New floorboards / DPM / insulation questions...

15 Jul 2016
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United Kingdom
Hello - long winded question / stream of consciousness re. installing new floorboards on suspended any advice appreciated if you don't get bored before the end...!

So - current situation: suspended timber floor with knackered floorboards in late Victorian house; intend to replace floorboards plus insulate whilst they're up. The sub-floor void is approx 0.5m to ground, ventilated with three airbricks (fairly sure B. Reg. requirements re. ventilation mm per m of wall are met / exceeded). Ground is not sealed. Joists are approx 80 x 100mm, spaced at approx 470mm centres, set on brick 'steps' (i.e. not embedded in wall but sitting on a brick at right angles to the wall) with a small piece of thick plastic DPM where the joist is in contact with the brick. The top of the joists are approximately level with the DPC (slate) in the walls. Joists appear in reasonable condition, but probably laid 15 years ago so uncertain of quality of wood used (pressure treated etc.). Moisture levels in the joists are perhaps a little higher than ideal (approx 16% on bottom of joist, decreasing to around 14% on the top) although there are no damp problems that I am aware of (but have not long lived in the house). The principle problem is that height is an issue with the flooring so installing new ply (etc.) base then new floorboards not easily achievable. Insulation will be wool fibre or similar, probably supported on netting, or solid supported on battens.

I'd really appreciate any advice re. the following...

- I am thinking of installing a DPM under the floor which (as I understand it) will (a) prevent moisture from the living area entering the subfloor and potentially forming interstitial condensation in / around the new insulation and (b) prevent moisture from the sub-floor adversely affecting the new floorboards above the DPM - is this interpretation correct?

- if the DPM is installed between the joists and flooring, does it need to be joined to the to the slate DPC, or can it just be run partly up the wall behind the skirting? Presumably the former would be ideal but would the latter work on the basis that the sub-floor is ventilated so (unlike a DPM in contact with concrete) there shouldn't be a significant 'water pressure' (sorry, not sure of correct term) pushing moisture to its edges (and so up behind the skirting)? Or would the alterations in water vapour movement caused by the DPM result in moist air being forced up behind the skirting in sufficient quantities to be an issue?

- and finally...given the height restrictions etc, how best to secure the new floorboards to the joists? Any screws or nails will obviously penetrate the DPM, but might this be acceptable given the above, specifically that the subfloor is ventilated and moisture levels in the subfloor and joists are only a little above typically targeted values? Would nailing / screwing through a a small amount of wet sealant help minimise the effects?

- any thoughts on the joist size / spacing? It's all solid and within the BR tolerances as far as I can work out for spans, although the spacing (470mm) is more than typically recommended for supporting floorboards - but not sure if this is BR requirement or flooring manufacturer thing. If I lift the floor, do I need to replace all the joists also...?

Any thoughts very much appreciated
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Personally I'd look to get some roof battens across the joists from underneath or use some reasonably waterproof ply to create a lip under the joist (or use nails spaced apart) and then buy either 50mm or 75mm Celotex board and cut to fit between the joists. This will do the job of insulating the floor a lot and keeping the moisture out without affecting the air flow to your existing timbers. If you find any radiator pipes do those too.

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Thanks - that's a neat job. Room looks pretty similar to mine. What flooring are you installing?

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