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Question about building envelope/insulation for garage area

8 years ago

Hello,

For our new construction in IL, we put 2 layers of foam (7" XPS and 2.5" EPS) above our 1:12 roofline so the attic is conditioned space. The exterior of our house will have 2" rigid foam and EIFS. Exterior 2x6 wall cavities will be filled with blown-in cellulose.

There are 3 different parts to our roof. My question is about how to insulate the area under the roof that covers the garage, pool bathroom, and part of the utility room as shown below. The second floor does not cover the area shown boxed in red (disregard notes in green).

The main floor has radiant floor heat, but the garage does not. Prior to placing all of our insulation above the roof, we planned on insulating the walls bordering the garage like other exterior walls. However, with all the insulation above the roof, there is no barrier between the ceiling of the utility room/pool bathroom and the garage (no attic insulation since all the insulation is above the roofline). So heat/air would escape the roof of the pool bath/utility room into the garage area. How should we handle this?

Our garage door is R19. How should we insulate garage walls (which will have 2" rigid foam on exterior)? The ceiling of the garage has been drywalled, but the insulation contractor has access to the attic area above the garage because the ceiling of the utility room and pool bath has not been drywalled. The side elevation plan shows the pitch of the roof above the garage.

There is 2 feet of attic space on the low end and about 6 feet of attic space above the pool bath/utility. I assume he needs to spray the vertical walls of the garage up to the roofline. Should we insulate the exterior walls bordering the garage like other exterior walls? Should we add attic insulation above the pool bath/utility room to prevent conditioned air from entering the garage area? Or should we wall off the attic space between the garage area and the pool/utility and insulate this separation wall up to the roof? Or should we not insulate the wall of the pool bath/utility that borders the garage and treat the garage walls as the building envelope?

Thanks for your help!

Kevin

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Comments (4)

PRO
Springtime Builders
8 years ago
No easy answers with attached garages and the dangers they introduce to otherwise healthy homes. At least you don't have the dreaded bonus room or living spaces above the garage to worry about.

I would encourage further insulating the garage, especially since you are already using exterior foam. Great choice by the way! Fiberglass batts might be fine for the cavity insulation with good air-sealing in place.

Assuming you are doing forced-air, I would try to expose the supply duct to the bathroom to put in an inline damper to better control that seldom used room. Might just be extreme cold weather, cracked open to keep the plumbing from freezing.

You need to be very meticulous with an air barrier to separate the garage air from the house air and need to install a garage exhaust fan to try to overcome the physics of the garage air mixing with your indoor air.

It depends on how complicated any existing framing is. If building a rigid air barrier up to the roofline is overly complicated or broken up, I think using spray foam at the garage flat ceiling and in the cavities of the shared house walls would be best. Ducts in the attic means keeping the thermal barrier at the roofline but if they come from below, I would be tempted to create redundant insulation at the flat ceiling level for those isolated areas. It again depends on what plane will be easier to airseal.

The most overlooked area is the mudsill sealer at these shared walls. Also be sure to air seal the door jamb to the framing and maintain the weatherstripping. It's a great question and concern that I wish more people building new were paying attention to.
PRO
Mark Bischak, Architect
8 years ago
What does your architect recommend?
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scone911
8 years ago
Ask the contractor, and/or insulation sub, what he would do if it were his own house. Or maybe his mom's house.
Ron Natalie
8 years ago
Insulated or not, as mentioned, you'll need to make sure any living space is properly sealed for monoxide from the car areas and most places want an some level of fire rating as well.

As for insulation, frankly NOT insulating my garage (in NC) was one of the mistakes I did make on my house. I have the garage and right next to it an airplane hangar. Since the hangar is heated, I spray foamed it. It's not air conditioned, but it took me a while of thought to figure out why the hangar stayed comfortable in the summer where the garage got beastly hot, then I remembered the insulation difference.

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