Guest User
3/25/2004
I started the thread below "Insulating a shop building". Since then I have framed my shop and am ready for the metal roofing.(I think) My method of insulating was to use a foam board on the underside of the purlins and the metal on top. When finished the purlins will be sandwiched between the two with a 1 1/2" airspace between each purlin. There will be no ventilation for this airspace. Will this cause a condensation problem?? JT
Allan Reid
Dura-Loc Roofing Systems, Inc.
3/26/2004
Yes especially if you intend to heat the interior and you are in a climate that goes below the dew point. The building code calls for a roof assembly that includes an interior air barrier such as a heavy poly sealed at the joints wich stops hot moist air loss which will condensate on the underside of the metal and without ventilation will drip. It also calls for a moisture barrier directly under any roof material. As you have no solid decking you should use one of the reiforced underlayments such as a Triflex 30 over the purlins. Now you still must ventilate the air space. You can easily do this with vented soffit of a vented eaves combined with a profiled vented closure strip under your ridge cap. Check with your material supplier to get their rcommendations.
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