Insulating Between Post Frame Building Purlins
Reader JUSTIN in Chelan writes:
“Hello, you have a lot of amazing advice, but no answers that seem to address my building exactly. I had a 24x40x16 put up last year, it has the R6.5 roll insulation blanketing the walls and ceiling with the metal sandwiching the insulation over the purlins and girts with the white vinyl face to the inside of the building. I would like to add more roof insulation while keeping the vaulted ceiling, the purlins are 2x8s on edge so I have 2ft by 10ft cavities that I would like to add more insulation to. I would love a simple inexpensive solution if you had one, but I’m assuming my best bet would be to cut 2″ foam board to fit in the cavity tight against the vinyl face, then use cans of spray foam to run a bead along all the seams to create an air impermeable surface. Once that cures I would either add 2 more layers of rigid foam, or maybe a bat of unfaced R-22 to fill the cavity and cover it with something breathable or a white faced house wrap, something like that?”
Thank you for your kind words.
To meet requirements of the Building Codes (R806.5(4)) “any air impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.”
Your Metal Building Insulation has a vapor retarder facing. It also has little net insulation value due to being compressed to nothing but fiberglass when it crosses each roof purlin. It makes for a very expensive method (if even properly installed) of attempting to control condensation.
If you are adding rigid foam boards, in Climate Zone 5, you should have no less than R-20 (IRC Table 806.5) in order to prevent condensation. Once you have installed R-20 (or more) is where your air sealing should occur to create an air impermeable surface. Any added insulation below this should be air permeable (like Rockwool batts) and remain unfaced without a vapor retarder or barrier below.
Please keep in mind, you are potentially trapping water vapor between two vapor retarders as area between white vinyl facing and sealing on underside of your air impermeable surface does not allow this region to dry to either exterior or interior. With your metal building insulation blanket covering roof and walls, it may require use of mechanical dehumidification in order to prevent mold and mildew from excessive interior humidity.
