Should I Insulate My Garage Ceiling or Garage Walls?

by Erik North on October 28, 2011

Should I Insulate My Garage Ceiling or Garage Walls?


This is what happens when you write a post with crazy enthusiasm. I ripped through this previous post and in my enthusiasm to tackle insulating finished rooms over the garage, I forgot to clearly state the garage ceiling vs. garage walls question.

So let’s be a little less opaque: both approaches can work but unless the garage slab is already insulated, retrofitting the ceiling makes more sense.

The two possibilities which need close attention when insulating are attached garages with a finished room over it or an attached garage without a finished space. In both cases, it is usually more difficult and more challenging to insulate the garage exterior walls. A garage is truly difficult to make into conditioned space.

Simple answer: If there is an finished space over the garage, insulate at the garage ceiling. If there is no finished space, insulate at the interior wall.

If you plan on using your garage for temporary activities, like a mechanic or woodworking shop, you would insulate both the interior and exterior walls. Yeah, you have to pay for your hobbies. The interior wall and ceiling insulation would be your year-round thermal control layer and the exterior wall insulation would allow the garage to be heated temporary when working.

Why Is It Hard To Insulate Garages?

It can be pretty difficult to insulate and air seal an attached garage. They have to overcome a lot of challenges:

  • Garages are almost never finished with the same rigorous attention to details as finished spaces. Garages are almost never finished and don’t employ ‘Airtight Drywall’ approaches (unless you have an extraordinarily conscientious contractor). This means a relatively air porous air barrier and if the interior drywall remained unpainted, it would be a small ‘v’ vapor retarder.
  • The concrete slabs on which garages are built are rarely insulated beneath them. The options for insulating them are either challenging and inadequate (trenching around the perimeter and installing edge insulation) or damn near impossible and ridiculously expensive (tearing up the whole slab, reinsulating then repouring). If the slab remains uninsulated, the concrete slab would be a massive heat sink defeating any effort to heat the space.
  • Garages are rarely connected either by duct or water pipe to the house’s heating system. If the garage space is used, homeowners install monitor or electric space heaters.
  • And one last fun point; garage doors. Standard doors are very hard to effectively air seal and insulate. Most doors are uninsulated. Foam board insulation could be added, but no amount of effort can make the doors acceptably airtight. There are some cleverly engineered track designs which produce a tighter door enclosure but at this time these are largely boutique products.

So, Should I Insulate The Ceiling or Walls?

What you do next depends on your plans for the space. If you only ever plan on storing your car, leafblower and old comic books, there’s no point in insulating the outside walls. And why are your comic books in your garage? Ok, so are mine! 🙂

If you intend on using this space for a workshop or mechanic garage, exterior wall insulation in addition to ceiling and interior wall insulation makes sense. Running a space heater will allow the workshop to be comfortable temporarily.

The other garage construction, where it is attached but without a living space overhead, should be considered in the same manner. All the same heating and insulating challenges apply so insulate at the wall between the house and garage. Because of the carbon monoxide concerns (covered in this article), this dividing partition wall needs to be carefully airsealed. Weatherization work should be performed while a blower door is running. Once the garage and living space are airsealed off, insulation can be installed. See this article for details of the needed work for a FROG (finished room over garage).

And I Insulate My Garage Ceiling (And The Finished Room) How?

Keeping the house warm where it connects with the garage isn’t easy. It faces a few challenges:

  • The heating system is often on the opposite side of the house, meaning the distribution system (which can be air or water) travels 70 or 80 feet before reaching the FROG.
  • A finished room over the garage is insulated on five sides, dealing with wind and cold from almost all directions.
  • Often the heat water pipes run through the garage ceiling. If the pipes are uninsulated, you’ve got hot water within a few inches of cold winter air.
  • Air leaks everywhere. The attic leaks conditioned air through recessed lights, chimney chases, light fixtures, etc. First floor soffets can allow cold air into the horizontal garage ceiling bays if unsealed. And where the garage ceiling joists meet the house is often open into the building framing. Yay, cold air!

What should the slipper wearing homeowner do? Insulate and air seal and block and insulate some more. Attic air sealing means caulking and foaming gaps around light fixtures, drywall seams and anywhere there is a gap in the ceiling air barrier.

And I Insulate My Garage Ceiling (And The Finished Room) – This Is How

In the garage, as in the attic, we need to airseal and insulate. Open first floor soffits and the ceiling joists open to the building frame can be sealed with foam blocks and foam sealants.

Beefing up the garage ceiling insulation can be a large project. My recommendation is usually first pulling down the drywall and existing fiberglass (why you ask? Here’s why). Then either air sealing the garage ceiling plane with a blower door running and dense packing with netted cellulose or with 2″ of close cell spray foam and fiberglass batts (if the homeowners will be away long enough for the foam curing process). The dry plywood flooring has a sufficiently low vapor permeance to act as the interior vapor retarder.

Finally, I always recommend installing a hard wired carbon monoxide detector with a battery back-up in garages and in rooms over garages. It’s a common sense way to avoid a potentially catastrophic problem.

Insulating The Garage Ceiling

Insulating garages is a challenging home project. Ultimately there are so many compromising elements that insulating the exterior walls are not particularly effective. Insulate and air seal the garage ceiling and interior walls and your house will be warm and comfortable during the winter.

{ 1 comment… read it below or add one }

Sue March 18, 2015 at 10:18 am

A local company is advertising insulation (film type) over a finished garage ceiling. Above the two car garage are the bedrooms. I’m thinking this is not that difficult to do. The question i have is: 1> Is it safe to install the film type insulator over the existing sheet rock/finished ceiling? I have no clue if the ceiling was insulated between the garage and bedrooms – just thinking of doing this for added protection. And 2> if this is recommended what type of insulation would be best suited for this job?


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