The building in question is a barn- far from the usual energy efficient dwellings we usually discuss. We are going to convert 1/3 of our barn into an occasional use guest house of sorts. Construction is T1-11 over 2X6 studs (no vapor barrier) on a concrete slab, It's one big room- 16 X 28 X 21' tall. I'll put in a couple double pane windows, but not many. The interior wall finish will be tongue and groove wood. I'll put R-19 batts in the walls, and blow in cellulose over the drywall ceiling.

The key is that we'll only use this room occasionally- maybe 10 days out of a year. Large get-togethers, sit by the wood burning stove on Christmas Eve, guests staying there for a few days. Obviously this room will never be very efficient. 'Climate control' will be a wood stove, and maybe a mini-split for the hot weather, or so overnight guests don't have to keep the woodstove going all night. We are in upstate SC- climate zone 3, bordering on 4.

My question is about the best strategy for insulation. Since I'm going to hang the paneling vertically, I'll have to put up horizontal battens over the studs. Is there enough to be gained by isolating the battens over foam for a thermal break? Do I want a vapor barrier in addition to the paper faced batts? The main goal is to minimize the size of the mini-split for the given parameters, and avoid creating an unhealthy wall structure that will grow mold or rot. I'm not sure about moisture control in such a light use building. Any thoughts?

Have you considered damp sprayed cellulose in the walls? The cost to have that installed in my area isn't much more than the cost of fiberglass batts alone, plus you don]t have to deal with that itchy fiberglass and you get a much better air seal. And with horizontal battens, you could fill the cavity completely out to the face of the battens. With cellulose, you wouldn't need a vapor barrier. With fiberglass batts, the kraft facing is sufficient. Also look into stabilized cellulose for the attic blow as well.

Another option given the horizontal battens would be to construct 2x4 walls with ripped 2x4 battens. That gives you a wall 5" thick which is 0.5" shy of a 2x6 wall. If you were to cut up some small squares of 1/2" thick rigid polyiso insulation and used those to shim the battens out where they attached to the studs using a single 3.5" screw, you would have a standard 5.5" thermally broken cavity so standard doors for 2x6 construction would work without modification. This option would work best with the sprayed cellulose in the walls. Wiring could be attached directly to the face of the 2x4 studs without having to drill through the studs.

Thanks, Arkie. The barn is already built, 2X6 studs. I guess I could get a price on the damp sprayed cellulose. That's what I had done in the attic of our house. The f'glass I used in my basement isn't itchy at all, but it ain't cheap, either. As you said, if the applied it out to the batten surface, I'd gain a little more R-value.

Are you going to climate control the space year round?

No- probably only 10 days or so out of a year. That's why I'm wondering what the best strategy is. Most of the time, it will be subject to ambient temperature and humidity.

Anyone know of they can do damp cellulose on a sloping surface? The barn is a gambrel roof, so there are two sections of 'wall' (roof) that are 30 degrees off of vertical. Can they get cellulose to stick to that, or should I plan on using batts in that area?

Here in Vermont, where we do get Hot and humid days (not as many as where you are I'm sure), dense sprayed cellulose with no vapor barrier is popular. Most people do NOT have air conditioning, however, we do open our windows, so it's not as if you end up with stagnant hot air and no air movement.

I wonder if you had a spot ERV in there and let it run all year if that would help ?