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Will adding insulation under floor make my room warmer in summer?

I have tried to get information on this by searching internet without success, hope someone might be able to help!

Our home was built in 1904.
We have restored original wood floor for ground floor. However, some drafts/ chill air come from unheated basement beneath. We thought that putting insulation in basement ceiling between wood joists, might help with the drafts and possibly save a little in heating (because rooms above basement might retain heat better from our forced air furnace).

HOWEVER - would the insulation make the room warmer in summer? We use a window a/c in the hottest part of summer. But I assume that cooler air in basement below might also keep room a little cooler than it would be otherwise. But insulating basement ceiling would keep that cool air from rising. Would it result in warmer room and more air conditioning energy expense in summer?

If you have any relevant knowledge to share we'd be very grateful!

Re: Will adding insulation under floor make my room warmer in summer?


By cutting off the 'chimney' effect' of the cold drafts you'll be saving on both heating and cooling. You'll fare best by using unfaced insulation in that space, snugly fitted, then cover all the basement ceiling with a vapor barrier.

This may have the unintended effect of skyrocketing the humidity in the basement as you are now trapping all the air in the basement with no air exchange. Mold, rust, and rotten wood can easily arise from excess moisture trapped in the basement.

I'd do a test first; 1- Buy a barometer and keep track of the barometric pressure. A humidity sensor will also be great. Check once a month for a year. 2- Staple sheet plastic all over the ceiling 3- run the testing again. 4- if the readings don't change, insulate as you wish.

If you don't want to invest the time or are into winging it, insulate and seal the air flow and see what happens, but be prepared to take it all down if you find the humidity is climbing.

Re: Will adding insulation under floor make my room warmer in summer?

Cool air does not rise, it settles so the insulation will help you in the summer as well by blocking the cool air air you are paying for from the AC from settling through the floor boards and into the basement.

Re: Will adding insulation under floor make my room warmer in summer?

Sealing and insulating any ducts that are in unconditioned space will provide greater cost savings than insulation under the floor. If your ducts are uninsulated metal and the seams aren't sealed (painted with a gray substance called duct mastic or covered with foil tape), do that first -- it will provide the greatest return on investment. The duct mastic is messy, but easy for a do-it-yourselfer. You smear it on with rubber gloves.

That said, insulation will probably help keep your house cooler in the summer, even if the space below is cool. You see, that space isn't as cool as the air traveling through your ducts.

Re: Will adding insulation under floor make my room warmer in summer?

Insulation isolates a conditioned area from unconditioned ones- the outside, or in this case your basement. Thus, it will aid the keeping of the heating or cooling in the places you want it, so it alone will not make a room warmer in thew summer. Vapor barriers (including the paper face on insulation) is designed to be used only between exteriors and interiors, so you don't want that under the insulation here- it belongs between the conditioned space and the insulation if it is used at all. A vapor barrier under the insulation here will allow moisture to collect in the insulation rendering it less effective and introducing other moisture problems. Impermeable insulation like closed-cell foam do not need a vapor barrier and cannot hold moisture, and with their not needing additional covering make them a good choice here. Note that there may be a requirement to cover if there is any heating or water heating equipment in the basement- you local code inspectors can answer that question for you.

Whatever approach you take, be sure that what you cover up is not going to need to be accessed later on. Seal the ducts now, and replace any galvanized plumbing before insulating (always replace all galvanized, it does not last). Also make sure the basement is ventilated well enough and mitigate that as a separate issue since the insulation does introduce a sealing of the house it didn't encounter before. You didn't mention it but insulation's value is best seen from the top down, in other words it's most effective in the attic, then the walls, and then under the floors so that if the rest is in order, insulating here will not reduce costs by a large amount but it will probably be cost-effective and it will certainly make for a more comfortable home which has a value of it's own apart from monetary benefits.


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