Insulate a Post Frame Building, Ventilation, and Column Sizes

This week the Pole Barn Guru answers reader questions about the most economical way to insulate a pole barn style building, advice on ventilation, and the best column size for a building in Michigan.

DEAR POLE BARN GURU: Hello, I am looking at either building a pole barn style or hybrid Steel column, post and joist with 2×6 purlins and girts. If I were to place 1/2″ sheathing on the walls and roof, install 30 felt paper then my metal on the walls and roof, what would be the most economical way to insulate it. It will be my hobby shop not a living space. ROBERT in OCEAN PARK

DEAR ROBERT: All of these recommendations are based upon meeting energy code requirements for conditioned buildings. Washington State will be a stickler for these.

1/2″ sheathing and felt is probably not your best design solution (besides being expensive).

Roof – order roof steel with a factory applied Integral Condensation Control

Order trusses with raised heels (deep enough to not compress blown in attic fiberglass insulation)

Order trusses designed to support weight of a ceiling and install one. Blow in R-60 fiberglass above ceiling. Vent eave and ridge in correct proportions

Walls: From out-to-in:

Steel siding; Weather Resistant Barrier (Housewrap); 2×8 bookshelf wall girts; R-30 unfaced Rockwool batts; 6mil clear plastic vapor barrier; Interior finish

Slab: If using radiant floor heat, R-10 under slab and at slab edge. Otherwise R-10 rigid boards down two feet, then out two feet.


DEAR POLE BARN GURU: I have an 18×20 metal building (my house) that’s covered with spray foam insulation at 1.5″ on the walls and 2″ on the inside roof. The entire space will be conditioned. I am now reading that I may need to ventilate the space. I thought the purpose of the spray foam was to make it air-tight as a closed envelope system. So, can you explain what I need to know about venting? Also I have a large attic fan I am wanting to install in the wall. It has louvers on it to hopefully seal the air and water out. I plan to use it to blow hot air out when I come home instead of cooling while at work. Then turn on the AC. Also plan to use on days that’s it’s not terribly hot as a method of cooling the whole house like my grandparents did. I live in East Texas so I won’t be able to do this too often. Is this ok or should I scrap this idea? Can I expect the louvers to seal good enough? With this would I need a mechanical ventilation system? Thank you for your awesome help. I’ve been reading your advice for years. CINDY in TYLER

DEAR CINDY: Thank you for your kind words and for being a loyal reader. Your solve is for your HVAC vendor to design and install a system to remove excess humidity from your interior air, otherwise you can expect to have some nasty condensation issues in your home.


DEAR POLE BARN GURU: What size posts should be used for 30′ x 40′ x 10 with 6/12 pitch in southern lower Michigan??? Storage/ garage / shop. STEVE in MARSHALL

DEAR STEVE: Looking at your most recent quote for a 30′ x 48′ truss bearing columns would be 6×6 solid sawn. We will be converting all of our roof supporting columns to glulams in a few months, then it would become 3ply 2×6 glulams.





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