Garage Insulation with Metal Roofing

Old 03-16-20, 06:24 AM
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Garage Insulation with Metal Roofing


I have a detached garage with metal roofing I want to insulate. The roofing is on by strapping attached to trusses with no vapor barrier installed before roofing went on.

I am trying to make a flat ceiling so insulate the bottom of the trusses, what I am worries about is water leaking onto the insulation if a screw leaks or something.

Is it ok to run vapor barrier in 2 parts (to allow air flow) on the bottom of top end of trusses to allow water to be redirected from insulation if it does leak?

I have attached an image (sorry in advance for crappy drawing)
Black = Metal Roof
Brown = trusses
Pink = Insulation
Red = vapor barrier I want to add?

How else would I resolve this issue I have?

Thank-you for help in advance.

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Old 03-16-20, 03:29 PM
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I think you are trying to do something that the building wasn't built for. If you plan to heat and finish that building as a shop, it should have started with the roof deck. What you have is unheated pole barn style roof construction. If it's going to be a heated building, it should have been sheathed with plywood followed by a hi-temp ice and water shield as the roof underlayment.

If you are concerned about condensation dripping off the metal, I would suggest you put 1x2 cleats along the length of the edges of all your rafters-both sides- then install styrofoam chute style baffles. (Stapled to the bottoms of the 1x2s) These are normally used for roof ventilation but in your case, they can be used to catch and channel any dripping moisture out beyond the exterior wall to your soffit area. You would just need to start at the top, and work your way down so that your chutes overlap shingle style.

Your insulation can then go in the bottom chord of the truss. You do not need any vapor barrier... but if you want to use kraft faced insulation so that you can staple the insulation up, there would be no problem with that. Kraft facing toward the warm (heated) side of your ceiling.

Once you put insulation up you would need to cover it with an air barrier, whether it be plywood or drywall. You want it to be air sealed so that your heated air cannot seep through your insulation into the cold space above, where it would condense in cold weather. And some ventilation would likely be good for summer months. An exhaust fan in one gable end with an intake louver in the other. (Preferably on the prevailing wind side, if possible.)

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