insulating a metal shed roof on perlins
I am building a 10×12 detached studio with a metal shed style roof. The roof has a 2 foot rise over a 10 foot run, so its a 2.4 pitch. I have the metal roof panels attached to 2×4 purlins on top of 2×6 rafters. I am in climate zone 4a.
I want to insulate before finishing the interior. I am now reading that for this low-pitched shed roof I probably should have used sheathing instead of perlins, with rigid foam between the sheathing and metal. I am building this myself and learning as I go… and this is not what I did (I have metal, perlins, rafters). So given where I’m at what are my options for insulating the roof? As I understand it, I can:
1) Have a vented roof – Install baffles under the perlins, add vents along the low and high walls, and place fiberglass batts beneath the baffles. But from what I gather, with this low pitch the roof won’t vent well?
2) Keep it unvented – Cut rigid foam to place against the metal between the perlins, then use fiberglass batts benetah the foam. But it seems like without a sheathing layer this may still cause condensation and then rot?
3) Any better options out there? I really don’t want use spray foam – as this was intended to be a low cost outbuilding.
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Replies
Nebuer,
Low sloped roofs don't typically vent well, but you have three things in your favour.
- The distance air has to travel from eaves to peak is very short.
- The panels are installed on purlins allowing some cross ventilation.
- Studios don't usually generate the amount of indoor moisture residences do.
I would use your first option. With a well air-sealed ceiling it will be fine. If you want more insulation than the rafters allow, you could either add a layer of foam on the underside, or fur them down with 2"x2"s before installing the ceiling.
This is great, thank you! As I was building I sealed things up, so I'll need to open it up for vents. For this kind of roof do you know how much venting is needed? I see for typical attics it is one square foot of ventilation per every 150 square feet of attic. Would that be the same here? So in my case, about 1 sqft of venting?
Yes. If you added blocking between the rafters, and don't want to backtrack too far, one easy way to vent the cavities is with these: https://www.homehardware.ca/en/3-round-white-plastic-mini-soffit-vent/p/2671255
Do the baffles need to form a continuous barrier between the air channel above and the insulation below? I've cut the holes and installed the vents and baffles, but there are clearly air gaps around the edges of the baffles. Should everything be taped and/or spray foam to seal it up? All the walls have house wrap and want to keep the integrity of the work I've already put in (but sealing up all these baffles will be a lot of work too!)
Nebuer,
If you have an air-barrier on the ceiling, not sealing the baffles just means a slight reduction in the R-value of the batts due to wind washing. Some people might do it. I wouldn't.
Ok - for air-barrier would that mean the ceiling material itself (like drywall), or another layer between the insulation and the ceiling? And if so what?
Nebuer,
Drywall works well, especially in a small building with no interior walls. If you choose a material like t&g wood which is not in itself air-tight, poly underneath can provide the air-barrier. Make sure to seal any penetrations - light boxes and the like.
Ok - for air-barrier would that mean the ceiling material itself (like drywall), or another layer between the insulation and the ceiling? And if so what?
I need to make sure you understand putting insulation in between the steel framing is a useless waste of materials as the steel will conduct the heat around the insulation. The only affective way to insulate a steel building is on the exterior.
Steel building are great places to keep your garden equipment dry but total failures as low-cost energy efficient living space.
How cold does it get in your location?
Walta
The only metal is the roof, it is stick built with dimensional lumber. There is no metal framing.