What R-value is, and where your number comes from
R-value is resistance to heat flow. Higher resists more, and the values of layers in an assembly add: two inches of R-5 board is R-10. That is the whole concept, and it is genuinely useful, but two things about it are routinely misread.
First, the R-value printed on a bag is for the material under test conditions, not for your wall. A wall framed at 16 inches on center is roughly a quarter wood by area, and wood is around R-1.25 per inch, so studs short-circuit the insulation between them. The assembly performs meaningfully worse than the number on the batt. Second, R-value measures conduction only. It says nothing about air moving through or around the insulation, which in most older houses is the larger loss.
As for how much you need: that comes from your climate zone, from the DOE zone map that runs from 1 in south Florida to 8 in interior Alaska, and from whatever energy code your jurisdiction has adopted and amended. Recommended attic values in a cold zone are far above what a mild coastal zone calls for. Look yours up rather than trusting a figure you read anywhere, including here — a number that is right for Minnesota is a waste of money in Georgia, and one that is right for Georgia leaves a Minnesota house cold. Once you know the target depth, the insulation calculator converts it into bags or batts.
R per inch, by material
| Material | R per inch | Where it belongs |
|---|---|---|
| Fiberglass batt | 3.1-3.4 | Open stud and joist bays. Cheap, and unforgiving of sloppy fitting — gaps and compression cost you real performance. |
| Blown fiberglass | 2.2-2.7 | Attic floors. Fills irregular space better than batts do. |
| Blown cellulose | 3.2-3.8 | Attics and dense-packed walls. Recycled paper, borate treated. Settles somewhat, so it is installed above the target depth. |
| Mineral wool | 3.0-3.3 | Where fire resistance or sound matters. Holds its shape and cuts cleanly. |
| EPS rigid board | 3.6-4.2 | Exterior sheathing, below grade |
| XPS rigid board | About 5.0 | Below grade, and where it will get damp |
| Polyisocyanurate board | 5.6-6.5 | Exterior continuous insulation. Loses some R in cold temperatures, which matters where you need it most. |
| Closed-cell spray foam | 6.0-7.0 | Rim joists, tight assemblies. Also an air and vapor barrier, which is most of its value. |
| Open-cell spray foam | 3.5-3.8 | Air sealing where vapor needs to pass |
| Wood framing, for comparison | About 1.25 | This is what a stud does to your wall |
Air sealing comes first
An uninsulated house loses heat through the assembly. A leaky house loses it as air, and adding fluffy insulation over a hole does not stop air — it filters it. This is why a house with a foot of attic insulation can still have ice dams and cold rooms.
The places to seal, roughly in order of what they are worth in a typical attic: the chases where plumbing stacks and ducts pass through the top plate; gaps at the top plate itself between the drywall and the framing; recessed light housings, which need an IC-rated fixture and fire-rated sealing rather than insulation piled on them; the attic hatch, which is usually a quarter-inch of plywood with no weatherstripping; a dropped soffit over kitchen cabinets, which is often open to the attic across its whole area; and the chimney chase, which needs sheet metal and high-temperature sealant, not foam.
Downstairs, the rim joist — the band of framing around the perimeter of the basement ceiling — is usually the single worst spot in the house and one of the easiest to fix, with rigid board cut to fit and sealed at the edges, or two inches of closed-cell foam. A blower door test, often free or cheap through a utility program, will rank all of this for your specific house rather than a generic one, and is worth arranging before you buy anything.
Where to spend, in order
| Location | Why it ranks here | Note |
|---|---|---|
| Attic air sealing, then attic insulation | Heat rises, the attic is the largest single loss, and it is the most accessible cavity in the house | Seal before you blow. Once there is loose fill on the floor you cannot find the bypasses. |
| Rim joist | Large air leakage, small material cost, and you can stand up while you do it | An afternoon and a couple hundred dollars of board and foam |
| Ducts in unconditioned space | Leaky attic or crawl space ducts throw a large share of what you paid to heat straight outdoors | Seal joints with mastic, not cloth duct tape, then insulate |
| Crawl space | Cold floors, humidity, and the moisture path into the house above | Current practice usually favors sealing and insulating the walls with a ground vapor barrier over insulating the floor above, but which is right depends on your climate |
| Windows | Real losses, but the worst dollars per unit of comfort of anything here | Weatherstripping, storm windows and insulating window film capture most of the benefit for a fraction of replacement cost |
| Existing walls | Last, because opening them is disruptive and dense-packing is a specialist job | Worth doing when the siding is coming off anyway |
To put a number on any of it before you commit, heating cost savings will show what a given reduction is worth against your actual fuel price, which is usually more sobering and more useful than a payback figure from a contractor.
Condensation, dew point, and not trapping water
Condensation is warm moist air touching a surface below its dew point. In winter that surface is the inside of a window, an exterior corner where framing concentrates and the insulation thins, the wall behind a dresser pushed tight against an exterior wall, or an uninsulated closet on an outside corner. It is not a leak, and treating it as one wastes money.
You have two levers. Raise the surface temperature, by insulating and air sealing so the interior face stays warmer, or by moving furniture a few inches off the wall so room air can wash it. Or lower the indoor humidity: aim for 30 to 50 percent, run bath and kitchen exhaust fans that actually vent outdoors, and be aware that in very cold weather even 40 percent will condense on single-pane glass. A hygrometer settles arguments here for ten dollars, and if the summer number is the problem, dehumidifier sizing is the next step.
The part that goes wrong: adding insulation to the interior side of a wall makes the outer part of that assembly colder, because less heat is now reaching it. Do that without controlling how much indoor moisture can get into the cavity and you can move the condensation from the wall surface, where you could see it, into the wall, where you cannot. The general rule is one vapor retarder per assembly and it goes on the warm side for your climate — never a plastic sheet on the inside and a foil-faced board on the outside sandwiching a cavity that can then never dry in either direction. The specifics genuinely vary by climate zone, and this is where a retrofit is worth an hour of a building-performance contractor's time before you close the wall.
Renting, or working with a small budget
Nearly everything with the best return is something a tenant can do and take with them. Weatherstripping on doors and a door sweep at the bottom. Rope caulk or removable film on drafty windows. Foam gaskets behind outlet and switch plates on exterior walls, which take about a minute each and are startlingly effective in an older house. Cellular shades, which trap a layer of air at the glass. A draft stopper at the base of an unused door. All of it is reversible and all of it is under a hundred dollars.
Beyond that, ask about programs before you spend: most utilities run rebates for insulation and air sealing, and the federal Weatherization Assistance Program covers income-qualified households at no cost, delivered through state and local agencies. Renters are eligible in many states with landlord consent. The application takes an afternoon and the work is worth thousands.
If you take one thing from this page, make it the order: seal the air leaks, then insulate, then think about windows. Doing it the other way around is how people spend twenty thousand dollars on replacement windows and still have a cold room over the garage.
Questions people ask
How much insulation do I actually need in my attic?
It depends on your climate zone and on the energy code your jurisdiction has adopted, so there is no honest single answer. The DOE climate zone map runs from 1 in south Florida to 8 in interior Alaska, and recommended attic R-values differ substantially across that range. Look up your zone and check what your local code requires, then convert the R-value into a depth using the R per inch of the material you are installing. Adding depth beyond the recommendation for your zone has diminishing returns quickly — the money is usually better spent on air sealing or on ducts.
Should I air seal or add insulation first?
Air seal first, and in an attic it is not close. Insulation slows conduction; it does not stop air moving through it, so blown fill over an open plumbing chase or an unsealed top plate just filters the air on its way out. There is also a practical reason: once there is a foot of loose fill on the attic floor you can no longer find the bypasses to seal them. Seal the chases, the top plates, the recessed cans, the attic hatch and any dropped soffit, then blow the insulation. If you can get a blower door test through a utility program, it will rank the leaks in your specific house.
Why does condensation form on my windows and walls in winter?
Warm indoor air holds moisture; when it touches a surface colder than its dew point, that moisture condenses out. Windows are the coldest surface in most rooms, so they go first. Exterior corners and the wall behind a dresser pushed against an outside wall are next, because framing concentrates there and air cannot circulate. You have two levers: warm the surface, through insulation, air sealing and pulling furniture a few inches off the wall, or lower the humidity, aiming for 30 to 50 percent indoors and running exhaust fans that genuinely vent outside. Very cold weather can produce condensation on single-pane glass at humidity levels that are otherwise perfectly reasonable.
Is spray foam always better than fiberglass?
It has a higher R per inch and it air seals, which is a real advantage in a rim joist or a tight cavity where you cannot fit enough of anything else. It also costs several times more, is difficult to remove or inspect behind, and creates assemblies that cannot dry in one direction, which has to be right for your climate. Where you have open joist bays and plenty of depth — an ordinary attic floor — blown cellulose or fiberglass gets you the same R for far less. Spend the difference on air sealing and duct sealing, which is where the money usually goes furthest.
I rent. What can I do that I get to keep?
A surprising amount, all of it reversible. Weatherstripping and a door sweep on exterior doors. Removable window film or rope caulk on drafty sashes. Foam gaskets behind outlet and switch plates on exterior walls, which take a minute each and matter more than they look like they should. Cellular shades, which hold a layer of still air at the glass. A hygrometer so you can see whether the humidity is your problem. Beyond that, ask your utility about rebates, and look into the federal Weatherization Assistance Program — many states serve renters with landlord consent, and the work is free to income-qualified households.