View Full Version : dewpoint while running AC inside walls
07-05-2012, 07:43 PM
Been reading threads that have referred to keeping a house to cold and causing moisture to form on the inside of walls. I personally set my temp to 70 and the inside humidity stays below 50%. My question is as temperature outside increases into the 90s and if humidity stays at 50% outdoors at what point should I start increasing my inside temp. From my calculations with an outdoor temp of 92 and a humidity of 50% I would be at the dewpoint inside my house if kept at 70 degrees. Can I get away with keeping cool indoor temps safely or should I be raising temps? We usually cool off at night so im not sure if that affects moisture in my walls. I did have insulation blown into my walls recently and do not want to cause mold growth. The insulation manufacture and installing company recomended against any form of vapor retarder since they said it would be better for the walls to "breath" in my type of aplication. If its true my walls do breath a little would the moisture be pulled into the house and safely removed by the AC?
Thanks for thoughts in advance.
07-05-2012, 08:53 PM
The claim is that insulating spray foam sealant will stop the hot moisture loaded air from getting to the cold interior wall so it works well to stop dew point condensation & any resultant mold.
If one wants to live in an ice box better use foam in walls & ceiling...
07-05-2012, 09:20 PM
If its 92 outside, and 70 inside your home. The drywall side toward the outside will be warmer then 70.
Next time its 92 outside, and you have your place at 70. Put a thermometer against an outside wall, it won't be 70 degrees.
07-06-2012, 12:33 AM
The scenario that can cause the most damage to a home in summer due to water condensing against interior wall materials goes like this:
Brick veneer or stucco exterior wall gets wet via rain or irrigation systems. Sun comes out and hits wall, warming it. Moisture is driven inward as well as outward. If there is no gap between brick and exterior sheathing, or if there is a gap but it is not adequately ventilated, the moisture keeps going through the permeable tarpaper (housewraps provide an exception to this) and into the wall cavity. From here things get a little tricky. If the indoor wall material is drywall, as most homes are, and it has been painted with a water based latex paint, the moisture may be able to diffuse through the drywall into the drier home without an issue of condensation on the backside of the drywall.
Beenthere is also correct in that most wall cavity insulation systems will about guarantee that the cavity side of the drywall is warmer than the indoor side. Wall cavities with fiberglass insulation that are not airtight on all six sides see air moving through the insulation due to convection and air leakage. This will keep the potential condensing surfaces above dew point.
However, if the drywall is covered with an impermeable paint or wall covering (such as vinyl wallpaper), then the risk of condensation on the backside of the drywall, combined with cold interior temperatures, is very real.
Have you ever been in a hotel room with vinyl wallpaper and seen pink spots on the vinyl? That's mold, induced by moisture, eating away at the wallpaper glue. Known as "Hilton Hives" or "Marriott Measles", it is because the wallpaper won't let the drywall "breathe", so moisture condenses on the material, wetting it, and leading to a climate favorable for mold growth.
07-06-2012, 03:32 AM
You should be fine, Oregon like California have drier summers. I am not sure where you are located but I picked Corvallis (worst case) has a design wet bulb is at around 69 F and dry bulb is 92 F, so when it gets to 92 F, you will get around 30% humidity.
Also in colder climates, you might have to worry more about condensation during the winter when your house is often more humid and hot than the colder winter air, thus the air moisture runs from interior to exterior.
Both cases, I really don't think you have to worry that much compared to the people in the southeast part of the country.
07-06-2012, 09:26 AM
Thanks for everyones input. It sounds like I dont have anything to worry about. Thanks for your time.
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