Winter condensation on windows in a house that is very well insulated.

This is my first post and I hope I have the formate correct, I live and build in central Pennsylvania. I built a house in 2004 using what I thought were the best practices at the time to insure it was tight and very well insulated. Took great pains to properly vent all bathrooms, dryer and range. I used Tyvek house wrap and a 6 mill vapor barrier on walls and ceiling. Used a pre-fab basement wall system that creates a very dry basement. Customer works for a propane gas co. and had a high efficiency gas hot air system installed.  They are having a very hard time with condensation on their windows in the winter. They are very unhappy with me because they feel the house is to tight and I should remove the plastic vapor barrier.  The only thing I changed was using Crestline windows instead of Anderson.                              As a side note I built 6 new homes using all of the same building techniques in my area and had none of these problems. We have discussed the use of exhaust fans allot and they assure me they use them. It is just Mom, Dad and a teenage girl so not an excessive amount water usage.     What should the RH be in the winter and should I just buy them a de-humidifier.  Oh, Yea they do have a heat recovery system they use when the temp reaches 50 degrees. Any help would be welcome.

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I can get you started Eric as I was asked to consult on a very similar situation.  The windows used will have a rating that will tell you at what temperature and humidity level condensation will form.  This will even happen to good windows under the right conditions.  First some questions.

Do they have curtains in front of the problem windows?

Are they monitoring the RH? And what is it?

What type of heat recovery unit is being used, HRV or ERV?

Have you done any pressure testing or duct leakage testing or calculated the necessary air exchange rate. 

Is there any Radon venting in place?

Building a house very tight also requires certain testing to avoid problems, condensation being one of the lesser of the issues.  Fill in what you can, but others will chime in with more specific advice.


Bud, first thanks for all you impute. They do have curtains in front of some of the windows but I do understand your concern with that and will discuss it with them. Not sure how closely the are monitoring the RH in the winter but we are going to address that for sure. I am pretty sure it is a is an HRV but I am going to check that. I am going to have a pressure test and blower door done soon for my own edification. Thanks again. Eric

The bottom line is that it's too humid in the house. I don't know if the vapor barrier is typical in your area, but it's hard to imagine it's the problem--far more moisture will be transported by air movement than by vapor diffusion, so while the poly might pose an issue if a wall gets wet and needs to dry to the interior, it's not so much an issue in terms of preventing the management of humidity within the house. Besides, how could you remove it without gutting the drywall?

The ventilation equipment is the answer. They need to use it more. Everyone will assure you that they use the bath fans, but they may need to run them for longer periods, say 30 minutes after each shower. Do they have timer switches? What's the chance a teenage daughter is taking long showers? Pretty good, IMO. What's the chance she's diligently running the bath fan? Not as good, also IMO.

And, what do you mean they have a heat recovery system, and use it when the temp reaches 50? Above 50 or below 50? Maybe it needs to run more, and maybe the control should be out of easy reach, so they don't turn it off. All of the ventilation equipment needs to be checked for proper operation (measure airflows, etc.) and it needs be operated enough to control this situation. 

People need to understand that they need a flow of outdoor air into their houses. Yes, this has an energy penalty. However, you did not build them a space shuttle where air is recycled. We use it, we vent it out, we replace it, we heat the new air. You could have done them a major disservice and built them a leaky house with uncontrolled infiltration--look Ma, no humidity problems. Instead, it sounds like they have a tight house and need to understand how to operate it.

Bud's point about blower door testing is good, you should have someone who is expert in to do that, and check all the other factors as well. I do a lot of blower door testing and periodically find a house that is exceedingly tight, at which point I talk to the owner about how they are going to manage it, since they are always moving from an older, less-tight house. It's an education issue.

All of this assumes you don't have something like a persistently wet basement that's slamming the house with humidity. You need to check all this and rule out all potential construction-related causes like that.

David,  I appreciate your information, The recovery system installer told them it was a waste of money to run it when the temperature was higher then 50 degrees I think I can get them to run it more often if that is the best way to run it. I did try to supply them an energy efficient house and they have been getting all this advise from friends, relatives and now even their insurance agent that the plastic vapor barrier is their problem and I am the bad guy. A last note the basement is the driest place in the house, it has a vapor barrier under the slab and sits on a bed of gravel and the pre-cast insulated walls are bone dry, there is also a radon system under the slab vented to the roof. I hope my local friend who does the blower door test and energy audits can help with some unbiassed information they will believe.  Thanks Eric

I think that some humidity measuring/monitoring is in order. You need to develop a picture of indoor humidity vs. outdoor, and if indoor is too high AND it's higher than outdoor, then ventilation is in order, regardless of what the installer said. My opinion is that RH at 40-45% @ 68F is the right range, except for perhaps humid periods during the summer when you can't get lower than that using outdoor air.

In dealing with these issues, it helps to have some tools for measuring humidity. The average no-brand $25 thermo-hygrometer from the hardware store could be quite inaccurate, and I would suggest investing somewhat more than that in a professional tool from Testo, Fluke, Protimeter, Fieldpiece, or another similar supplier. You need to be able to go in, get a reading, and know that it's within a few percent. It's also helpful to get one for your clients--there is one from Aube that's affordable and accurate, and if you can get them to use it, it may help.

If you really want to go for it, invest in a set of USB dataloggers so that you can track temperature and humidity over a period of time and then look for clues in the data.

I like what Robert is saying below. Verify the performance of the ventilation equipment, and then run it in accordance with ASHRAE 62.2 standards. Track humidity over time as the system runs and see what happens. Your blower door tech may have some tools for helping with these issues.

Which model Crestline did you use?

Is the condensation all over the window, bottom, or around the edges? 

Here is an example of a condensation chart:

What it is telling you is the point at which the inside RH and outside temperature will result in condensation.  Crestline should have a chart for the windows you installed.

Curtains or drapes or anything you place in front of a window will lower the surface temperature of that window and increase the outside temperature at which condensation will occur.  Note the example chart above is based upon 70° inside temp, thus everything shifts if the space between the shade and the window is 60° air, the window gets colder.

Remember, when dealing with Relative Humidity, the word relative is of prime importance.  70° inside air at 50% RH contains much more moisture than 40° outside air at 50% RH.  Even inside a home, it can matter where you take your RH reading as temperatures may vary, like behind those curtains.  There are charts and websites that will convert temp and RH to a new temp and RH.

The home I mentioned was well insulated and medium tight with only 2 older adults, no long showers and no steamy cooking.  No HRV, but they preferred 75° and humid.  Without adding any moisture the house ran between 50% and 65% RH.  The window mfg insisted they get it down to 35% before they would look at their complaint.  They installed dehumidification and made the 35% and the condensation continued.  These were builder's grade (model 200) name brand windows, the brand you didn't go with.  The condensation chart confirmed the windows would show condensation in our climate.  The builder selected the wrong window and especially the wrong one for this couple as they preferred 50%RH.

I hope some of this helps,


People tend to like RH between 30% and 70%, but cold-climate houses like between 20% and 40%, so the compromise zone is 30%-40% RH. In Canada, the code requires no more than 30% indoor RH. I shoot for 40% as an upper limit in northern New England.

If there's regular condensation on good double-glazed lowE wood or fiberglass windows, then there is a potential for moisture problems in the building envelope, including mold and rot.


I'll differ with Bud a bit in that window condensation (assuming at least double-glazed, non-metallic sash) is a sign of major indoor air quality and house management problems, as high indoor humidity means insufficient air exchange and high humidity can increase formaldehyde outgassing from building/cabinet materials and furnishings.

And I'll go a bit beyond David's comment about a "persistently wet basement" as even a persistently damp basement with no or little visible moisture, for instance from a slab with no vapor barrier, can cause humidity problems in the entire house.

If the house has an ERV, rather than HRV, then it's recycling moisture when it should be evacuating it (ERVs are more appropriate for hot, humid climates and HRVs for colder climates).

But no tight house can function properly with just intermittent spot ventilation - it requires whole-house ventilation whenever the windows are closed, either 24/7 or intermittent if properly programmed to meet ASHRAE 62.2 standards.

The vapor barrier, while unnecessary and potentially problematic for interstitial moisture accumulation within the envelope, will not contribute to an indoor humidity problem.

Hello Robert, great to get your feedback, are you saying the HRV should probably run allot more or continually? They have central air but it felt humid in the house the other day when I was checking out their concerns.

Whether or not your local building code required it back in 2004, as the builder it was your responsibility to include whole house ventilation providing fresh air 24/7 at the ASHRAE 62.2 rate of 0.01 CFM/100 SF plus 7.5 CFM/(# of bedrooms +1). That can be provided by a balanced system such as HRV or supply-only or exhaust only, but it's an essential part of any energy efficient house. Is the HRV integrated into the air handler or a separately ducted system?

At what outdoor temperatures are they experiencing window condensation and what type of glazing and frames did you install? The HVAC system should have been commissioned for proper installation and operation. Either something is introducing excess moisture into the house or it is not being sufficiently evacuated and diluted through ventilation.

The vapor barrier has no role in this, but clearly something is amiss and it sounds as if it's a lack of whole house ventilation, probably combined with insufficient use of spot ventilation (the bath fan must be on a timer so that it runs at least 15 minutes after a shower).

Thanks again for your input, we are meeting soon with the clients my HVAC guy and we will determine how the HVAC is installed, and my local Energy Audit Co. is going to a blower door test. (Side note I built his house ten years ago). The condensation happens in the winter when its cold but I really do not what temp. it needs to go down to to begin condensing. I am guessing in the twenties but I will check with home owner. The windows by the way are double glazed low-E in wooded frames vinyl clad exterior, wood interior. We are also going to go over all the exhaust fans again and upgrade or add timers etc. as needed. I do want to solve this problem.

Hello Robert, I first want to thank you for insight into this issue and responses. First I was wrong in that the house was built in 2000. Time flys. I followed the advise of getting a blower door test done and found that I had constructed the tightest home they had ever measured. Basement was dry and all insulation was correct. The problem was the HRC system was not being used properly and was tied to a hydrostate and the installer told them to only run it when the temperature dropped below 50 degrees. I took your input that as the builder I should have installed the HRV when I supplied them with such a tight house. I have made arrangements to reimbursed them for the cost of the HRV installation and also to have my HVAC guy properly tie it into the furnace and with the correct monitor system.    Hopefully this will correct the problem, HVAC and energy audit guys felt this would do it. They also helped my customer realize I had built the type of house they are trying to head their clients toward and that I was not a bad guy. 


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