If a home is too tight and does not have provided mechanical ventilation, can it draw in humidity and excessive heat during peak times?
matt.tucker
| Posted in Mechanicals on
I have a customer that purchased a home eight months ago and all summer long he has complained to the builder that the temperature and humidity levels are way too high. I performed a blower door test and found that the home is 2000 cfm lower on infiltration than what BPI recommends.
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Replies
Matt,
First of all, all tight homes need a mechanical ventilation system. For more information on this topic, see Designing a Good Ventilation System.
Second, building a tight house is a good thing. It limits the infiltration of humid exterior air into the house.
Third, in most cases, the best way to control the indoor humidity during the summer is with an air conditioner.
If the homeowners don't have an air conditioner, the usual practice is to open their windows -- in which case the tightness of the home and the absence of a ventilation system becomes irrelevant.
In a few very humid locations in the U.S., tight air-conditioned homes sometimes need supplemental dehumidification. For more information on this issue, see All About Dehumidifiers.
Finally, high indoor humidity is more of a problem during the first year of occupancy of a new home. That's because it takes a while for the construction moisture (from damp concrete and damp drywall compound) to dry out. Everything should get better during the second year of occupancy.
Martin,
Do you have any numbers on how much construction moisture in a typical house? We happen to building a house during a time of record breaking rainfall in the South and I figure it will be significant.
Thank you.
Lucy,
Here's a source of information:
http://www.nationalfiber.com/docs/SprayAppliedCelluloseInsulationandMoistureFacts0909.pdf
An "average 2,500 sq. ft. building contains: 100 yards of concrete, 10,000 board feet of lumber, 6,000 square feet of drywall, paint and other finishes, adding up to 3,000+ gallons of water released over the first few years of occupancy."
Lucy:
I am in the same boat, and my Earthcraft rater is recommending we heat the house to 85 degrees for two weeks prior to moving in. He says the heat will also accelerate outgassing of construction materials. I imagine it will also dry things as well The tricky part will be raising the temperature to 85 degrees in December, when the house should be far enough along for that step.
Matt Risinger also has a great post on moisture and humidity control during construction (http://risinger.blogspot.com/2013/02/drying-wet-framing-lumber.html?q=moisture). He uses portable HVAC on some of his projects to bring down moisture levels. I'd like to try it with my own project, but the portable HVACs can be expensive to rent.
All you need is a portable dehumidifier like a Drizair 110--not a household model but the type used by remediation contractors. Any rental shop should have them. Keep the building heated and use a few fans to circulate air inside and you will remove a lot of moisture.
I guess I should have mentioned that I live in "Famously Hot" Columbia, SC so the customer does have AC, two systems in fact, one up and one down. The one upstairs is the main trouble maker. I did a load calculation, he is a little undersized, but that would cause more dehumidification than anything.
I did not think/know about the construction wetness, that is a very valid point. Thank you all for commenting!
Matt,
I am up the road a little way in Greenville, SC. Our weather is a little more moderate, but still hot and humid.