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Window Condensation Calculator

Work out whether your windows will run with condensation, and the maximum indoor humidity they can take at any outdoor temperature.

How to use this calculator

  1. 1Get the U-factor from the NFRC label if the windows are modern, or use the type. It is centre-of-glass, which is why the page also works out the edge.
  2. 2Measure the indoor humidity with a hygrometer rather than guessing. Houses with furnace humidifiers routinely run ten points higher than the occupants think.
  3. 3Read the edge figure, not the centre. Condensation always starts as a band along the bottom of the glass.
  4. 4Use the table to find the humidity setting that works through your coldest week rather than through an average day.

How the calculation works

T_glass = T_in - (T_in - T_out) x 0.68 x U Dew point: gamma = ln(RH/100) + 17.625 T / (243.04 + T); Td = 243.04 gamma / (17.625 - gamma) Max RH = 100 x exp( 17.625 Td / (243.04 + Td) - 17.625 T / (243.04 + T) ) Condensation when T_glass < Td
U
Window U-factor from the NFRC label, in BTU per hour per square foot per degree F. Lower is better
0.68
Resistance of the still air film clinging to the inside face of the glass. Essentially fixed, and the reason glass is always colder than the room
Td
Dew point - the temperature at which the air in the room becomes saturated and starts giving up water
Spacer factor
How much better the perimeter of a sealed unit conducts than its centre. About 1.45 for aluminium, 1.15 for a warm-edge spacer

The Magnus relation is the standard engineering approximation for saturation vapour pressure over water, accurate to a few hundredths of a degree across ordinary indoor conditions.

Inverting it is what produces the winter humidity table. Every published version of that table is the same calculation with an unstated window U-factor baked in, which is why they disagree with one another.

The interior film resistance is what makes the glass colder than the room even on a perfect window. A U-0.20 triple unit at 70 F inside and 0 F outside still has its inside surface around 60 F.

Worked example

Double low-e windows at 10 F outside

  1. 1.A 60 degree difference across a U-0.35 window: the centre of the glass sits at 70 - 60 x 0.68 x 0.35 = 55.7 F.
  2. 2.The aluminium spacer makes the edge behave like U-0.51, so the edge is at 70 - 60 x 0.68 x 0.51 = 49.3 F.
  3. 3.Room air at 70 F and 40% relative humidity has a dew point of 44.6 F.
  4. 4.The edge is 4.7 degrees above that, so no condensation - but the margin is thin, and a colder night or a load of laundry drying indoors would close it.

Result: Dry, with about 4.7 degrees of margin

Single pane at 0 F - the classic streaming window

  1. 1.Single glazing at U-1.04 with a 70 degree difference: the glass sits at 70 - 70 x 0.68 x 1.04 = 20.5 F.
  2. 2.That is below freezing, so what forms is not condensation but frost.
  3. 3.For a dew point of 20.5 F the room could only be at about 15% relative humidity - drier than most deserts and not somewhere anyone wants to live.
  4. 4.This is why single-glazed houses in cold climates frost up whatever the occupants do, and why an interior storm window or even a film kit makes such a visible difference.

Result: Frost. The humidity limit is around 15%, which is unliveable

What a warm-edge triple unit buys

  1. 1.Triple glazing at U-0.20 with a warm-edge spacer: the edge behaves like U-0.23.
  2. 2.At -10 F outside the difference is 80 degrees, so the edge sits at 70 - 80 x 0.68 x 0.23 = 57.5 F.
  3. 3.Room air at 70 F and 45% has a dew point of 47.7 F - nearly ten degrees of margin at minus ten outside.
  4. 4.The same house with aluminium-spacer double low-e would be streaming at that humidity. The spacer alone is worth several points of allowable humidity, and it costs almost nothing at the time of manufacture.

Result: Comfortable humidity at -10 F, with ten degrees to spare

Why glass is always colder than the room

A thin layer of still air clings to every interior surface, and it insulates. Its resistance is about 0.68 in the units building physics uses, and it is effectively the same on every window in every house - it depends on the surface and the air, not on the glazing.

That film is why an interior surface always sits below room temperature when it is cold outside, and why the drop is proportional to how much heat the assembly lets through. A poor window passes a great deal of heat, so a large temperature gradient develops across that fixed film and the glass ends up close to outdoor conditions. A good window passes very little, so the film hardly has to work and the glass stays near room temperature.

It also sets a floor nothing can beat. Even a theoretical perfect window would have its interior surface at room temperature, and every real one is below it. There is no glazing that eliminates condensation at any humidity - only glazing that pushes the limit high enough to be irrelevant.

The edge is the whole story

An insulating glass unit works by separating two panes with a gas that conducts poorly. Around the entire perimeter, though, those panes are joined by a spacer bar that holds them apart and seals the cavity - and for most of the history of the product that bar was aluminium, which conducts extremely well.

The result is a thermal bridge running around the whole edge of every pane. The centre of the glass may behave like the U-0.30 unit it was sold as; the outer inch or two behaves like something far worse. In cold weather that band runs several degrees below the middle, and it is where the dew point is crossed first.

This explains the experience almost everybody has with published humidity tables: the table says 40% is fine at 10 degrees, the hygrometer says 38%, and there is a puddle on the sill every morning. The table was calculated at centre-of-glass. Warm-edge spacers - foam, structural silicone, thin stainless - largely close the gap, and on a unit being specified new they are among the cheapest upgrades available.

  • Inside the room — A humidity problem. The air is too damp for the glass temperature. Ventilate, or lower the humidity.
  • Between the panes — A failed seal. The unit has lost its gas fill and its desiccant is saturated. Only replacement fixes it.
  • On the outside face — Not a problem at all - a sign of a very good window. The outer pane is staying cold because so little heat reaches it.
  • A band at the bottom only — The classic edge effect. The spacer is bridging the unit and the centre is still above the dew point.

The tension between comfort and condensation

Dry indoor air in winter is genuinely unpleasant. Below about 30% relative humidity people report static shocks, dry skin, sore throats and irritated airways, and timber floors, furniture and instruments all shrink and move. There are good reasons to want a house at 40% in January.

The trouble is that a house with older windows cannot have one. At 10 degrees outside, a double-glazed aluminium-spacer unit caps the humidity somewhere in the mid-thirties; single glazing caps it far lower than anyone would tolerate. Running a humidifier against that ceiling does not make the house comfortable - it makes the windows wet and eventually the frames rotten.

The order of operations is therefore: find and remove the moisture sources you do not want, which is almost always bathrooms and cooking and drying laundry indoors; set the humidifier, if there is one, to what the coldest week actually allows rather than to a fixed number; and treat the humidity ceiling as one of the real returns from better glazing, alongside the heating bill. A warm-edge triple unit is not only cheaper to run - it lets the house be a more comfortable place to be.

What this assumes, and where it stops

Assumptions

  • The interior air film is taken at 0.68 hr sq ft F per BTU, the standard value for a vertical surface with ordinary room air movement.
  • U-factors are typical centre-of-glass values by window type. The NFRC label on a specific window is authoritative.
  • Spacer factors are broad multipliers on the centre-of-glass U-factor to represent the edge condition.
  • Room air is taken as well mixed. In practice it is not, and the air against a cold window is often damper and cooler than the middle of the room.

Limitations

  • It models the glass. Frames, sashes and sills have their own temperatures - metal frames without a thermal break are usually the coldest surface in the assembly and condense first.
  • Curtains, blinds and deep sills make matters worse by cutting off room air from the glass, which lowers the surface temperature further. A window that is dry with the blinds up can stream with them down.
  • It does not account for room-to-room variation. Bedrooms with the door closed overnight, and rooms above unheated spaces, run damper and colder than the house average.
  • Solar gain, wind and night-sky radiation all move the outer surface temperature and are not included.
  • It says nothing about condensation inside wall assemblies, which is a more serious problem than window condensation and is governed by vapour retarders and air sealing.

Common questions

What should indoor humidity be in winter?

Whatever your windows can take, which changes with the outdoor temperature. As a rough guide with ordinary double glazing: around 40% at 20 F outside, 35% at 10 F, 30% at 0 F, and lower again below that. The table on this page is that guide calculated for your actual window rather than an assumed one.

Why do my windows sweat only at the bottom?

Because the spacer bar around the perimeter of a sealed unit conducts heat far better than the gas between the panes, so the outer inch or two of glass runs several degrees colder than the middle. The dew point is crossed there first. Warm-edge spacers largely fix it; on existing windows the answer is lower humidity or better air movement across the glass.

Is condensation on my windows damaging?

The water on the glass is not, but where it goes is. It runs onto the sill, soaks into the timber, lifts the paint, and over a few winters supports mould at the frame and in the wall below. It is worth acting on the first winter it appears rather than treating it as a seasonal nuisance.

Will new windows stop condensation?

They will raise the humidity at which it starts, sometimes dramatically - triple glazing with a warm-edge spacer might take a house from a 30% ceiling to a 50% one. They will not make condensation impossible, because the interior air film means glass is always below room temperature. If the underlying problem is a humidifier set too high or a bathroom fan that vents into the loft, new windows are an expensive way to not fix it.

There is condensation between the panes. What can I do?

Replace the unit. Moisture between the panes means the perimeter seal has failed, the gas fill has escaped and the desiccant in the spacer is saturated. The insulating value has largely gone with the gas. Defogging services exist that drill and vent the unit, but they do not restore the fill and the result is a vented double pane rather than a sealed unit.

Sources

Formula and content last reviewed on .

Results are estimates for information only, not professional advice.

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