Looking for the best insulation option for a new detached garage. I’ll heat it to 45 most of the time and warmer when I’m working. I’ll have steel on the ceiling. I’m thinking of sealing the perimeter j-channel and all steel sheets at the overlap, then insulating with blown cellulose. Most of the spray foam guys I’ve talked to about doing the walls want to spray foam on the roof sheathing and just say “trust me you’ll like it better”. The warmest part of the garage would be the 4700cu. ft. attic that I won’t spend any time in or store anything in. The ceiling is at 12′ so that’s gotta be alot warmer near the 20′ peak than where I’m working. My shingles are black. Would that shorten their life or void the warranty? Also I already have soffet and ridge vent. Is there any good reason to spray foam on the roof sheathing?
Thanks for your help
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If it's just the (unused) attic you're talking about, and you have the height, it would be silly to not use cellulose. Yes, foam has about twice the R value of cellulose, but generally you'll find it significantly cheaper to blow 2x the cellulose vs the spray foam.
Just be sure to install "chutes" to maintain the soffit venting with the cellulose in place.
I used to suggest nothing but cellulos and lots of it....but that's only if your labor is free or you have access to a blower much larger than those at HD or Lowes. I recently blew in 36" of fiberglass and it went 4x as fast as the last cellulose blower - instead of an entire day in the attic I was in and out in a little over 2=1/2 hours. I didn't have to move around as much and it blew in and around weird shapes better.
Having said that I'd still use cellulos if it were my own home or if sound deadening is a high prioritiy (it works fantastic in floors to deaden sound).
Keep in mind that more than 18" or so of cellulos needs additional support or it will bow your sheetrock over time because it weighs just enough to be a problem if you don't do something about it. I simply save narrow plywood/osb rips and add them every 2' across the top of the ceiling joists. Insulation can work its way under the rips but most of the weight is supported above. It's also a great way to keep scraps out of the landfill. Of course any scrap wood can be used.
Foam is great for cathedral ceilings and unvented attics or areas that have limited depth, but bang for the buck nothing beats cellulose.
Where are you located? I know Newpro specializes in attic barriers and installation. If you are in New England definitely give them a call. Used them for replacement windows years ago and was very pleased with the service.