U-factor, R-value, and why windows use the other one
Windows are rated in U-factor and everything else in the building is rated in R-value, which makes comparison awkward until you notice they are reciprocals. U is one divided by R. A window at U-0.30 is about R-3.3. A wall at R-20 is about U-0.05. The window is roughly seven times worse per square foot, which is the whole reason windows come up in energy conversations at all.
The number that matters is the whole-window U-factor from the product label. It accounts for the glass, the spacer at the edge of the glass, the frame material and the frame area, all combined and area-weighted. Centre-of-glass figures ignore the frame and the edge and are always better, sometimes substantially, and they are not comparable across products with different frames. When two quotes give you different kinds of number, make them give you the same kind.
| Glazing | Whole-window U, typical range | Equivalent R |
|---|---|---|
| Clear single glazing, wood or metal frame | 0.9 - 1.2 | 0.8 - 1.1 |
| Single with an exterior storm | 0.5 - 0.6 | 1.7 - 2.0 |
| Older clear double glazing | 0.45 - 0.55 | 1.8 - 2.2 |
| Modern low-e double, gas filled | 0.27 - 0.35 | 2.9 - 3.7 |
| Triple glazed | 0.15 - 0.25 | 4.0 - 6.7 |
Those ranges are orientation, not specification. The label on the product you are being sold is the number to use, and the number for what you already have is usually an estimate rather than a measurement, which is worth remembering because it is the input this calculation is most sensitive to.
Turning a U-factor into a year
Heat flow through an element is area times U times temperature difference, giving BTU per hour. To get a year you need the temperature difference summed over the whole heating season, and that is what heating degree days are: one degree day is one day at one degree below the base temperature. Multiply degree days by 24 and you have degree-hours. So annual loss is 24 times area times U times HDD.
Two hundred square feet of glass at U-0.85 in a 5,000 degree day climate loses about 20.4 million BTU a year. The same area at U-0.30 loses about 7.2 million. The difference is 13.2 million BTU, which at 85 percent seasonal efficiency means the furnace burns about 15.5 million BTU less, which is about 155 therms, which at 1.40 a therm is around 217 dollars a year. Against a 12,000 dollar project that is a 55 year payback. The heating degree day calculator will get you a degree day figure for your own weather and calibrate it against your past bills.
Why the answer is nearly always disappointing
Two hundred square feet of glass in a house with 1,600 square feet of wall and 1,500 of ceiling is a small share of the shell. Even at seven times the heat flow per square foot, the windows are a minority of the total loss. Meanwhile they are among the most expensive things per square foot that go into a building, because each one is a manufactured assembly that has to be installed, flashed, trimmed and made watertight.
That combination — modest share of the loss, large share of the cost — is why the arithmetic comes out the way it does, and why replacing sound windows for energy reasons is usually the wrong call. The same 12,000 dollars spent on air sealing and attic insulation buys several times the annual saving in most existing houses, which is what the air sealing payback calculator and the attic insulation top-up calculator will show for your own building.
The cases where replacement is justified are about condition and use rather than energy: sashes that are rotten, seals that have failed and fogged, windows painted shut or unsafe to operate, egress that does not comply, noise on a busy road, or a renovation that is opening the walls anyway. The energy saving is real and it is worth counting. It is not usually the reason.
The cheaper move that the arithmetic likes
Most of the conductive improvement in a window happens between one layer of glass and two. Going from U-0.85 to U-0.50 with a storm window captures roughly two thirds of the saving that going all the way to U-0.30 delivers, for a small fraction of the cost, which is why the comparison lane on this page usually favours it on payback even though nobody sells it as hard.
What a storm does not give you is a new frame, working hardware, a fixed sill or an easier window to open, and interior storms in particular have to be installed with care because sealing humid room air into a cold gap moves the condensation somewhere you cannot wipe it. For the film and interior panel version of the same idea, the window film calculator covers material and payback directly. For what happens to the glass surface temperature and the humidity that condenses on it, the wall condensation risk organizer handles the gradient and the humidifier sizing calculator covers the room humidity that feeds it.
Questions people ask
Do new windows pay for themselves?
On energy alone, in most existing houses, no. Run your own numbers and the annual saving typically lands in the low hundreds of dollars while the project costs five figures, which puts simple payback somewhere between twenty and sixty years — at or beyond the service life of the product, before any discounting. That is the ordinary honest result and it is not a sign the inputs are wrong. New windows are justified by condition, comfort at the glass, operation, security, noise and appearance, with the energy saving as a genuine but secondary benefit. If a quotation is built primarily on energy savings, ask to see the arithmetic and check what existing U-factor it assumed.
What is the difference between U-factor and R-value?
They are reciprocals of the same property. U-factor is conductance, how much heat passes per square foot per degree per hour, so lower is better. R-value is resistance, its inverse, so higher is better. U-0.30 is R-3.3, U-0.50 is R-2.0, U-1.0 is R-1.0. Windows are labelled in U because the industry rates whole assemblies that way and because the numbers are small enough to be awkward as R-values. When you feed a window into a heat loss calculation, use area times U directly rather than converting back and forth, which is what the heat loss calculator does.
Are storm windows a good substitute for replacement?
On the energy arithmetic they usually look better, because most of the conductive gain is in going from one layer of glass to two and a storm delivers that for a fraction of the price. A single-glazed window with a decent storm lands around U-0.5 against U-0.85 or worse for bare single glazing, and it captures a large share of what a full replacement would save. What it does not do is fix a rotten sash, a frame that leaks air around its perimeter, hardware that no longer works, or a window that will not open. If the existing windows are structurally sound and the complaint is cold glass and heating cost, a storm is the move the numbers support. If they are failing, it is a patch on a failing thing.
Should I pick a low solar gain window?
It depends on the orientation and the climate, and it is a real trade rather than a straight upgrade. A low solar heat gain coefficient keeps summer sun out, which helps a cooling-dominated climate and a west-facing room considerably. It also keeps winter sun out, and on a south-facing window in a heating-dominated climate that free heat is worth having. Many houses are best served by different specifications on different elevations, which manufacturers will do and salespeople rarely propose. None of this appears in the payback on this page, which is conduction only, and it can be a larger effect than the U-factor difference on a sunny elevation.
Why does my new window still feel cold?
Because a window at U-0.30 is still about R-3.3 sitting in a wall at R-15 or better, so the interior glass surface runs colder than the wall beside it on any cold day and your body radiates heat toward it. That is physics rather than a defect. What is worth checking is whether the cold you feel is radiant or moving: hold a hand near the frame perimeter and near the sill, and if there is air movement the installation, the flashing or the gap between the frame and the rough opening is the problem rather than the window itself. Draughts at a new window are an installation question, and the installer should come back for them.