Insulation, flat roofs & mini splits in the midwest

[justinmc]

So working on a heating/ac option for my 21W x 30D garage in the midwest (Kansas City). A bit of a different animal as I have a "flat" (low slope) roof and the garage is an enclosed former carport.

Insulation...
Floor is a bare slab with no insulation under it or at the perimeter.

Ceiling/Roof... was replaced as part of the purchase and I believe has R19 foam (above deck) creating a "hot/warm" roof. However the previous owner had installed R13 bat fiberglass which I think might cause some moisture issues if I condition this space. Also being a enclosed carport with a low slope roof the ceilings start at 8' and taper to about the 7' mark on the outside wall.

Walls... appear to be R13-19? Bat with a plastic barrier on the top (inside) and then pegboard over top instead of drywall. The exterior walls on this portion of the house are a form of vinyl? siding.

Doors... 2 roll up garage doors 8W x 7H which are newer insulated pieces from Overhead Door Co.. but honestly aren't super insulated.

One wall is the exterior to the house (so conditioned on the inside obviously) the other 3 are open to the elements. I get a lot of shade on the structure and most sunlight hits the roof during peak daylight hours.

I'm thinking mini split because we get quite a bit of humidity and during the spring when temps get higher and the slab is still cold I already get quite a bit of moisture/condensation on items in the garage (tools, benches, etc) until the temps even out. So I'd like to have a bit more regulated temperature year round. Our temps average mid 20's for the lows (with some peak colder days of course) and mid 90's for the highs. I'm thinking I will have to remove the bat insulation in the ceiling as a hot/warm roof with loose bat insulation under the decking can cause moisture problems in the roof decking and I don't want that. So my quick calculations with some of the online calculators say that a 18k BTU unit should be sufficient. Looking at Mr. Cool because of the price, various good feedback for DIY install and what appears to be decent warranty and luck from most people who have used them.

Any input/additions? I'm thinking I need to at least insulate the end of the eaves with foam board and obviously seal up any gaps/etc. The front wall needs some insulation and better weather strip around the doors for sure. I have a sub panel in the garage already and I think there is a empty 20amp breaker I can use.

Photos of interior/exterior shortly after move in.....unfortunately not that clean anymore!

 

[strutaeng]

So working on a heating/ac option for my 21W x 30D garage in the midwest (Kansas City). A bit of a different animal as I have a "flat" (low slope) roof and the garage is an enclosed former carport.

Insulation...
Floor is a bare slab with no insulation under it or at the perimeter.

Ceiling/Roof... was replaced as part of the purchase and I believe has R19 foam (above deck) creating a "hot/warm" roof. However the previous owner had installed R13 bat fiberglass which I think might cause some moisture issues if I condition this space. Also being a enclosed carport with a low slope roof the ceilings start at 8' and taper to about the 7' mark on the outside wall.

Walls... appear to be R13-19? Bat with a plastic barrier on the top (inside) and then pegboard over top instead of drywall. The exterior walls on this portion of the house are a form of vinyl? siding.

Doors... 2 roll up garage doors 8W x 7H which are newer insulated pieces from Overhead Door Co.. but honestly aren't super insulated.

One wall is the exterior to the house (so conditioned on the inside obviously) the other 3 are open to the elements. I get a lot of shade on the structure and most sunlight hits the roof during peak daylight hours.

I'm thinking mini split because we get quite a bit of humidity and during the spring when temps get higher and the slab is still cold I already get quite a bit of moisture/condensation on items in the garage (tools, benches, etc) until the temps even out. So I'd like to have a bit more regulated temperature year round. Our temps average mid 20's for the lows (with some peak colder days of course) and mid 90's for the highs. I'm thinking I will have to remove the bat insulation in the ceiling as a hot/warm roof with loose bat insulation under the decking can cause moisture problems in the roof decking and I don't want that. So my quick calculations with some of the online calculators say that a 18k BTU unit should be sufficient. Looking at Mr. Cool because of the price, various good feedback for DIY install and what appears to be decent warranty and luck from most people who have used them.

Any input/additions? I'm thinking I need to at least insulate the end of the eaves with foam board and obviously seal up any gaps/etc. The front wall needs some insulation and better weather strip around the doors for sure. I have a sub panel in the garage already and I think there is a empty 20amp breaker I can use.

Photos of interior/exterior shortly after move in.....unfortunately not that clean anymore!

I don't think the R13 will hurt you from a moisture perspective. It will help from a thermal perspective. The rigid foam seals from air leakage, which is what causes the condensation and moisture problems. I would do the gap sealing.

You have an unvented assembly. Any big box commercial store that's conditioned has a similar insulation above the roof deck and no venting.

Has the space been conditioned before? With that much insulation, I would think so.

I can't tell from the picture; is your roof modified bitumen?

 

[justinmc]

I don't think the R13 will hurt you from a moisture perspective. It will help from a thermal perspective. The rigid foam seals from air leakage, which is what causes the condensation and moisture problems. I would do the gap sealing.

You have an unvented assembly. Any big box commercial store that's conditioned has a similar insulation above the roof deck and no venting.

Has the space been conditioned before? With that much insulation, I would think so.

I can't tell from the picture; is your roof modified bitumen?

It's a TPO setup. Everything I've read about flat warm roofs and insulation seems to indicate not having a fiberglass bat style insulation below the deck as it can cause moisture to form in the bats (when the roof is warm and the inside air is cold or vice versa) and cause mold/rot/etc.

The TPO is the top outer layer... foam under that and a moisture barrier below that against the decking itself.

 

[strutaeng]

I'm certainly not an expert on the subject, but positive it won't cause condensation problems for the amount of rigid you have. I think if you had something like 2" or less of rigid it might do that.

Please go check out greenbuildinadvisor.com. They have some really good articles about this setup.