Negative Pressure?

[Ash]

Hello all,
I'm just wondering if anyone knows what "negative pressure" in a house is? We put a 18x24 two floor addition on our house and the 2nd floor is one large bedroom with a 10x8, 6x6 and 5 other windows. We get condensation on the 5 sliding windows upstairs but none on the solid windows or the ones on the main floor and I'm curious if this could be caused by negative pressure?

 

[DickRussell]

First off, "negative pressure" would mean the house is slightly depressurized, as it could be if you have a ventilation exhaust fan on or if you have an HRV (ventilation with heat recovery) and the input & exhaust are out of balance. Depressurization would enhance air leakage into the house.

On the sliders with condensation, if the condensation is inside the glass unit (I'm presume new, with double pane IGU), then the IGU edge seal has failed, and you need to replace it. More likely you are referring to condensation on the interior side of the glass, which you can wipe with a towel. That means that the glass surface is below the dew point temperature of the inside air. This can be caused, for example, by having drapes over the windows (for privacy or just sleeping in darkness) in the bedroom. That would add another layer of insulation on the inside, which would lower the surface temperature of the glass, yet interior air would be free to flow behind the drape or at least interior humidity in the air could migrate to the glass and condense. Finally, you could be running to high a RH in the house, with fish aquariums, lots of plants, a lot of cooking, etc.

I suspect the problem is not with the new windows up there leaking cold air, because that outside air would be dry and would tend to lower the RH in the vicinity of the window (inside).

The only other I can think of that would cause condensation on just those windows is perhaps there is dampness in the structure around the windows, from roof leakage, improperly flashed windows, etc.

 

[Ash]

Dick, thanks for the explanation. Never thought of it before but those are the only windows covered with drapes and we don't have issues with any others. I'll try pull the drapes away and see how that works. Thanks again

 

[DougA]

If the addition is new, this is perfectly normal. It takes awhile for cement, drywall, etc. to dry.

If the windows are covered with curtains, you are trapping colder air between and it will always want to condense on the coldest surface which is the glass. Warm air will hold moisture in it, cold air not. Wipe it down every few days and if you see mold on it, wipe it with a cloth that is soaked with water and a tiny bit of bleach.

 

[Ash]

Thanks for the tips Doug

 

[gzecc]

You guys are good.

 

[jebatty]

Sliders also leak cold air into the house around the sliding weather stripping, fixed windows do not. The leak may be very small, try a lighter around the edges and see if you can notice the flame flickering. Probably not be a big deal. Overall the new construction drying out process or window coverings. Plus as a bedroom with people sleeping, lots of water vapor is exhausted through breathing. If condensation does not disappear over time, you may need more ventilation between the bedroom and the rest of the house.