Quote:
Originally Posted by NHAero
Building engineer here...FWIW...
41" to peak seems marginal storage for the cost of putting the thermal boundary at the roof rather than the ceiling joists. If you have stuff to store up there that gets used once/year, think about an end wall access in the gable, above the thermal boundary.
You want to make this enclosure airtight (and then consider a small energy recovery ventilator). The occupancy you describe won't generate much moisture and a vapor retarder is less critical than an air barrier, especially at the ceiling level. If you have the floor to ceiling height available, and because you have minimal insulation depth available under the existing storage "floor" in the attic, unless you remove them, I might put up a 1" layer of foil-faced polyisocyanurate insulation on the underside of the ceiling, tape the joints as an air barrier (foil takes care of the vapor issue too), strap over the foam with a true 1" thick strapping to run wiring in without penetrating the foam. Avoid recessed lights - use surface mount LED lights, some don't even require a shallow electrical box.
I'd rather blow-in cellulose than batts, fills all the nooks and crannies. If using batts, mineral wool batts are stiffer and fill cavities better than fiberglass - Roxul is one brand. Same for the walls. Cut the batts carefully around wiring, and use an outlet cover gasket to limit air leakage.
1" of foam on the floor will make a big difference in comfort. You can get Huber Zip-R with OSB laminated to 1" foam (or more) and maybe glue it down. You'll need an additional surface on top of that for welding though! Maybe 1/2" cementboard, such as Durock.
If you use a vapor retarder, in your climate I would choose Certainteed MemBrain, which is a variable permeability product. Tape it if it's the air barrier, otherwise not needed.
Finally, a single zone minisplit heat pump will provide heating and cooling, and if you do a decent job on the thermal enclosure you can likely get away with a 9,000 BTU/hour unit, or even a 6,000 BTU/hour. The brands I use are Mitsubishi and Fujitsu.
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If we could vacation without the thule box I'd get rid of it, but don't have another space to store it off the car. So while the amount of space isn't much, its what I have to work with. If I cut into the gable wall, I will probably need to replace the siding on the whole garage... which also matches the house. That is not a project we want to tackle right now. It is older, faded and somewhat brittle, and I don't think it would survive removing some sections to cut an exterior access door on the gable. There are also diagonal braces that go from the gable wall sill plate to the ridge beam that would block an access door. I was considering sistering a couple of the ceiling joists, cutting one joist in between and making a drop down access door to gain access, but need to do a little more thinking on that regard.
I dont know that I'll be able to make it completely airtight on the first phase as the windows aren't in the plan and aren't exactly airtight. I will be replacing the metal garage door with an insulated door, but windows will be phase two. The hope is just to make it comfortable enough to work in efficiently for now and leave some upgrades, like flooring and a mini-split system for later. There is only 95" from floor to bottom of the ceiling joists so can't do too much layering of floor and ceiling without bumping my head, I'd also need to reframe the side door to clear any raised flooring.
I am not planning on recessed lighting. Its a workshop but was going to look into some more basic LED light options.
Thank you for the options to consider. I appreciate it. Maybe I need to just knock it down and make it bigger... that seems the most economically sound(less) choice, but would let me net what I need from the space more easily. All it takes is money. lol.