Home & Garden

Cold Basement. (6-8'c difference with main and 2nd floor)

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  • Jun 26th, 2022 8:06 pm
[OP]
Member
Sep 19, 2014
488 posts
254 upvotes
Markham, ON

Cold Basement. (6-8'c difference with main and 2nd floor)

We had our basement finished last year.

Before the basement were finished, the temp is usually around 20'C. (The main floor is always set to 23'C year round)

But now, when the main floor is at 23'C, the basement is constantly at 16'C.
This is the same during winter when it is -20'C outside, or summer like now when it is 30+'C outside. (So I dont' think it is due to insufficient insulation)

I know cold air sink and hot air raise, but 6'C difference seems like a lot...also why would the difference became so much more after the basement is finished?

I tried shutting off all the vents now since it is blowing cold air now (The shut off is not air tight), but temp is still hovering around 16'C...

Any idea why that is and how I can minimize the temp difference?
17 replies
Deal Fanatic
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Jun 13, 2010
5407 posts
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GTA
Since you insulated your cold conditioned air is staying in your house (basement) and not leaking out anywhere near as much. So your basement will be colder in the summer with the AC running than it was before. To minimize the temp difference run the furnace fan 24/7 to circulate the air in the house.
Last edited by tew on Jun 25th, 2022 12:18 pm, edited 1 time in total.
Deal Addict
Sep 13, 2016
3376 posts
2121 upvotes
Mississauga
You can only do so much. Basement will always be the coldest floor. And a diff of 7-8 degrees is not out of ordinary.

This is what my sensors are reporting at the moment.

See how basement is a good 10 degrees lesser than a bedroom upstairs which is in front of the house and gets a lot of direct sunlight for most part of the day.
Screenshot_20220625-121516_Home Assistant.jpg
Deal Addict
Dec 19, 2009
4846 posts
2770 upvotes
Basements will tend to be cooler than the main/2nd floors because they do not gain as much heat from the hot summer temperatures outside because most of the basement is BELOW GROUND where the ground is cooler than the air.
[OP]
Member
Sep 19, 2014
488 posts
254 upvotes
Markham, ON
I just thought the difference is a bit much (6'C with main and 7-8'C with 2nd Floor)
And the difference wasn't that much before the basement was finished. (3-4'C difference)

tbh 16'C makes the basement almost unusable...
Sr. Member
Jun 23, 2019
501 posts
330 upvotes
Also in the summer keep the door to the basement closed so the A/C air from the main floor doesn't go down into the basement.
Deal Fanatic
Aug 29, 2011
7986 posts
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Mississauga
Try running the furnace fan continuously so that air gets circulated all the time throughout the house. I do that and combined with closing off the vents to our finished basement, I see at most a 2-3C temperature difference between the floors.
Deal Fanatic
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Oct 16, 2008
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^^^ Try that ^^^ also close door to basement in summer. Open in winter.
...
Member
Jun 2, 2007
439 posts
80 upvotes
Toronto
I agree with the comment to keep your furnace fan running 24/7.

Some furnaces allow you to adjust the air flow CFM for the "fan only" mode. If you dig into the manual you might be able to increase the rate, which would further help.
Deal Guru
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Feb 23, 2008
12571 posts
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Brampton
Run a dehumidifier to remove the dampness which does make it feel colder.
Also open basement windows to let in the warm outside air.
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Deal Fanatic
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Oct 12, 2007
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Ottawa
As above posters have said: keep the furnace fan running continuously. As another said, use a dehumidifier (or two). Also, make sure your cold air returns in the basement are open and check for the duct dampers - you want the air to have less restrictions against climbing to the upper stories. Finally, don't use a filter above MERV8 (MERV 6 is better in the summer in my experience) and keep it clean.
Deal Addict
Apr 3, 2006
1241 posts
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When having your basement finished perhaps they closed or blocked the cold air return. You need at least one in the basement with 6” duct to pull the cold damp air and will also more easily cool your house.

If your basement was finished properly with Vapor barrier on floor and walls you shouldn’t need a dehumidifier, from my understanding. Make sure your floor vents on top floor are all open in the summer, so not to keep air being kept in basement.
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Deal Expert
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Feb 11, 2007
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GTA
tew wrote: Since you insulated your cold conditioned air is staying in your house (basement) and not leaking out anywhere near as much. So your basement will be colder in the summer with the AC running than it was before. To minimize the temp difference run the furnace fan 24/7 to circulate the air in the house.
+1
blackn54 wrote: I just thought the difference is a bit much (6'C with main and 7-8'C with 2nd Floor)
And the difference wasn't that much before the basement was finished. (3-4'C difference)
tbh 16'C makes the basement almost unusable...
See above ^^^
Also, put some tape over your basement vents to seal them up, or shove a bag behind the vent.
Also be sure to open any return vents there, and if there aren't, you can just open up a return access panel, or your humidifier, which will suck in cold air and blow it upstairs.
Also seal any leaks in your duct work in the basement with foil tape or mastic paste.
mrweather wrote: Try running the furnace fan continuously so that air gets circulated all the time throughout the house. I do that and combined with closing off the vents to our finished basement, I see at most a 2-3C temperature difference between the floors.
+1

I do all of the above and my basement is still 5-8'C colder than the 2nd floor.
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Sr. Member
Jan 8, 2009
868 posts
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Simcoe County
What's the humidity level in the basement? Ours (finished) was that much of a difference until we checked the humidity (using a hygrometer) and started using a dehumidifier. Now it's a lot more bearable in the summer.
Deal Fanatic
Aug 29, 2011
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Mississauga
engineered wrote: I do all of the above and my basement is still 5-8'C colder than the 2nd floor.
Not sure what else to suggest. Maybe you have a lot of leakage in the existing ductwork? Apparently, somthing like up to 20% air loss due to leaks is acceptable. Before we finished our basement, I sealed every connection I could find with foil tape.

Right now, the basement is 22, the main floor is 24 and the second floor is 25.
Deal Fanatic
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Oct 13, 2008
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Durham
Well ... I just checked the Temps. All normal to me. And I work in the basement (home office in the basement).


Second Floor Bedroom ... south facing at the front of the house above the garage ... 25 degrees

IMG_20220626_142101.jpg

Main Floor Dining/Living Room ... middle of the house ... 23.5 degrees

IMG_20220626_142227.jpg

Basement ... unfinished ... 18 degrees

IMG_20220626_142156.jpg
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Feb 11, 2007
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mrweather wrote: Not sure what else to suggest. Maybe you have a lot of leakage in the existing ductwork? Apparently, somthing like up to 20% air loss due to leaks is acceptable. Before we finished our basement, I sealed every connection I could find with foil tape.

Right now, the basement is 22, the main floor is 24 and the second floor is 25.
Mine's also a 5 level split, so I think the temperature difference is exaggerated.
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Deal Fanatic
Jun 7, 2005
9579 posts
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Close all vents in the basement.

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