the rafters. This would leave ~1/2″ of space between the bottom of the polyiso and the top of the ceiling covering. (The Reflectix might be subject to some damage in a shop but has lighting benefits and provides an additional radiant barrier for what that’s worth.)
If I am calculating correctly this would give me R-12 from the polyiso plus a bit more from the other stuff, which is adequate considering that I only heat it sometimes. I can’t really do more because the headroom is very limited so I don’t want to put furring strips below the rafters.
Here are my questions:
* Is this a good approach for my situation? If not, what should I change?
* Will something like this work in a sporadically heated building? At most I would tend to heat it 15 – 20 hours per week, often less.
* Do I need to seal the polyiso / rafter joints with foam to prevent significant heat loss?
* Do I need to either seal the polyiso to the rafters or install a moisture barrier below it to prevent moisture infiltration? Or can I just rely on the top side venting to remove moisture if it gets in there?
And here is (I think) a separate issue: In the summer I have a different condensation problem because the floor is poorly sealed and the building is over dirt — no slab (the corners sit on blocks). Moisture rises from the ground, especially on warm spring days, and condenses at the peak inside. I have remote humidity sensors, there can be a 20% difference between the top of the wall and the peak of the ceiling, just 18″ or so higher! I have seen it dripping. Just bought a humidistat which I plan to connect to an exhaust fan currently installed at the peak at one end; there’s a vent at the other end. Any other thoughts on what to do about this problem?
Thanks for any help.
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