Spray Foam or Batt Insulation?

[jobruba37]

Finishing a loft for my eventual golf simulator room I have above my detached 3-car garage and am trying to decide how to insulate it. I could go the DIY route and put in mineral wool batts but that would technically not bring it up to code as the R value would then only be about 23 along the roof. Note that I do not want to extend the insulation beyond the 2x6 rafters because it is already a tight enough space (11 ft high at the peak). However, if I were to use closed cell spray foam it would bring up the R value to 39 (which is up to code).

The issue then becomes this: the estimated cost for the mineral wool insulation (if I do it myself) is $1.5-2k, but the quote I received for having the area spray foamed is about $8k. While I am able to afford this, it seems like an excessive amount more to use for a slightly higher R-value of insulation. Furthermore, the stairway to the garage will be left open and the bottom part of the garage is not (and will not) be insulated, so is it really worth it to pay an extra 6k to slightly better insulate the area? Has anyone else had a similar circumstance to this? Note that I do live in southeast Michigan, I would likely only be spending a few hours per week in the loft, and do have a wood stove that will be used for heating the area. TIA!

[wildcatfan9698]

seems like it would negatively impact resale value of the property if it's a finished space which isn't to code.

[Stetson]

R-23 ceiling insulation in the winter will be less than ideal. It can be done with great air sealing of the structure but from a comfort standpoint it requires a lot of energy. As for code … a detached garage is not something folks care about code when resale comes up. Atleast not around here.

how are you venting that wood stove? That could be quite an issue

[jobruba37]

Apologies for the delayed response. Yeah R-23 is less than ideal for sure, but if the staircase is going to be open (which my wife will not allow me to change), then I just don't see the value in something like spray foam. Additionally all my electrical work cannot be easily changed if spray foam is used, another reason I'm thinking of sticking with the batts. And you're right, a detached garage that's not a living space doesn't actually have to be up to code. For the wood stove there is already a vertical vent in place, but I will likely remove it and put in two 240V electrical heaters.

[Bobby]

Assuming that roof ridge is not vented you’d need to do closed cell spray foam to air seal it else you’d probably get mold between the roof and the batts eventually. Google ventless or unvented roof system for more info.

[aja]

Absolutely essential to ensure that there is a ventilated air space between the roofing (sheathing) and fiberglass insulation. I know of one instance where the fiberglass was pushed up right to the sheathing (no ventilation space) - in less than 5 years the roof's wood construction was rotted out and had to be replaced.

[jobruba37]

Agreed about the venting and it looks like it would be easy enough to put in baffles to allow for air flow.

[Brettster]

jobruba37 I have a similar situation, see my post about my Gambrel Roof shed.

What I am considering is Polyiso board (1” thick, foil side down applied to roof areas between rafters for waterproofing), then some type of batting over that (either Rock wool, or fiberglass - but probably rock wool as I can’t stand the smell of fiberglass), and finally a thin Luan board over the top screwed or nailed to rafters as ceiling.

This has high R value with multiple layers and is light weight - will not add too much to roof, and still provides a covered ceiling I can paint over instead of bare rafters.

I plan on similar layers over gable ends of end walls and the side wall insulation, though with side walls will use 1/2” drywall for better insulation value and thicker walls.

I know the biggest thing is price right now! I’ve been taking $300+ trips to Home Depot about 2-3 times a week! It’s getting old.

Good luck - take pics with your decisions so we can see how it worked out for you.

Thanks,
-Brett

[jobruba37]

Thank Brett, interesting approach but I will probably be using baffles with the batts. Additionally, because I'm only going to heat the area when I use it, it will experience temperature fluctuations and it was suggested to me that instead of using drywall I should consider wood paneling. Right now that is likely my path forward. Will update w/ pics once it finally comes together, but honestly it might not be until the spring as I am having my first kid in January and that will take up a good amount of my time!

[Brettster]

@joruba37 - I'm going to heat the area also when I use it, but I'm going to keep it around 50 degrees plus when I'm not using it because of the electronic equipment inside... so I suppose it has to do with the area you live in, it can get to minus 25 here in Winter. The wood paneling you mention is what I would call the Luan board I will use nailed to rafters in roof.

[aja]

If you have no plans on insulating the first storey of the garage, there is little benefit in overdoing the insulation in the roof. Your biggest area of heat loss will be through the floor to the first floor of the garage. Having an open stairwell won't help matters either.

Use some of the savings of not installing spray foam to address closing off the open stairs with a trap door or other solution. Another idea is to insulate the floor of your second storey space.

[jobruba37]

Any suggestions for insulating the floor of the second storey? I found some 5 mm underlayment (quietwalk I believe?) I was considering. And the stairs cannot be covered as my wife will not allow it, so that's a no go. I will try and provide more insulation for the first level, including on the garage doors and certain areas to use canned spray foam. Also I planned to get rid of the wood stove and use two 240V electric heaters on the second floor for the few hours I plan to be in the area each week.

[jobruba37]

Status update: I do plan to now use spray foam along the top of the structure. While it will cost more than I would like, the fact is I do plan to live at my current place for the next 25-30 years and it's not going to break the bank.

Two questions I do have for folks:

1) Given that it is 700 sq. ft., what's the best suggested electrical heater (240V) I should use? I intend to remove the wood stove as it takes up too much room and I want to re-wire the garage anyways to eventually allow for my car to be powered via a 240V line. Currently I was thinking of the King Electric 7,500W 240V heater, which says it should be good for up to 750 sq. ft. or buy two of the Heat Storm HS-6000-GC heaters. Any thoughts?

2) What's the best way i can insulate the first floor of the garage? I plan to put in R8 batts on the garage door and cover up the windows with a thick acrylic. Currently thinking about putting in mineral wool batts on the ceiling of the first floor to help insulate it, but the fact is the stairway will be left open. Note that the two car ports are slightly sectioned off from where the stairs are (which is my shed area more or less.

[Brettster]

You're going to have to deal with the open stairway - most likely by finding a way to seal it with a door. The downstairs is unheated and will remain that way, is that correct?

[jobruba37]

That's correct, the first floor will be unheated, but it opens up to my shed portion of the garage and there is no way I could add a door there. I have been doing whatever I can to insulate the first floor, including putting on R-8 batts on the garage doors.

[Bakflip]

I recently finished a 24x36x10' tall shop and I spray foamed a 1" seal on the walls and 3" on the ceiling. I then added r-13 batts to the walls. All in for spray foam and batts was just under $4,500. You could possibly do the same to save a little money while achieving the needed R-value.

[jobruba37]

I contemplated doing something similar, bit just recently received a much more reasonable quote that would make doing all spray foam not much more than going that route.