Cause of window condensation?

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Posted by: from Calgary
2/2/2015 at 5:48:01 PM

Our back windows are sweating during this winter. Do we need to replace them? Or could we simply put insulation or other methods that can solve this problem?

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Date/Time2/2/2015 at 6:10:02 PM


Your problem is caused by a lack of fresh air. You could install an HRV to solve this problem. Try opening a window over night, you will find no moisture. HRV's take fresh air from outside and cycle it through your furnace.



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Manuel from A.A.U. Construction in Toronto
Date/Time2/2/2015 at 7:35:58 PM

Hi Lina,

I agree with Jeff. HRV's will work. But also, if your windows are really old, they could definately be a problem.

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Rod. from Artall R.M. in Toronto
Date/Time2/2/2015 at 11:30:42 PM

Hi Lina,

Condensation on windows is caused by moisture in the air.

Set your home humidifier to lower setting, and it should be fine.



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Date/Time2/3/2015 at 10:34:36 AM

Hi Lina,

It is important to know more about the sweating before the issue can be diagnosed. If the windows are double or triple pane units, meaning there is two or three layers of glass in each glazing and the sweating is occurring between these layers, then your seal has been broken on the unit. If this is the case, you can either replace the window or there are companies out there that will refill the void between the layers of glass with an inert gas like Argon or Krypton and reseal the unit.

If the sweating is occurring on the room side of the glass then that is simply humidity in the house condensing on the cooler glass.

If your house has Forced Air Gas heating (duct work and grills on the floor) there should be a register (grill) located below the window units which permits warm air movement up the interior face of the window to alleviate this issue and to combat the lowest R-Value in the building envelope.

As it is February and this is about as dry as the air is going to get outside, it is unlikely you will be comfortable equalizing the relative humidity to that of outside, however if your relative humidity inside your house is above about 55% you will develop problems in your house both with potential mould and with hardwood floors and millwork.

An HRV is an excellent piece of equipment and very energy efficient as it pre-heats the incoming frigid air with the outgoing heated exhaust (while not mixing the two) and provides filtration of the incoming air. This is better for everyone inside the house and reduces the load on your furnace.

While I absolutely recommend an HRV in your HVAC system, if your house is older or unfortunately one of most subdivision homes, it is quite likely that your home is exchanging air with the outside in an uncontrolled manner, as much as 4 or 5 times an hour (100 times a day).

If you are going to invest in an HRV system, I would first recommend addressing sealing issues like caulking, insulation around windows and doors, etc.

Best of luck.

Jason Irving

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