Pole Barn Loft details
We are putting a loft apartment in a pole barn being built currently in northern MI. Climate zone 6A. Will use as a small cabin for a few years then as guest house after I win the lottery and build a house there. A contractor did the barn shell, I will do much of the inside.
I am concerned about moisture issues in the walls and ceiling. I have read input from GBA and FHB but still am unclear if my planned wall assembly is a reasonable approach or if I will have moisture issues.
The building is built with 6×6 posts 8’OC with 2×4 horizontal purlins, house wrap under steel siding, 12″ soffit vents all around building and ridge vented roof. Scissor trusses, steel roof with some kind of anti-condensation material on underside of the steel. NO sheathing under steel roof or siding. Obviously not a great setup for an insulated, cold-climate space but that’s what it is.
Loft will be 36×16, a 3/4 bath and a small kitchen, 1 walled off bedroom. Entry to loft space from inside the unheated, uninsulated (for now) storage barn. Plan to heat with heat pump mini split and possibly wood or pellet fireplace for backup/ambience.
I have read input on this topic on GBA and FHB but still am unclear if my planned wall assembly is a reasonable approach or if I will have moisture issues.
Wall air seal, insulation options being considered:
1. 3″ or 4″ rigid foam between posts against purlins (leaves a gap of purlin depth between foam board and siding), taped and caulked. Not sure if 1.5″ foam board (purlin depth) should also go between purlins directly against the steel, taped or caulked around the edges of each one to fill the gap. 2×4 or 2×6 framed walls filled with something fluffy. Painted 1/2″ drywall, carefully taped.
2. Newer, slightly less offensive closed-cell spray foam directly onto the steel siding and over the purlins. Then frame walls with fluffy inside that. Is this a better option vs polyiso boards? Is that better for preventing moisture issues?
3. A totally different option I am not considering, other than pulling roof and siding to add sheathing?
Vaulted ceiling (interior pitch 4/12, 12″ heel truss, no sheathing under steel roof): Figured some combination of rigid foam and fluffy, but not sure where to install the foam and how to create a sufficient air barrier. Again, concerned about moisture issues from the steel roof even though the builder said that’s why they add the material on underside of steel roof.
Floor: The loft floor will be about about 6 feet above the concrete barn floor. 8’x16′ area of slab has 2″of foam under and around perimeter. That will be the insulated and heated mechanical room under the loft. I figured the floor assembly would be 10″ joists filled with fluffy and rigid foam (how much?) under the joists on the outside, 3/4″ TG OSB on interior. Maybe something between subfloor and joists if recommended? Everything carefully taped and caulked wherever possible. Interior flooring either wood or vinyl planks.
Sorry for the long post and thanks for any input.
Jim
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Replies
Every time I see the words “pole barn,” I think of my colleague Patrick McCombe—senior editor at Fine Homebuilding magazine and all-around awesome guy. I’m giving your question a bump, while introducing you to Patrick’s pole barn, a lesson-rich project that I think you will appreciate. Patrick might be a good resource for you, and he is very responsive to questions—especially about pole barns.
Thank you, Kiley. I look forward to meeting Patrick and I will check out his project today.
Jim, a few thoughts--
Walls: in cold climates, the safest wall assemblies can dry to the exterior. Steel siding doesn't dry directly but its built-in ribs allow vertical drying, if left open. I would not use spray foam against the siding because it will block this path to drying.
Somewhere in the wall assembly you need an air control layer. Painted drywall can work but you have to take extra care at electrical outlets, windows and other penetrations. Rigid foam against the purlins could work but it's hard to get and keep a perfect seal, plus foam has a larger carbon footprint than other insulation options. With similar effort I would consider building a double stud wall, with an air and vapor control membrane between the two layers, and blown or batt fluffy insulation in the framing cavities. Fluffy insulation performs best in fully airtight cavities, so you could add a membrane such as Tyvek on the outer side of the framing.
Roof: I would want to know exactly what the condensation-resistant material is and how it's supposed to work, but even then I wouldn't trust it. A vented metal roof will likely condense water on cool, clear nights after warm, humid days. Your best bet, unfortunately, is probably to spray closed cell foam against the bottom of the roofing. You could do a flash-and-fill approach. Or do cut-and-cobble with rigid foam, but that is time consuming and less reliable. If you do closed-cell spray foam, HFO-blown has a significantly lower climate impact than conventional HFC-blown. If you do flash-and-fill, in zone 6 you need at least 50% of the R-value to be in the closed cell foam layer. Ignore the R-values they tell you; closed cell foam ages to around R-5.5 to 6 per inch. The "fill" layer can get tricky to install around trusses; you'll probably find that open-cell foam is the simplest way to get the rest of the required R-value. Just don't insulate the entire roof with open cell alone, or you'll get moisture accumulation.
Floor: elevated floors are pretty forgiving, especially in enclosed spaces like this. The IRC calls for R-30 in zone 6 floors, which you can get in a 2x10 space without adding foam. If you want to use foam you can use any thickness at the bottom. XPS is by far the most climate-damaging of the rigid foams, unless you can get recycled material. Instead of foam, if I wanted more than R-30 I would add additional framing below the floor, with more fluffy insulation.
Thanks, Mike, for taking the time to reply. two quick follow-up questions:
1. There will still be here thermal bridge in the floor between the joists in the subfloor to the outside. I was thinking maybe the bottom of the floor deck could be zip sheathing with the outside joists covered in foam. Any rigid foam I use will be reclaimed.
For the roof I was considering a cold roof and just insulating the ceiling side of the roof area. I realize that the air sealing in the ceiling details needs to be well done to keep moist air from reaching the exposed cold metal roof. Thinking much of the moisture that might pass through the ceiling insulation and air seal would also out the roof vent. Even with this approach do you think it is wise to spray foam the metal roof?
Thanks again.
Jim
Jim,
I'm not sure I'm following about your floor. You could install the wall framing first and hang the joists from the wall, which would allow continuous insulation and little thermal bridging. A sketch of what you're thinking would be helpful.
For the roof, the main concern is not interior air reaching the metal, but warm, moist outdoor air condensing on the bottom of the roofing at night when the roof is exposed to the sky, due to a phenomenon called night sky radiation, which cools objects below the air temperature. Here's a Q+A about it, and some linked articles: https://www.greenbuildingadvisor.com/question/how-to-deal-with-metal-roof-condensation. The roofing basically becomes a still. If it's installed directly over sheathing it's not an issue, but if it's not over sheathing you will likely get drips occasionally. There is a chance that the drips would be rare enough that exposed cellulose insulation could absorb and release the moisture safely, but it's not an approach I can recommend. With the roof already on I can't think of a reliably safe way to insulate this other than with closed cell foam. Maybe someone else will have an idea.
Mike,
Understood on roof. Thank you.
For the floor, I was envisioning building the floor deck then building the walls above and below. Are you suggesting I build the the double walls from slab to truss and build the floor decking for the loft inside of that?
Jim
Jim, yes, that's just an idea to limit thermal bridging and also to make things like air sealing easier. But platform framing, which you are describing, is fine too. If you balloon frame as I suggest you need fireblocking every 10' vertically and where vertical cavities meet horizontal cavities.
Mike, here is a drawing of what I am thinking.
I could build wall assemblies to fit between the posts. Outside of each wall would be 2 inches of reclaimed rigid foam, 2 x 6 filled with batts or dense packed cellulose, interior 1/2” drywall. If I wrap the posts in 2 inch foam board and put the wall assemblies flush with the girts the interior of the wall will be flush with the wrapped posts and ready for drywall.
And alternative I suppose could be OSB on the outside of the wall assembly, an inch or two of closed cell spray foam and then some fluffy stuff.
Third idea would be double 2x4 stud walls inside the existing structure but that would take up a lot of floorspace and seems like overkill.
Any of these sing to you?
Thank you.
Jim