Lucas Blass
2/12/2018

I have a 14x26 cabin, the roof is 2x6 10-pitch rafters with horizontal 2x4 purlins. Corrugated metal panels are screwed directly to the purlins with no sheathing. I'm wondering what the best way to insulate this roof would be? The building is unheated now, and the condensation is not too bad. However, I want to add a wood stove for next winter. I've considered maybe putting 1/2" inch rigid insulation under the purlins, then fiberglass batts, then maybe a vapor barrier closest to the living space? Does anyone have any pro tips or suggestions? Additionally, is there a better way to flash a chimney pipe through a metal roof than the silicone boot flashings?
Thanks in advance!

Todd Miller
Isaiah Industries, Inc.
2/12/2018

Thanks for your question. If you insulate as you have described, the vapor barrier must be done very well. If any moisture at all gets around or through it, it will condense between the insulation and the metal. The other option is to spray the back of the roofing with closed cell polyurethane foam. This will provide R Value and act as a vapor barrier. As far as the chimney flashing, unless it is a double wall and well insulated chimney pipe, I think you need an all metal flashing.

Lucas Blass
2/12/2018

Thanks for the reply! I was wondering, since the rigid insulation in my proposed scenario would be contacting the back of the purlins but leaving an open 1 1/2" in between purlins to the metal, would that be enough of an airspace to prevent condensation? I plan on using double-wall chimney pipe, but am open to all-metal flashing. Any recommendations on one?

Todd Miller
Isaiah Industries, Inc.
2/12/2018

Unless the 1-1/2" is actually vented, if moisture gets into that area (due to a breach in the vapor barrier), it will condense on the back of the metal roofing. Not sure I can help you with the chimney flashing source. Sorry. Have you asked your metal roofing supplier what they suggest?

Lucas Blass
2/13/2018

Thanks so much for the replies, this is great info. I'm planning on having a continuous soffitt and ridge vent if I go this route. Do you think that would provide enough ventilation? When I called my metal supplier, they said the boot flashing was what they sold.
Thanks again!

Todd Miller
Isaiah Industries, Inc.
2/13/2018

Thanks. Sorry, for some reason I thought that ventilation was out of the question. Yes, venting the airspace is what you'd need but the problem is that the horizontal purlins will prevent air flow ... you will get a little through the ribs in the metal roof but not enough. One option would be vertical battens with your insulation and and air gap and then horizontal battens ... need to make sure that the air can get to the vented area.

Lucas Blass
2/13/2018

Gotcha. With all those considerations, do you think sprayed closed cell foam would be the best option? If I went the spray foam route, would that eliminate the need for a ridge vent?

Todd Miller
Isaiah Industries, Inc.
2/13/2018

Overall, I feel that is best.


7/25/2018

I also have a metal roof fastened to purlins in my metal building. There is so much heat transferring from the metal panels (148 degrees), that I would like to insulate. Building was constructed with soffit and ridge vent. The garage is not heated or insulated. Should I spray foam or use a insulation blanket. I am located in Wisconsin. Any suggestion are helpful.

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