Underfloor joist pocket conundrum - to fill or not to fill?

Hi Whoever can help
We are currently carrying out retrofit on a 1910 house. Specifically insulating suspended timber floors from the crawl space below.
The timbers and floorboards are all in good condition.

The plan is to:

  1. draught proof: seal holes around central heating pipes; parge bricks between joists upto and behind floorboards with Diathonite
  2. use Thermafleece between joists
  3. cover the Thermafleece on the cold side using a windproof membrane (Thermafleece breather membrane ) taped (Tescon Vana) to the parge (Diathonite) on bricks.

Our conundrum is what to do where the joists sit in the brick wall.
Currently the brick pocket is open all around the joist (except where the floorboards sit above and the joist sits on slate or brick below) i.e. the timber is well ventilated.

Our thinking is that the insulation steps above will effectively put the open space around the joists that were on the cold-side into the ‘warm-side’. At the end of the joist there is only a single brick of the outside wall, so if the space is left open are we creating the potential for condensation?

Would appreciate a view on the following:
Should we leave the space open or fill around the joist?
If we fill what is best to fill with? (in some cases access to the outer most end of the joist is limited).
Hope someone can help, thanks
Don & Helen

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Hi Don & Helen, and welcome to the Carbon Co-op Community!

This is a great question, but unfortunately there are quite a lot of complicated factors that make it impossible for us to give specific advice on this through the forum.

Hopefully someone else is able to share some general information which might be useful for you going forward, and we wish you the best of luck with your project.

We have had our floor insulated from underneath with access via the crawl space (as you described).
I guess there is a potential risk for interstitial condensation around the joist ends and this was a decision we took on consciously. Our subfloor void was quite dry as were the joists. So basically we took the risk.
So far 6yrs on all seems to be OK.

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Hi @don_helen,

I just attended an airtightness webinar where the same, thorny joist question was asked by a well known eco builder merchant… the response was:

a) cut the joists and use a perimeter joist wherever possible
b) use a putty to seal the joist end

I didn’t catch the product name, but the webinar recordings are available to members (£60/yr, well worth it), though the second approach sounds similar to parging round the joist ends and sealing with an airtightness tape.

I’ve sealed floorboards from above, taping to walls under skirting boards for well ventilated voids, but would be tempted in future to place a moisture sensor into the timber before sealing up.

It is worth highlighting that Carbon Co-op members have access to free AECB membership, which you can access through our member hub. You should be able to follow-through the link to the discount from there, but let me know if you have any trouble!

You might also want to look at the AECB members page where you could find someone to draw up a design?

Can;t help with the joists but as a word of warning we used thermafleece (wool) in our underfloor and one of the batches we used was obviously not good as mold started to grow on the wool.

I have just put sheepwool in my loft…I haven’t heard of mould growing on it before. Is this something that anyone else has experienced??

@pottyone72 I fitted the first area of sheepwool in the very early days of this product and it is only that batch that we have had a problem with other batches either sub floor or in loft. To be honest we hadn’t noticed till Marianne came round to do our retrofit survey and she spotted the growth. I have removed the affected area and the joists are all sound so it seems it was just something in the wool. My guess is that the production process has changed and that anything you get now will be OK.

A question for anyone who has fitted underfloor insulation on a suspended floor.
Is it a good idea to include the central heating pipes within the insulated area.
Currently our central heating pipes are hung about 6 inches below the joists with just some pipe insulation round them but I am wondering if I should fit a new plastic system and incorporate it as part of the insulated area?

The essential thing is to make sure the pipe is insulated. Plastic pipes will undergo movement i.e. expansion, straightening and contraction so might be best outside the insulation - depending on how it is secured (stapled, glued, taped). The critical point is to know where the joints are on the heating system and the points of access should you need to go back in. T