Why insulate the walls rather than the floor joists in a crawlspace?

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Why insulate the walls rather than the floor joists in a crawlspace?

Ron Monk June 2, 2021Last updated: June 3, 2021

I find many crawlspaces where they are insulated between the floor joists rather than the walls. Is there any benefit to this or should the walls be insulated instead?

Responses (3)

Murray Kipp
Murray Kipp Sept. 16, 2021, 3:18 p.m.

Hi i have a small cottage in sothern ontario (Canada) than i am going to rent out for the winter, it was never used in the winter and we have installed a furnace in the small basement, the rest of the cottage is crawl space with dirt(sand) floor, there is a couple heat ducts set up to blow heat down there,, I will install insulation over the water supply lines with heat tape, nothing over the drains or pee traps. the walls in the crawl space has been insulated with 1" rigid insulation the cottage itself has been well sealed, the floor joists on one 1/2 of the cottage has R12 between the Joists, the other 1/2 looks like it has ceiling tiles between the joists for a bit of insulation, question is , should i remove the insulation between the joists and install rigid insulation over the dirt floor? or insulate between all the Joists? or would just vapour barier above the ground sufice? will the 2 heat ducts be enough to keep everything from freezing down below for the winter, also should anything be done with the drain lines?

also as a second thought the area that the floor is insulated with R12 is where there is no water lines would it make sense to curtain that area off and just focus on keeping the side with the water lines warmer.... the cottage is only 670 sq feet.... so not large

Mike Reynolds Sept. 20, 2021, 11:24 a.m.

Hi Murray, 

I think you have a pretty good idea of what matters here, so I would say yes, only keep the areas warm that you need to keep warm, where plumbing and any other mechanical components are. If you have insulation in the joists already that’s probably not a bad thing, that would enable you to keep it just warm enough down there to keep it operational, but not feel to much cold on your feet.

Most important is a vapour barrier on the dirt floor to keep moisture down. Next best if possible is insulating the floor as you will lose heat to the ground. Sounds like you are exploring that idea, so if its flat enough to lay some rigid foam sheets down on top of the poly that's great. Great if you can still walk on it carefully and not crack it due to uneven ground.

As long as the space is kept dry ( poly on the dirt and a dehumidifier) then the insulation in the joists will help save energy if the two spaces are different temperatures, not enough so that I would make a grand investment in material and labour to separate the spaces if they aren’t already, but it shouldn’t be a problem to leave it in place.