Insulate uder floorboards, Air Brick & is this 'damp' normal?

16 Jul 2011
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United Kingdom
Hi all,

One day I will post more on here with aswers rather than questions, but hope someoe has come accross this before,

I have taken up the floorboards in our lounge because of woodworm and it being horrendously cold we are goig to put in PIR boards between the joists, 2 part query:

In the first couple of pictures there has costantly been a 'wet bit' along the floor next to the wall, the ground outside is just above this and am thinking I will need to dig outside a bit to fix? - Or is this just normal and it comes and goes, or should the sub-floor really be bone dry?

The second set of pictures shows an air brick with the joist in that hole, I am using 50mm PIR, the joists are 100mm. Any advice about how to make this as air-tight as possible without compromising the air brick. My current thoughts is to staple some DPM at a sloped angle away from the air brick at roughly a 45 degree angle and then start the PIR from that point onwards, I would have the DPM going up the wall a bit too so hopefully any draughts go under the floor rather than into the room?
Second part of question 2: Also, anyone who has used PIR boards before, any suggestios for the really small gaps betwee joist and wall - just expanding mouse, then tape over the top?

Thank you as always in advace for any and all resposes.



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6 Feb 2019
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United Kingdom
It looks as though the one wet joist is touching the wall below the DPM. Protect that side of the joist or prevent it touching the wall.
It looks like the joist ends have been protected by a preserver of some sort. You could lift them a little and ensure they are sitting on a DPM, even a DPM wrapped up the sides and ends of the joist.

Your air brick can accommodate a telescopic/periscope type air brick.
If necessary, build a part dwarf wall inside of the existing to support the joist (Like the existing dwarf wall half way across the room.), tied into the main wall.

You could use a fibre insulation (cheaper) by fixing netting to the sides of the joists allowing it to hang down to the bottom of the joists.

Yes, expanding foam.

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