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Garage wall insulation

26x40 attached garage with radiant floor heat in the slab. Standard exterior construction, i.e. 2x6 walls, sheathing and Tyvek with vinyl siding.

I am trying to plan the most effective way to keep my radiant heat in the garage so it can stay warm and help warm the bonus room above. I have several ways of doing this and would like an opinion on them:

1. Spray 2-3 inches of open cell foam into the stud cavities, then drywall.

2. Spray 2-3 inches of CLOSED CELL foam in the studs and drywall.

3. Install Roxul batts, then vapor barrier (or smart barrier whichever is better) and drywall.

Cost is a big consideration here so I do not want to break the bank.

Asked by Anthony Hughes
Posted Sep 4, 2014 7:55 AM ET
Edited Sep 4, 2014 9:05 AM ET


9 Answers

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You didn't mention your climate zone. In general, it makes sense to install better insulation in a cold climate than in a warm climate.

You are asking an elementary question about wall insulation. If you want to educate yourself on available options, you might want to read the articles on the topic in the GBA Encyclopedia:

Insulating Roofs, Walls, and Floors

Insulation Choices

Rigid Foam Insulation

Spray Foam Insulation: Open and Closed Cell

Batt and Blanket Insulation

Before making a decision, you have to answer the following questions:

What R-value goal am I aiming for?

Do I care about thermal bridging through the studs?

What is my budget?

Do I care about environmental concerns?

Who will be doing the work -- the homeowner or a contractor?

What types of insulation are locally available?

Answered by Martin Holladay
Posted Sep 4, 2014 9:00 AM ET


Martin I have already posted a bunch about the energy efficient home I am building, the garage is an area that I am not super concerned about simply because I have radiant heat and just want to keep the heat IN, not going for crazy efficient garage here. So i'm very familiar with the articles you suggest but was wondering what folks are doing in garages. Thanks.

Answered by Anthony Hughes
Posted Sep 4, 2014 10:04 AM ET


If your garage is heated, then the insulation methods used for the garage wall are exactly the same methods used for any other insulated wall.

Answered by Martin Holladay
Posted Sep 4, 2014 10:17 AM ET


Right, thanks.

Answered by Anthony Hughes
Posted Sep 4, 2014 10:32 AM ET


Sounds more like a double edged sword. The garage must be isolated from the bonus room above if it's designed to be living space. How do you plan to equalize that floor heat above the garage unless your plan is not to isolate the bonus room from the garage?

Considering it is a garage, in my opinion it would make more sense to air seal from the inside, install Roxul mineral wool and install two taped layers of Type-X 5/8" fire rated drywall from a fire and IAQ perspective.

Answered by Richard Beyer
Posted Sep 4, 2014 10:35 AM ET


I am using 3 inches of open cell foam in the bonus room floor to isolate it but not completely block everythign from a radiant heat perspective
I plan to spray either open cell at 2-3 inches deep or closed cell at 1-2 inches deep in the walls of the garage for the airseal, I am undecided if I will install a Batt in there or not after the spray foam.

Answered by Anthony Hughes
Posted Sep 4, 2014 10:59 AM ET


What's your climate zone? Where are you located?

Your plan to install between R-6.5 and R-13 in your wall is insufficient in most climate zones. If you install a higher R-value, the energy savings will be enough to quickly repay the cost of the thicker insulation.

Answered by Martin Holladay
Posted Sep 4, 2014 11:17 AM ET
Edited Sep 4, 2014 11:20 AM ET.


R23 Roxul and a smart vapor retarder is probably cheaper than ~R11 (3" of open cell foam) or R12 (2" of closed cell foam), and will be higher performance too.

Since it's a garage (air-leaky garage door for ventilation, and not many humidity sources, which means the garage air humidity is normally low in winter) and it has vinyl siding (inherently back-ventilated) you can safely skip the vapor retarder in ZIP code 47460, since it meets the "Vented cladding over wood structural panels" exception found in IRC 2012 chapter 7:


Answered by Dana Dorsett
Posted Sep 4, 2014 11:38 AM ET


So just a plain R-23 ruxol batt installed in there will be just fine, even without any airsealing of the stud cavities? I don't want to underdo it but I certainly don't want to overdo it either. I like the sounds of that. Roxul is a great product.

Answered by Anthony Hughes
Posted Sep 4, 2014 11:46 AM ET

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