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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Permalink Reply by William Reid on September 12, 2012 at 8:08am

I think you must be using more elaborate HVAC systems in the US than we use here in the UK. An average whole house ventilation system here with HR (<90% efficiency) will use about 120 kW/h per annum - that is for 22 hours a day at trickle and 2 hours a day boosted. That costs about £20 per annum before the heat recovery saving is accounted for. In highly insulated houses, ventilation loss can be the largest heat loss component if no HR is provided.

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Permalink Reply by Robert Riversong on September 12, 2012 at 9:37am

Everything in the US is more complicated. For one thing, we have either longer heating seasons or longer cooling seasons. We have much bigger homes. Most homes have an air-handler HVAC system and the ventilation system is often integrated into the existing ductwork which often requires a large, inefficient air-handler fan to operate in order to distribute fresh air.

In the CARB study, with three new highly-efficient and tight houses, the three tested ventilation systems (including associated air-handler consumption) used 4309, 3714 and 263 kWh/year respectively, with the last being two 20W bath exhaust fans running 67% of the time, which translated to $398, $379, $76 annual cost deducting for gas savings from heat recovery for the first (with electricity at $0.10/kWh).

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Permalink Reply by Chad Robinson on September 12, 2012 at 8:00pm

This may be obvious, but the worst humidity problem I've seen in a tight house was caused by a whole house humidifier set on high.

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Permalink Reply by Robert Riversong on September 12, 2012 at 8:06pm

A house in winter is dry only if it's leaky. So "correcting" the problem by running a humidifier to add moisture to the air that's leaking out, will increase interstitial condensation and potentially cause mold and rot.

Other prime culprits are damp basements or crawlspaces, unvented clothes dryers, lots of cooking without using a range hood, lots of showering without using an exhaust fan, lots of aquariums or potted plants, or drying firewood indoors. Jacuzzi parties also don't help.

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Permalink Reply by Chad Robinson on September 12, 2012 at 8:21pm

My point was, the humidifier was running when the house was already too humid. The house was tight.

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Permalink Reply by Robert Riversong on September 12, 2012 at 8:26pm

Why?

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Permalink Reply by tedkidd on September 13, 2012 at 8:10am

Chad, you beat me too it! How many times you go into a house with humidifiers running on the first and second floors, and a dehumidifier running in the basement!

I was at a house once, rainy day, windows open, and humidifier running. Woman blamed us for condensation on her windows.

People cheap out when buying equipment then have all these avoidable problems. Communicating equipment will manage rh based upon outdoor temp to keep condensation off windows.

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Permalink Reply by Larry Nissman on September 13, 2012 at 9:04am

I was going to say what Chad said. Somebody should make sure that they don't have a humidifier running.

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Permalink Reply by Robert Riversong on September 13, 2012 at 9:12am

I'd still like an answer to the question that Chad hasn't offered. Why do people run humidifiers if the house is already too humid? There must be a reason they think it's too dry.

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Permalink Reply by Larry Nissman on September 13, 2012 at 9:26am

Robert, I think Chad was suggeting that the humidifier might be running and they don't realize it is running. I would assume somebody has checked this out, but just in case it is worth confirming.

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Permalink Reply by Chad Robinson on September 13, 2012 at 11:49am

Robert, as everyone here knows, people do stupid things. Running a humidifier when its already too humid is not exempt. I would bet there are more people that don't know what their humidity level should bethan those that do.

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Permalink Reply by Robert Riversong on September 13, 2012 at 11:59am

I agree. I suspect there's not one in a million who could give a number for proper indoor relative humidity (including most building professionals), but I would also think that most people would know that when there's water running down your windows it's too humid inside.

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