How do I prevent winter window condensation on garage windows?
Data: The garage is unheated, uninsulated. The apartment above is both heated and insulated. The stairway from the garage to the apartment is insulated but not heated. There is a gasketed, self-closing door between the garage and the stairway. The Whisperlite fan in the garage is on a motion sensor and turns on when a car enters. The fan delay is set to maximum and stays on for about 20 minutes. The windows are are on the north and west elevations. They are fiberglass, and there is no low e or argon in these unheated locations. It’s been two years since construction, so the slab has cured. The contractor used a conventional 10mil vapor retarder beneath the slab.
The client reports that there is a lot of condensation on the glazing- both in the garage and the stairway – so much that the water flows down and the interior wood sills are rotting.
I can imagine a car bringing water into the garage, and I can imagine that the garage is slightly warmer than the exterior so that vapor is condensing on the interior face of the colder glass. I can’t see how the vapor is migrating up the stairwell.
Do any solutions come to mind?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Lydia,
There are only two ways to prevent condensation on the inside of windows. Either:
1. Reduce the indoor humidity level.
2. Raise the temperature of the window glass by adding another layer of glazing.
Have you checked the grading around the exterior of the garage to be sure that rain coming off the roof is directed away from the garage slab?
Storm windows should help.
Don't idle the car in the garage.
Robert and Martin,
The owner does not keep her car running in the garage for any significant period of time. The windows were installed in 2008 and are standard double paned. There is a significant overhang on the north, but we'll check the east side of the garage, this is an interesting thought.
Lydia
This is a difficult problem to solve. As the temperature of the unheated area drops, the relative humidity (RH) can increase dramatically. This means that air that has say 30% RH at room temp, can have a much higher RH at say 35 degrees F. Even fairly dry air can result in condensation on windows at temperatures just above freezing. You might try directing a small fan at the windows to help bring the inside glass temp up.
Interesting.
I just had another idea. Panasonic makes a continuous running low cfm fan. If we replaced the exhaust fan we have with one that runs at 30cfm all the time, it might keep the air moving enough not to condense out? I'm beginning to see the problem with your statement above. Our Pacific Northwest climate has typically moist air.
Lydia,
If the make-up air for the exhaust fan is outside ambient (high RH) air, then there may be little value in terms of condensation reduction in continuous ventilation. It may be useful to exhaust car fumes, however, as it is now functioning on a motion sensor.
It sounds like you'll need either some heat or a dehumidifier in the garage.
A dehumidifier might help, but if the temperature of the interior glass is in the 32-40 degree range,(roughly the same temperature as the condensing coils in the dehumidifier) the windows will still show condensation. Adding heat will help, but since the building is uninsulated, it will be costly and wasteful.
Lydia,
There are "solutions" that require almost no energy.
They can be found in Old Books
http://books.google.com/books?id=plY1AAAAMAAJ&pg=PA167&dq=William+a+radford+old+house+drawings#v=onepage&q&f=false
Predict where the condensation might occur, collect it,drain it....dehumidify for free.
They did it in "the old days"
mini-gutters,weeps,etc.,page 96,97....
Perhaps Add a sacrificial single glazed window with water collection?
Use a window treatment to make the glass even colder?
Nice Old book, Eh?
They even had triple glazed windows!
Moveable Insulation.....
And superinsulated walls!
Sure. Leave the garage doors open at night. If the inside air and the glass drop in temperature at the same rate, then there can be no condensation. That's why you'll never see condensation on the outside of a window (unless the space is air conditioned).
Actually,
Parking the car outside is not a bad idea.
According to Bill Rose Cold Cars attract water vapor.
Is the problem the unsightly condensation or the rotting sill?
I guess it just strikes me as funny....
Running a heater or dehumidifier inside a garage.. for a car?
Martin's Storm window idea sounded reasonable.
Is the stairwell connected to the interior space?
How bout warm air exfiltrating out of the home into the stairwell?
Could be a problem...
Thanks All,
I LOVE the old books link, thanks so much John! So far the no energy approach of adding something to keep the window glass warmer seems most sensible after all of this discussion. Brett's question about warm (moist) air from the apartment above is something we've wondered about, but both the garage person door to the interior stairwell to the apartment entry door are weatherstripped. We need to double check it to be sure it's working. This is our first certified green residential project (we're the architects), so it is a building science question we'd love to solve...why a well air sealed well insulated second story over an uninsulated but tightly built lower story connected by a stairwell and ventilated periodically by an exhaust fan might be having this problem. We're into year 2, so the possibility of rotten window sills is on the horizon if we don't figure the problem out.
Lydia
The fact that there is no low E coating or argon gas in the windows has a lot to do with your problem. The window glass is losing heat via radiation too quickly. If the budget allows, you could replace the glazing.....
Lydia,
How is the Air Control layer constructed?
Did you blower door test?
How tight is the living/stairwell , garage/living , garage to stairwell?
What is the controlled ventilation system for the living space?
When the garage is depressurized...where does the make-up air come from?
Is the slab insulated below the stairwell?
How about the underside of the stair?..how is it insulated and air-sealed?
The slab to me most likely needs to be sealed. Also a low temperature dehumidifier set to run at highest setting will help. I have installed two dehumidifiers for very cold unheated cellars in Northern NY and they are handling the humidity well. The only issue I have with units installed is they do not turn off completely, the fan always runs. That said, all is safe from moisture now.
I would try the dehumidifier first as it is any easy test even if the slab is not the cause. If you have one of your own, try it first. Big box stores also allow returns.
Further thoughts... your fan... good idea for venting car fumes... but... it's make up air where you are is full of moisture. Leave the fan set to run via motion and then turn off. Add the test dehumdifier and all will be fine IMO.