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Window condensation/frost despite 30%RH

I am located in Ottawa Canada - so 30% RH is recommended when the weather is below -10'C.
We had condensation problems and older windows. Last year we replaced our windows and I expected the problem to go away. If it is mild/cool out there is little or no problem. With the cold weather there is a lot of moisture on the windows - enough that needs to be wiped up, and frost built up on the North side windows overnight. The humidity level was 35% and with a dehumidifier I brought it down to 30%. Last night the dehumidifier didn't collect any water and the humidity level has stayed at 30%, but there is just as much moisture/frost on the windows as before.
We have wood windows so moisture is a big issue.
- Keeping the blinds open doesn't help
- We heat with natural gas
- Removing the window screens doesn't help
Is there something I am missing here?

Re: Window condensation/frost despite 30%RH

How are you measuring the humidity?

What is the indoor temp?

30%RH @ 75F will create a lot more moisture on windows than even 35%RH @68F.

What was the overnight low temperature. On my last house with double pane Low-E windows, below 10F, I was limited to around 35%RH.

On my current home I have single panel wood frame windows form 1925. Surprisingly the surface temperature with Low-E storm windows isn't much lower.

It all has to do with glass surface temperatures. A lot of that on double pane glass has to do with the metal spacers. IF they are not well isolated, condensation will occur there.

I also suspect your humidity levels are higher than you think. The measuring device you're using may not be that accurate.

Finally, even a short term spike in humidity from showering or breathing while you sleep can cause condensation. It will be slow to evaporate as the indoor RH lowers.

Timothy Miller
Re: Window condensation/frost despite 30%RH

Howdy do you have a humidifier on the hvac? if so turn it off and see what the result. Check any basement of crawlspace for evidence of moisture too. Run fans when bathing and cooking to exhaust the moisture out of the home and leave fans on 215 minutes after you done..
Is the imposture on the glass or between pains - failed thermo pains?

Re: Window condensation/frost despite 30%RH

You heat with natural gas. If these are unvented or improperly vented units within the living space, then the water produced by the combustion could be a source of the excess moisture.

Re: Window condensation/frost despite 30%RH

You need to understand dew point as well as relative humidity. The key word is relative, that means that it is a variable and it depends on temperature and pressure. Since in the overall view, air pressure in the atmosphere doesn't change all that much, we will not consider it for now.

The amount of water that air can hold is dependent on the temperature. The warmer the air, the more water vapor it can hold. The amount of water is measured in grams per cubic meter. The following link is a chart of the amount of vapor in air by temperature and RH (relative humidity).


At 65˚F and 30% RH, there id 48.1 grams of water vapor per cubic meter. If you look down the 30% column, you can see the amount of vapor decreasing with temperature. Now go down the 100% column until you find the values above and below 48.1 and you will see that that is between 35 and 40F. The exact temperature at which 48.1 g/cubic meter is 100% is called the dew point. At or below that temperature is where condensation occurs.

There is another site that has a calculator that will give you the dew point when you enter the temperature and the RH in the room.


I think what you will find is that the surface temperature of the inside pane of glass is below the dew point for the temperature and RH in the room.

Re: Window condensation/frost despite 30%RH

eEre's the best dewpoint calculator I've found. Easy to use... and using the sliders, you can see the relationship.


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