Pole barn post frame insulation
Hello, I am going to be building a pole barn garage/house in Montgomery, Alabama. I have a few questions. The building will be red with a white roof (trying to keep roof cool), vented soffits, full length ridge vent, double pane e glass windows, insulated doors. 36x48x12 building, posts 8′ on center, conventional framing between posts 24″ on center adding 6″ R19 mineral wool, no interior air or vapor barrier. 10-12″ insulation in the ceiling. It will be half house half garage.
1: can I use 5/8″ drywall throughout the house and 3/4″ plywood plus steel ceiling for garage – both will have hvac and be insulated.
2: should I use separate hvac systems for each because of fumes in garage
3: I will build 2×6 wall to separate the two sides, should I sheath the truss above the dividing wall for fire break or hvac reasons.
4: Can my insulation in central Alabama consist of exterior steel-housewrap (air vapor retarder)-mineral wool- interior wall?
5: Should I let them use double bubble radiant air and water (barrier) instead of housewrap (retarder)? Nevermind the R value.
6: The walls will be 7.5 inches thick excluding the steel and interior wall material. True 6×6 posts plus exterior 2x4s.
7: Because of the horizontal 2x4s on the exterior of most pole barns, does it matter if the insulation leaves air gaps around that area as long as the area between the 6×6 posts is full? Otherwise you would need to fill that 1.5″ gap that the 2x4s create plus the 6×6 depth of the posts. That’s a ton of insulation in the south.
I see where some have recommended housewrap before exterior girt installation, is this possible? It doesn’t seem like the posts would be firm enough unless maybe installing the top header plates first. Seems difficult, but I see the sealing potential rather than wrapping outside the girts.
This is not a northern house, I use A/C almost year round. Any suggestions will be greatly appreciated. No I simply can’t afford closed cell spray foam – several quotes in the 11-14k range.
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Replies
You definitely want separate hvac. Even having an attached garage at all is considered sketchy in regards to air quality. Pay special attention to the air barrier between the garage and living space.
Garage,
There are several aspects to your project that need better consideration. First, I'd like to point out that airtightness is essential -- between your garage and the outdoors, between your house and the outdoors, and between your house and your garage. Considering the stakes, I would want all of the necessary air barriers to be tested with a blower door.
Second, you wrote that there will be "no interior air barrier." Why? In most homes, builders use drywall as the interior air barrier. Will you have interior drywall? If so, I hope you strive to make it as airtight as possible.
You certainly don't want a single forced-air heating and cooling system that mixes return air from the garage with return air from your house, and then sends the conditioned air to both sides. So you'll need separate heating and cooling systems for your house and your garage.
I think of a pole building a as the lowest cost structure that will keep your tractor dry and slow down the wind.
This design is driven by cost alone, every choice is the polar opposite of the one that should be made when building a high performance building.
If you want a high performance building IE one that you can heat and cool while spending the least amount of money on fuel and construction over time. Then you will need to spend the money to build one.
You can spend lots of money putting lip stick on a pig of a pole building. The question is will it be what you want in the end and can you afforded what you want.
Walta
Thanks guys for your responses. I am located in Southern Alabama so I was going to use for the full exterior wall system metal siding / housewrap / Rock wool. I want to use drywall in the house walls and roof which will be airtight. In the garage I would rather use plywood walls and steel roof. I was planning on using a four unit mini split a/c. Two in the garage, two in the house. My wife and I are both disabled but we plan on doing some of this work because of our extremely low budget. It won't be the most efficient, I just don't want it to mold. Can I use an air barrier above the steel in the garage? If I use drywall in the garage ceiling too I will have to buy several more trusses. The dividing wall from garage side to house side 3/4" plywood / rock wool / drywall.
I'd just like to point out that the garage will have (2) 10x10 doors that will be opened with the a/c or heat off periodically. So, yes I want the house side tight and efficient but this garage will have welding, woodworking, automotive restoring, everything guy related. A true workshop not a place to park cars.
Garage,
You didn't mention whether or not your walls and roof assembly will include sheathing (OSB or plywood). Without OSB or plywood sheathing, it's really hard to make this type of building airtight -- and with an attached garage, airtight methods are essential.
For more information on this issue, see "Insulating a Pole Barn."