Insulating & HVAC Options -- 24x26

[JGoodish]

I live in Zone 5a and have a detached garage which I'd like to insulate and heat (electric only). The stats:

-> 24x26 exterior dimensions
-> brick veneer exterior, all above grade
-> 2x4 stick framed walls, 2x8 framed ceiling and roof
-> 8.5' floor to ceiling height, attic floor above
-> one insulated man door, two 9x7 insulated (2") garage doors
-> two small windows, double-pane

The interior is presently unfinished, and I am preparing to insulate it before the colder temperatures arrive. I'd appreciate some validation and/or feedback on my plans.

Ceiling insulation options:

1 -- Unfaced R30 batts + 1" XPS or PolyISO + Type X drywall. That would give me ~R30 or so in the ceiling (I'd lose some R-value under the attic floor since the batt would be slightly compressed in that area).

2 -- Blown cellulose + 1" XPS or PolyIso + Type X drywall. Should give me the same ~R30, but might be tricky to ensure adequate distribution of the cellulose under the attic floor.

3 -- Blown cellulose + Type X drywall. By my calculation, probably less that R30 with the same distribution concerns as option 2 above.

I could pull up the attic floor to ensure better distribution of the blown in stuff, but that would likely require unloading part of the attic, which would be pain for seemingly questionable benefit over batts in my situation.

For the walls, I was planning on R13 kraft-faced batts.

I am skeptical of the ROI for spray foam in this application, and for that reason am not presently considering it as an option.

Also, I was planning on the electric heater route for warmth, but am now wondering whether a mini split would be worth the investment given the additional efficiency that it would provide. Not sure of required sizing for this type of a space given my insulation plans. If someone tells me that electric heat via any option is going to be outrageously expensive, digging a trench for gas is an option but one that I had hoped to avoid.

Any thoughts and feedback would be appreciated.


Thanks,
JKG

 

[fastjohnny]

#3 for ceiling
R15 batts or wetblow cellulose for the walls
12k BTU minisplit for heat/cool, prefer mitsubishi or fujitsu.

 

[DC73]

You might get a price on mineral wool insulation (Roxul is one brand). Batts for the walls are R15. Mineral wool blocks airflow much better than fiberglass, is sound resistance and fire resistant. I did my entire shop with R15 batts of mineral wool and then used R19 fiberglass batts on top of the mineral wool to get the additional R-value needed for the ceiling. Lowe's around here carries Roxul but I got much better pricing from some of the smaller lumber companies that service the contractors. I ended up getting mine from 84 Lumber.

Yes, electric heat can be very expensive. The more you plan on heating the space, the more you should consider other options. A mini-split would be a good choice. Depending on your cost of electricity, natural gas should be a more cost effective option but the mini-split would certainly be much easier than digging a trench for the gas line. Again, the more you intend to heat the space, the more likely you should consider natural gas.

 

[JGoodish]

Thanks for the feedback. I've had a couple of HVAC contractors tell me they'd go with natural gas over a mini split, unless I wanted A/C, and even then they seemed to suggest that the mini split wasn't the right solution for the heating side. Both HVAC contractors I've talked to have claimed that the mini split won't provide any cost efficiency over a standard electric resistance heater, though I'm not sure if that's accurate.

Regarding the insulation, my thought is to maximize R-value in the ceiling so as to minimize heat loss. The only way that I can get to R30+ is to use foam board, though that will add a few hundred dollars to the project. Not sure how long it would take me to recoup that investment even with more expensive electric heat.

My plan is to keep the space above freezing in the winter months (maybe 5-6 months a year) and then turn it up when I'm working out there, which may be only a few hours on the weekend. I don't really NEED A/C, though during the 2-3 hot months in the summer it might be "nice." If mini splits aren't really that great for heating, that probably makes the natural gas a better choice for solving most of my needs.


Thanks,
JKG

 

[TractorJeff]

You should say what state you are in. Zone 5A doesn't mean much to some people.

 

[sands35]

Are you going to keep your garage at ~65-70? or 45-50? If your garage temps are held lower, the delta T to the outside will also be lower. So lower heating bills - and no need for residential insulation levels (i.e., R60 in the attic).

If you want to improve your wall insulation, install 2x3s horizontally at 24" on centers from floor to ceiling, then do cellulose. This will give you an extra inch or so of insulation and reduce thermal bridges to the outside. This will make the window and door trim more involved and electrical a bit more interesting - but still very do-able.

It's hard to get better than blown in for the ceiling as far as cost effectiveness.

When I quoted spray foam, it was ~2x the price or so compared to DIY batts. Sure, more effective, but still.

Make sure you caulk / foam up any wall penetrations.

 

[DC73]

Both HVAC contractors I've talked to have claimed that the mini split won't provide any cost efficiency over a standard electric resistance heater, though I'm not sure if that's accurate.

It's not accurate. Standard electric resistance heat will be the highest cost to operate but should be cheaper to install. If the building is super-insulated and air sealed, and you don't need a lot of heat anyway, a mini split may not provide enough savings over the life of the equipment to pay back the additional upfront cost. The higher your electric costs and the colder your climate, the more you should consider the mini split. If the cost is not an issue, I'd put the mini split in anyway. You'll have AC in addition to heat.

If mini splits aren't really that great for heating, that probably makes the natural gas a better choice for solving most of my needs.

Mini splits are effective for heating even in some fairly cold climates. Do some research on the lowest effective temperature for any unit you are considering. But, I do like natural gas for heating if it is available. Having hot air blowing out of a vent is preferable to me over the luke warm air provided by a heat pump like the mini split. Both will warm the space, they just get there in different ways.

DC

 

[Falcon67]

You should say what state you are in. Zone 5A doesn't mean much to some people.

Googling says "think Chicago" LOL.

Heat pumps work down to some X temp then it's resistance heating. Around here, our heat pump works pretty much all the time except for a couple of weeks maybe. I personally use plain old resistance heat in the shop because it works and it's cheap. It'd be a hell of a lot of hours of run time to justify a $1500~2000 expense to not spend $0.22/hr (approximate cost on only the very coldest days) for maybe 8~10 total run time hours a month.