My question is about insulating the ground floor. I have a crawlspace under the floor which is vented and therefore quite cold (average temp in the winter is 4 °C and in the summer 13 °C plus there are loads of cold winds).

I think I would like to do it without stripping all the flooring - I would like to use the crawlspace to insulate it from below.

I have spoken to some 'well versed colleagues' and there is no consensus - options that have been suggested so far are

  1. Squeeze blocks of styrofoam between the joists from below
  2. 'Hug' entire floor with insulation foil from below
  3. Take glass wool and 'hang it' between the joists with some foil or panels

Now I am not sure which method is best (or none of the above?).

Can you please suggest best course of action?

  • i would suggest you put insulation along the vertical walls, preferably down to 0.5m deep and 1m horizontally along the sides of the rectangular floor. Here is an interesting article of what they do in America ornl.gov/sci/roofs+walls/insulation/fact%20sheets/… – user13556 Jun 19 '13 at 11:16

There's nothing wrong with fiberglass insulation beneath the floors. It will be, most likely, the most cost-efficient solution.

If you have a situation where critters like the insulation, well you have a vented crawl space! Critters get in there.

If you have an issue where the insulation gets moist and sags, the problem is not the insulation, it's a moisture/humidity/temperature issue! If you have, especially during the summer, humidity circulate beneath the home and begin to soak into the insulation, you have a more serious problem and using a different insulation only ignores or covers it up.

Not knowing exactly where you are, putting plastic sheeting on the bottom of the joists is a potentially dangerous idea! You are setting up a temperature difference between subfloor and bottom of joist, and then trapping moisture in that area. A plastic 'vapor barrier' in this area can cause a substantial amount of condensation on the joists leading to wood rot.

Plastic on the soil.

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  • Welcome to the site; please see the faq for the rules on self-promotion. – Niall C. May 3 '13 at 15:44
  • I disagree with straight fiberglass in these situations. You are building nests for all of the nearby animals. Currently owner has nothing in the joists and no rodent issue. I am sure if you are a reputable contractor you would fix the new rodent issue a year later right? Plastic sheeting on joists wasn't meant to be a vapor barrier, it was meant to hold the blow in insulation. Add lots of holes if you want for proper breathing - if there ever becomes a moisture issue. Also what you are doing is putting fiberglass on the joists and subfloor. Both will be wet now if moisture issue. – DMoore May 3 '13 at 18:49
  • And this is my educated opinion - this would be the last thing I would do. The XPS and dricore is a much better solution. I offered something that I have done because I have heard nothing but good feedback and it was much cheaper and easier to do than XPS/dricore. You need to read the Building Science mold moisture rentention findings (mold/rot) before you start stapling fiberglass to everything - and that isn't getting into new critter problems. – DMoore May 3 '13 at 18:53

I think I would be inclined to use XPS foam cut and inserted between the joists. Caulk around all of the seams. This is similar to how you insulate rim joists in a basement. It would also act as a vapor and air barrier.

Another option would be closed cell spray foam but this is usually expensive and not a DIY job.

If you were willing to do it from the top of the floor then you might look at a product like DriCore:


(Not affiliated with the product - just something that is commonly used in basements in my region).

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    ok that is a ton of work and money. Might as well spray foam. – Tom Apr 25 '13 at 16:06
  • XPS is generally cheaper then spray foam, but that is definitely an available option, though not usually DIY. – Steven Apr 25 '13 at 18:01

You need to have plastic sheeting - thicker the better stapled to your joists on the bottom with the insulation sandwiched in between. This is a must to first keep wind out and second to protect your insulation.

Additional info - I don't think I would press any insulation accept spray foam up to the subfloor. Using the Building Science findings and applying it to this I would keep a couple of inches of breathing room between your subfloor and insulation. I took the spray foam out because it is generally a vapor barrier in itself. You could get some spray that contracts yet that still is OK since it will not harbor water.

Even with the plastic sheeting I would not want to chance my insulation getting wet and transferring the water to the subfloor. Currently home owner has no water issue, so why start one. So I would definitely leave a 1-2 inch gap between insulation and subfloor.

And it is a crawlspace. Any insulation you add is going to be an attractive home for rodents (mice, rates, squirrels, chipmunks... and also some bugs). They want to enjoy the warmth too.

So my final advice on this would be this. You cover the joists with a plastic sheet barrier. Then blow in cellulose insulation treated with boric acid - this is usually used for attics. You might be able to do your whole house for a few hundred dollars and won't be causing water issues or pest problems. The other option is spray foam - and I would still protect it with a plastic sheet.

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  • Why no love for fiberglass? Properly installed with a good vapor barrier, fiberglass is an effective insulating material. – Evan Johnson Apr 25 '13 at 15:27
  • Sags and condenses too much with moisture. Experience tells me no fiberglass when water/moisture is involved. – DMoore Apr 25 '13 at 15:55
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    Thanks for satisfying my curiosity. A lot people in my region recommending fiberglass under floors, but we do have a very dry climate. – Evan Johnson Apr 25 '13 at 18:15
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    This is bad advice. Placing a plastic barrier on the unconditioned side is creating a moisture trap that could result in condensation inside the insulation. The reason tyvek and other house wraps are used on the exterior of the home is that they breath water vapor (especially in the winter) while still blocking liquid water that may get behind the siding. A house wrap material would be much better on the unconditioned side of the insulation under the house. You still need a vapor barrier on the conditioned side of the joists. – BMitch Jun 19 '13 at 11:49
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    @DMoore Mike Holmes says to always put the vapor barrier on the warm side and house wrap on the cold (i.e. unconditioned) side: theglobeandmail.com/life/home-and-garden/real-estate/… and nationalpost.com/homes/Mike+Holmes+down+crawl+spaces/3958820/…. The only time I've seen him put put poly on the cold side was as a backer for spray foam, and spray foam is a vapor barrier itself so you don't have the condensation issue. – BMitch Jun 19 '13 at 22:15

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