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Community and Q&A

Insulating a Houston garage

Jay_Yelvington | Posted in General Questions on

As you well know it is unbearably hot here in Houston a minimum of 7 months of the year.

About two years ago we moved into a prebuilt but new home with a 3 car attached garage.

The garage/house faces west and both the two bay garage door and single bay door are “factory insulated”. The construction of the walls is a rock/brick veneer /air gap / 1/2″ Dow Styrofoam Residential sheathing ( taped and flashed to form moisture barrier) stud cavity/ 5/8 gypsum painted with a latex paint which will be repainted sometime soon. The exterior walls as well as the area above the garage is un-insulated. The garage stays about 10 degrees cooler in the summer with the doors closed than the outside air temp.

The garage is about 650 sq. ft. with 10 ft ceilings
I would love to add a mini-split to this garage (I am thinking 18k BTU) in order to make this area bearable for my two sons to work in the evenings and on weekends.

Obviously insulation above the garage is easy to do and should be done. All I need to do in prep for that is add the baffles so the sofit vents aren’t covered.

It is the walls I am concerned about. I would love to blow or have cellulose blown into the walls. Since it is a garage I am fine with it being done from the inside. I am however concerned that the foam is the only sheathing and it’s ability to hold the pressure of dense packing cellulose in retrofit application.

Is the wall insulation something that can be done. Is it a necessity for this use application.

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  1. GBA Editor
    Martin Holladay | | #1

    User 7092850,
    First of all, can you tell us your name? (I'm Martin.)

    1. If you plan to air condition your garage, insulation is a good idea.

    2. I agree that your wimpy sheathing makes dense-packed cellulose a poor choice for your wall insulation. Either mineral wool batts or fiberglass batts, carefully installed, make sense -- assuming, of course, that your stud bays are exposed from the interior.

    If your garage has already been drywalled, you don't really want to pull down the drywall just to insulate. But you may have to, unless you want to accept the risk of the 1/2-inch foam sheathing bellying out and blocking the air gap behind your brick veneer and stone veneer.

  2. jackofalltrades777 | | #2

    My attached garage is insulated and I am in a very hot desert climate. When it's 98F outside, it is about 78F inside my garage. No windows and a steel insulated door. It's ICF walls with a R-40 roof.

    What helps also is my heat pump water heater blows cold air when running. That does help in the summer. In the winter it can cool the garage down to the mid 40's in the coldest of winter but it never gets into the 30s

  3. Jay_Yelvington | | #3

    In my particular case the walls already had the drywall installed. In some cases the walls already have cabinets on them. Most of the cabinets can be removed making drywall removal only slightly less invasive. Discarding the obvious drawback of energy consumption of AC in an insulated space; What about order of install of AC vs insulation? I am a fairly good at drywall but I would work much faster if the garage had AC. Can I insulate the ceiling, install AC, and then tear out drywall in sections and insulate? Is there any drawbacks to this order?

  4. GBA Editor
    Martin Holladay | | #4

    User 7092850,
    It would still be nice if you would tell us your name.

    You can install the AC first, before insulating the walls, if you want -- as long as whatever registers or indoor ductless units you intend to install are mounted in a way that doesn't interfere with your insulation and drywall work.

  5. Jay_Yelvington | | #5

    Martin, I am sorry to have missed that particular question! My name is Jay. I am actually planning on using a ceiling cassette style minis split so doing the walls later would not be that big of deal.

    Since it looks like I am looking at having to place batting in the walls what is my best choice , or is there one, on bats? I would love to get as much sound proofing as possible along with the insulation characteristic.

  6. GBA Editor
    Martin Holladay | | #6

    No need to apologize; glad to know your name.

    Either mineral wool batts or fiberglass batts can work. In general, mineral wool batts are usually denser than fiberglass batts, and mineral wool batts are generlly a little better at sound control than fiberglass batts. Whether these differences are important enough to justify the higher cost -- especially for a garage -- is hard to say.

    If you choose fiberglass batts, chose the densest available batts (in other words, batts with the highest available R-value for whatever thickness you need).

  7. Jay_Yelvington | | #7

    Thanks Martin for your help. I am assuming the batts should not be faced due to the wall construction.

  8. GBA Editor
    Martin Holladay | | #8

    Unfaced batts are fine. But if you accidentally install kraft-faced batts, it won't make much of a difference. No real harm, either way.

  9. Jay_Yelvington | | #9

    Your statement about unfaced or kraft faced batts not making much of a difference has me curious and confused. I thought with the Styrofoam sheathing (taped and sealed) providing a moisture barrier that any additional vapor retarder or barrier would cause the potential to trap moisture in the wall? Would you put the facing to the inside or outside of the wall? I still plan to use unfaced batts....just trying to wrap my head around some concepts that are unclear to me.

  10. GBA Editor
    Martin Holladay | | #10

    Your sheathing is 1/2-inch XPS. That has a vapor permeance of about 2 perms. In other words, it's somewhat permeable.

    Kraft facing is a "smart" retarder with variable vapor permeance. It has a vapor permeance of about 0.4 perm when dry, but the vapor permeance rises to about 4.2 perms when damp.

    In short, water vapor is unlikely to get "trapped" between these two layers, since neither layer is a vapor barrier.

  11. Jon_R | | #11

    The economics of wall insulation will depend greatly on how often the space needs to be cooled.

  12. Jay_Yelvington | | #12

    Thanks for that clarification, it helps!

    Trust me, I know. I would need it cool a couple evenings a week and perhaps half a day Saturdays and Sundays. There may be the occasional periods where more time is required but they would be limited. I have/ am seriously considering not doing (or at least postponing) the walls and only blowing in insulation in the ceiling along with getting better seals around the garage doors which are already insulated. I guess as long as I don't create a situation that harms the building it really comes down to the cost of the energy vs the cost of wall insulation and what the payback is.

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