how best to insulate under metal roof
OK, I am lost … the more I ask and read, the more confused I get. I am hoping someone can help straighten me out.
I am building a large workshop … 4500 sq ft … 16 ft ceilings.
I am just finishing up putting a standing seam metal roof over purlins.
Problem … on a day when the metal gets cold at night (like last night we had a good frost), the metal panels are wet on the bottom.
This makes sense … moisture in the air condenses on the cold panels.
Now, how best to prevent this.
I have full vented eaves and a full vented ridge cap.
I plan on insulating the ceiling and putting up a vapor barrier.
I was going to just insulate along the bottom of the ceiling but that gives a large air pocket below the roof.
I could insulate just below the roof (leave say a 3 or 4 inch air gap). This would leave a minimum air pocket but is much much harder to do.
I have drawn (attached) a quick picture to show these top options.
Foam is out as it is way over the budget (plus you could not spray it on the bottom of the panels as the roof “moves” … expanding and contracting with the temp).
I was thinking fiberglass batts or blow in cellulose.
I have been told everything from “don’t worry about it .. it will go away” … to “your building is going to rot”
Just trying to minimize any issues.
Thanks ..
You can see pictures and videos of the project on my facebook page:
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Replies
Once you have a ceiling in place with good air ceiling and a vapor barrier on the drywall side the air above the ceiling won't be as humid. I can't tell from the picture, but does the roof have a vent at the ridge? If air can get in at the eave and exit at the ridge you should get a natural air flow and that area should be at the same humidity as outdoor air.
DC,
https://www.greenbuildingadvisor.com/question/moisture-on-bottom-of-roof-panels
Yah ... roof is almost done and I and finally getting to close the insulation time (I can work on that all winter and finish the siding in the spring).
As you can see from those answers, some say it is not an issue ... some say it is).
Just hoping to see if I can get a more definitive answer.
Thanks ...
I do have a fully vented ridge.
I was just not sure if having a minimal air space above the two bay ceilings would make a big difference.
Thanks ...