Below the attic, the work is about finding the edge of the heated house and insulating that edge, not everything in sight. A vented crawl space puts the edge at the floor above it; a sealed one puts it at the foundation walls. A basement’s edge is usually its walls. A garage’s is the wall and ceiling it shares with the house. The order is the same everywhere: water first, then air leaks, then insulation, then a covering over what can’t stay bare, then a venting check.
General information, not professional advice. Building codes are adopted locally, with changes, and your building department decides what applies to your house. Stop and call a qualified professional for anything this page marks as a stop.
The six guides below take their figures from ENERGY STAR, the Department of Energy (DOE), EPA, the 2024 model codes and makers’ own sheets, each named and dated on the page. ENERGY STAR’s do-it-yourself guide does the attic first, because warm air leaking out at the top of a house pulls cold air in at the bottom, so start with the attic and air sealing it if you haven’t yet. Down here, the rim joist is where most people can begin, and closed walls come last.
Stops that come before any of these jobs
Each guide has its own stop list. These recur across them:
- Water. Standing water, stains, damp or moldy insulation, soft or crumbling wood. DOE’s basement guide says moisture “MUST be addressed before insulating a basement”.
- Old pipe, duct or boiler wrap, or loose pebble-like fill. It may contain asbestos, and you can’t tell by looking. EPA: “If you suspect material contains asbestos, don’t touch it.” For vermiculite, EPA says to assume it contains asbestos, leave it undisturbed and not try to remove it yourself. See asbestos insulation and vermiculite insulation.
- Knob-and-tube wiring. Never insulate over or around it. A licensed electrician comes first; see knob-and-tube wiring and insulation.
- A furnace, boiler or water heater that burns gas, oil or propane in the basement, crawl space or garage. Sealing around it can make it backdraft. A heating technician looks before the work and again after it.
- Signs of termites or carpenter ants. A pest control professional before anything covers the wall or sill.
1. The rim joist: where most people can start
The rim joist is the band of wood that sits on top of the foundation and closes off the ends of the floor joists. It is the one job in this part of the house for which ENERGY STAR gives homeowners step-by-step instructions (ENERGY STAR, basement and crawlspace project page). The method has two steps: seal the top and bottom of every cavity with caulk or canned foam, then fit rigid foam board or a batt against the wood. Our guide to rim joist insulation gives those steps and the three conditions under which the 2024 model building code lets foam stay uncovered there.
2. Crawl spaces: floor or walls, never both
If the crawl space is vented to the outdoors, the insulation belongs in the floor above it. If it is unvented, the insulation belongs on the foundation walls and the floor above stays uninsulated. That is DOE’s guidance. Closing the vents is not enough: a guide written for DOE’s Building America program warns that it “could make the situation worse”, and the 2024 model residential code asks an unvented crawl space for a sealed ground cover and a way of keeping the air dry. Crawl space insulation: floor or walls? helps you tell which kind you have, and covers dirt floors and sagging batts.
3. Floors over crawl spaces, garages and open air
A floor needs insulation when unheated space is under it: a vented crawl space, an unheated garage, or open air under an overhang or bay window. DOE’s guides ask for three things: the subfloor air sealed first, because batts do not stop air; the insulation in full contact with the underside of the subfloor; and supports that hold it there for good. If the batt has a paper facing, the makers’ sheets say it goes up against the floor and is not left exposed. Our floor insulation guide has the batt thicknesses to check against your joist depth and what is different over a garage.
4. Basement walls
DOE prefers insulating basement walls to insulating the basement ceiling, because even an unheated basement is more connected to the rooms above than to the outdoors. Its guides for an existing house come down to this: get the basement dry, put insulation that water won’t harm, usually rigid foam board, tight against the inside of the concrete, seal it so room air can’t get behind it, and cover the foam with ½-inch drywall. Batts or wood pressed straight against bare concrete, and plastic sheeting over batts, are the layouts DOE’s research warns about. Basement wall insulation sets out the finished walls DOE describes, the cases where the floor above is the better choice, and a checklist for a contractor’s proposal. Board types are compared on our foam board insulation page.
5. Garages
In an attached garage, the surfaces to work on are the ones it shares with the house: the wall between them, and the ceiling if a room sits above. Seal them against air first. The garage’s own outside walls earn insulation if you heat the garage. The drywall between garage and house is a fire separation in the model code, so every piece that comes off goes back. Garage insulation: walls and ceiling covers that separation and heated garages.
6. Walls you don’t want to open
An empty wood-framed wall can be insulated without taking it apart. An installer drills a hole into each stud cavity and blows loose insulation in until it is packed tight. By the makers’ own charts, that gives R-13 with cellulose and R-14 or R-15 with blown fiberglass in a 2×4 wall. It is a contractor’s job: DOE’s archived guidance says adding insulation to exterior walls “usually requires a contractor”. Knob-and-tube wiring, vermiculite in the wall and a wall that gets wet each stop the job before anyone drills. Insulating existing walls without opening them explains how the work is done, what to ask, and how the signed receipt the Federal Trade Commission requires lets you check the bag count for a wall you can’t see into.
How much: ENERGY STAR’s levels by climate zone
R-value measures resistance to heat flow; higher is more. Climate zones run from 1 (hottest) to 8 (coldest), by county. These are the levels ENERGY STAR’s table calls cost-effective for existing wood-framed homes. They are a recommendation, not a rule about your house; each guide sets them beside the 2024 model energy code’s minimums for new homes.
| Climate zone | Floor over unheated space | Basement or crawl space wall |
|---|---|---|
| 1 and 2 | R13 | None given |
| 3 | R19 | R5 insulative wall sheathing or R13 batt |
| 4A and 4B | R19 | R10 insulative wall sheathing or R13 batt |
| 4C (marine), 5 and 6 | R30 | R15 insulative wall sheathing or R19 batt |
| 7 and 8 | R38 | R15 insulative wall sheathing or R19 batt |
Our how much insulation do I need tool finds your zone, and the R-value to thickness tool turns a target into inches for the materials whose sheets we have read.
Foam and paper facings can’t stay bare
Foam plastic burns, and the makers’ sheets say so. The 2024 International Residential Code, a model code, separates foam from the inside of a house with a thermal barrier such as ½-inch gypsum wallboard (section R303.4; R316 in earlier editions). Its narrow allowances for the rim joist and for crawl spaces entered only for repairs do not reach a basement you walk into or a garage you park in. Kraft and standard foil facings on batts are not left exposed either.
After any of these jobs
Sealing and insulating make a house tighter. ENERGY STAR’s guide says to have your heating and cooling technician then check the gas- or oil-fired furnace, water heater and dryer for proper venting. Keep working carbon monoxide alarms; the Consumer Product Safety Commission recommends them on each level of the home, outside each sleeping area. And sealing is not a radon fix: EPA does not recommend sealing alone to reduce radon, and says all homes should be tested. EPA’s radon pages explain how.
Your next step
- Look, with a flashlight, and touch nothing old: what is under each floor, whether the crawl space has vents, and what is wet.
- Check the stops above. Any one of them comes before insulation.
- Find your climate zone, read your row in the table, and ask your building department which code edition it enforces and whether the job needs a permit.
Every figure here is cited to its source in the guides linked above. Sources checked on 10 November 2026.
