We have a room projecting above our front door, with a concrete floor and 0.24m below that wooden rafters (if that is the right term, horizontal, 7 cm × 5 cm) beneath which a wooden roof (2.3m × 1.7m) is fixed, forming a small covered but not enclosed area in front of the door. The room above gets cold, so I would like to insulate the floor1 and was thinking of filling the 0.24m gap with glass wool (in two layers of 0.12m). I see that small amounts of water have in the past penetrated this space2; do I need to take precautions against a build-up of moisture, or can I expect it to disperse naturally?

1 I make take other steps, but am here only concerned with how to insulate this space (which I have open anyway).
2 There is some staining of the wooden strips, but no significant damage.

  • That first sentence is a doozy (pardon the early 20th century American automotive reference). Wooden rafters above a wooden roof? Maybe you can break it up for clarity.
    – isherwood
    Commented Oct 13, 2017 at 13:16
  • @isherwood: Sorry if it is not clear, but since the room overhangs the front door area, the roof is fixed to the bottom of the rafters – if rafters is what they are.
    – PJTraill
    Commented Oct 13, 2017 at 13:20

1 Answer 1


I'm guessing that by glass wool you mean fiberglass insulation. Assuming that is the case, you have a couple of options. The fiberglass itself doesn't care about water, it won't rot or grow much of anything, the problem is the paper backer. You could use backer-less fiberglass.

If you choose to do this you will need to be aware of a couple of problems. While the fiberglass doesn't care about water, it will hold water in contact with the wood of the structure. It also takes a long time to dry which will increase the potential damage to the wooden structure. To prevent this you must add a sealed vapor barrier between the insulation and wherever the water is coming from. If this is done well, you should be good to go.

Now my choice would be to use closed cell spray foam insulation instead of fiberglass. Closed cell foam is impervious to water and acts as it's own vapor barrier. It also generally has a higher insulation value than a similar amount of fiberglass insulation. The downside is that it is typically more expensive. Good Luck!

  • Since, in WIkipedia, Fiberglass insulation redirects to Glass wool I am sure you are right – thanks for your help. The glass wool I already have is a roll without backing, so that sounds OK. I may look into closed cell spray foam insulation, but I am almost there with the glass wool.
    – PJTraill
    Commented Oct 13, 2017 at 16:20
  • If that's your plan then make sure you get a good vapor barrier in place before you close up the space.
    – Haendler
    Commented Oct 13, 2017 at 16:21

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