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Community and Q&A

House Heats up at night on warm days

Jeff Classen | Posted in Energy Efficiency and Durability on

I have a 918 SQFT home on a crawlspace in Kansas City area. Attic air sealed and insulated to R-40+ Walls dense packed with Cellulose to R-15, .26 air changes. furnace/air handler is in the attic with insulated ductwork.

Last night I went to bed and it was a reasonably nice 77 degrees. I have not turned on the Air conditioner. Outdoor temperature got to 69 last night, but when I woke up the house was at 81 degrees. I am struggling to figure out why a 4 degree temperature increase overnight even when it is mild outside.

Should I be looking at attic ventilation or to the crawlspace?


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  1. GBA Editor
    Martin Holladay | | #1

    The most likely reason that your house heats up is that it is relatively well insulated, so that internal loads (your refrigerator, especially, along with all other electrical loads that stay on all night, including such items as your internet router, electric clocks, doorbell transformers, night lights, etc.) -- all of which give off heat -- are able to raise the indoor temperature.

    Solutions include turning off all unnecessary electrical devices, opening a few windows, and installing a whole-house fan in your ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    I"m also guessing that you get up after dawn, and you have some amount of east facing windows with a lot of AM solar gain. Exterior shades on east facing windows should make a difference if that's the case.

  3. Sonny Chatum | | #3

    During the shoulder seasons, Spring and Fall, a tight, well insulated house can work just a little bit against you, especially if you really have achieved 0.26 ACH. In addition to Martin's and Dana's comments, there is body heat from occupants that can sort of work against you this time of year.

    Perhaps more importantly, any passive solar energy gain this time of year tends to work somewhat against you at night. If you have south facing windows, even if they are passively shaded to the proper amount, that shading isn't perfect during shoulder seasons. There is always some amount of thermal mass in the house that can store up a little bit of that solar from the day and then release it at night, causing a bit of temperature rise. Bottom line, as already noted, you can manage this somewhat with opening windows. Be careful of whole house fans. They can be great for the relatively short shoulder seasons, but can sometimes just sit around and be leaky for most of the year. In addition, if climatic humidity is anything of an issue in KC, then, with a whole-house fan, before you know it you are pulling in humid air which feels terrible, even though it may be cooler.

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