How to insulate a garage

$675 of fixes. Half the heater.

Seal the leaks, treat the door, then the ceiling — in that order, because each one pays back faster than the next. On BayHeat's standard worked example (a 24×24 ft attached two-car garage, Chicago design day), that order turns a 31,900 BTU/hCComputed by our enginefixFirst() via lib/planner/roi.tsReport a problem load (grade DCComputed by our enginefixFirst()Report a problem) into 13,200 BTU/hCComputed by our enginefixFirst()Report a problem (grade BCComputed by our enginefixFirst()Report a problem) — a 10 kW class heater becomes a 5 kW class heater, and the breaker drops from 60 ACComputed by our enginefixFirst()Report a problem to 30 ACComputed by our enginefixFirst()Report a problem. None of that is a guess — it's the same insulateFirst() / bundleCheapMeasures() model behind the calculator, run live on this page.

Savings vary. Find out why in the seller's fact sheet on R-values. Higher R-values mean greater insulating power.

The order, and why it's this order

Fixing the envelope before buying a heater changes two things, not one: the energy it takes to run the heater, and the size (and price) of the heater itself. A garage that needs a 5 kW class heater instead of a 10 kW class one saves on the equipment too, every year it's owned — not just on the electric bill. The table below is sorted by fastest electric payback first.

Savings vary. Find out why in the seller's fact sheet on R-values. Higher R-values mean greater insulating power.

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Shop the fixes

No specific product for the ceiling (a rented blower and bagged insulation from any home center) or a new door (get local quotes) — those two are hire-or-DIY jobs, not a single part to buy.

1. Seal the leaks first

Weatherstripping the garage door's bottom, sides and top, plus the service door, is the cheapest single step and the fastest payback on the list. At $125 it cuts 3,500 BTU/hCComputed by our engineinsulateFirst().weatherstrip.dQDesignReport a problem off the design load. Going from "average" drafts to "tight" doesn't require new materials science — it's a bottom seal, a perimeter stop, and closing the gaps around the service door.

2. Treat the big door

The garage door is usually the single largest area of uninsulated surface in the room. An EPS or fiberglass kit (a typical kit lands in the R-6 to R-10 class) drops in without replacing the door itself. It cuts about 4,700 BTU/hCComputed by our engineinsulateFirst().door_kit_eps.dQDesignReport a problem. A reflective/radiant-barrier kit is cheaper but does less on a cold-climate design day — it works best against summer heat gain, not winter heat loss, so skip it if winter is the problem you're solving.

3. Then the ceiling

Blowing R-30 over a bare drywall ceiling (roughly R-2 as-is) is the single biggest fix on this list, because a vented attic sits directly above an uninsulated ceiling with almost nothing slowing the loss. It cuts 8,700 BTU/hCComputed by our engineinsulateFirst().ceiling_r30.dQDesignReport a problem — about 30% of the whole load — for a materials cost most DIYers can rent a blower to install in an afternoon.

What's optional, and what to skip for now

• An attic hatch gasket is a small, cheap add-on — worth doing at the same time as the ceiling, but it's a minor line (1% of the load), not a headline fix.
• Skip the reflective kit if you're already doing the EPS kit above — they treat the same door, so only one belongs in your cart.
• Skip a full $2,200 polyurethane door replacement for now. It saves less heat per dollar than the $120 kit on the door you already have — put it on the list for when the door panel itself needs replacing, not before.

Next step

Run your own garage through the calculator → to see your own load, grade and heater class before or after these fixes — the worked example above is one specific garage in Chicago, not yours.