Author Topic: humidity control for new home construction  (Read 2510 times)

Offline oldguy2

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humidity control for new home construction
« on: February 15, 2015, 05:37:53 PM »
wondering if anyone else runs into the odd customer that just cant understand what you tell them. Builder tells me they have a problem with a house where they get condensation on windows. I go over to look at what's going on . t-stat set at 22/ 30%rh on hrv setting.Find out blinds are closed . tell them they need to open them to get air circulation across window and with cold temps they need to lower RH setting below 30%. Well bitch and complain. house is to dry windows are crap. hardwood floors will dry out . the hardwood floor people are telling them to maintain 35-50% the window people are telling them nothing wrong with windows humidity is too high. builder tells them to open blinds up at night.and owner tells me its too dry in the house with RH at 30% I know I can't raise the RH with the extremely cold temps if they open their blinds it will solve the problem but they won't. they are basing everything around the hardwood floors. so I'm ready to bang my head against the wall I give up drives me nuts

Offline Admin

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Re: humidity control for new home construction
« Reply #1 on: February 15, 2015, 11:07:26 PM »
This is pretty common with new homes in my area.  The new homes can have a lot of moisture in building materials than can take time to dry.

All we can do is verify our exhaust fans, dryer vent and HRV are working.  If there's a humidifier installed refer the client to the user manual for proper settings.  Everyone says to keep the house between 30-50% RH, but this will lead to moisture on windows.  On very cold days below 10F/-12C you should have less than 30% RH otherwise the air will contain too much moisture and will condense on cold windows, and sometimes form ice.  The client has to understand that with today's tight homes this is to be expected.  They can't have an indoor RH above 30% and expect to have dry windows.  At -20F/-29C we need the RH at 15% or less to avoid this problem. 

One solution is we can use an automatic humidistat control with an outdoor sensor.  If the control has built in window frost protection technology we should be able to avoid moisture on windows and never drop below 30% RH.  Keep in mind there is no avoiding moisture on windows when the outside temperature changes drastically.

In my area during the winter months the outdoor RH is usually above 50%.  An HRV will only effectively remove moisture when the outdoor RH is lower than the indoor RH.  The dial on some HRV controls can add to the confusion.  For example with the Vanee Bronze control, the dial simply switches the low speed fan to high speed and the dial only works if the control is set to MIN.

I agree, advise the client to keep the curtains and blinds up and remove the screens in the winter months.  They may also want to use stand alone dehumidifiers.  See if they are using a thermostat setback of more than 2C.  If the setback is greater than 2C it could be leading to the excess moisture problem.  They can also try running the furnace fan continuously, but if they are using the HRV it's usually interlocked with the furnace fan anyway.

If the client wants to protect their hardwood flooring warranty then they need to live with moisture on the windows.

Refer the client or builder to The Tarion Construction Performance Guidelines Section 8.5,


Acceptable Performance/Condition
Condensation may occur on interior window surfaces.


Damage caused by dampness or condensation due to failure by the homeowner to maintain adequate ventilation is excluded from the statutory warranty.


Condensation occurs when water vapour in indoor air contacts cool surfaces such as window glass. Condensation on interior window surfaces is common during cold seasons. When outdoor temperatures are well below freezing, ice may form at the bottom of the window.

Since it is important for homeowners to maintain proper humidity levels within the home to prevent damage to other components such as hardwood flooring and for physical health, some condensation on windows may be expected.
Indoor humidity levels may be controlled by dehumidification, ventilation or air conditioning.

Interior air moving over the windows can help control condensation. Heavy draperies or window coverings that cover windows and block heat diffusers can prevent air flow. Running the furnace fan continuously in conjunction with the principal exhaust fan, can also help to control condensation on windows.

See Appendix A3 “Moisture and Windows”

I attached Appendix A3 and it shows that a low RH is required to prevent moisture on windows.  You can read the NRC and CMHC documents the Tarion guide references in the attachment below.

Offline walker

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Re: humidity control for new home construction
« Reply #2 on: February 16, 2015, 03:54:24 PM »
its going to have to be a trade off, if you want proper humidity levels, you're going to have to deal with the moisture on the windows, if you don't want moisture on the windows then you're going to have to deal with dryer air, and the problems that come along with dry air.  It's a pretty simple question to the customer, which do you prefer, no moisture on the windows or dry air that could lead to cracking in the hardwood, and other health problems?