insulating draughty cavity under ground floor boards?

[Ala]

However, I have not heard a word suggesting that this will happen or not happen

[geraldthehamster]

Yes, but it's 25% of that element, not of all the elements of that type in the house. ie if you expose more than a quarter of the floor in one room. It's all academic anyway, who the hell is going to notify Building Control if they're insulating a floor, and what are Building Control going to do anyway, tell you to take it out again? Really not worth worrying about.

Cheers Richard

[The Natural Philosopher]

Basically talked this over with a mate, who is doing up house to sell when things improve.. No BCO, no ticket. No ticket no resale value increase.

No insulation!

[Tim Watts]

I never understand this - I don't fix houses for resale value, I fix it so I like it...

In your mate's case, if there really was a resale value potential increase, and he felt the BNA sign off was actually worth something, why not just get a BNA - for that value of work it would only be a couple of hundred?

[Tabby]

Maybe both. The ground is always damp though, in Britain. Even when it looks dry it isnt really.

NT

[The Natural Philosopher]

He hasn't got even a couple of hundred.

And the estate agent was dubious as to whether even so, it would increase the house value.

[The Natural Philosopher]

You haven't been under a house then?

Usually bone dry unless there is some sort of water table there.

[Tim Watts]

I guess that brings it back to - do it if *you* want to and stop thinking about estate agents :)

[Tabby]

Chuck a piece of plastic membrane down, come back next day and you'll have a wet patch under it.

NT

[The Natural Philosopher]

In his case it was simple cost benefit. He's been ill for years and cant do full time work. Wife works in estate agents, retirement looms, kids gone, so everything is 'what is the least amount of money I can spend using my spare time to maximise the nest egg'

Cost benefit of underfloor insulation was negative, on best advice.

In terms of resale value.

We all agree in terms of heating bills and comfort its a winner all the way.

[The Natural Philosopher]

On top of it.

Condensation.

Dew

[chris]

There has been an awful lot of misinformation and with some small amount of good corrections. It is best to insulate. Do so such that the timber remains warm and thus not at risk of condensation. If it is warm it does not need ventilation of the sub-floor airvents but they should be left in regardless for keeping moisture levels low in the abutting masonry and oversite. This is what I would recommend on our pro bono terms. lift all fbs labelling so they go back in same places use a proper floorboard lifter if you can hire or or borrow one if not use long long levers with flat iron heads glue any that split

When lifting use a hammer and fulcrum to knock them out with the nails rather than trying to lift the nailed boards up direct (like knocking the other end of a see saw) Put in polythene sheeting on the oversite and put on some sand to keep it down in place (reduces water vapour entry into void)

put in PU 4" foam with a careful cut filling any gaps with foam gun get under and put one or two inches of foil-backed PU under the lot cross ways using large plastic washers and long enough nails (or battens) and leave just a small hole for you to get up back inside and then put the last bit in and hold it up with string or whatever from the inside fill the last gap between FBs put on a vapour check such as thin poly Fbs back down either in right places or cramped together with a replacement to take up the slack

Job done Wall plates not at risk if on a DPC joists all warm - you warm as toast

can have boards or carpet as you wish U Value probably (guess) 0.25 bobs yer uncle

no decay risk from oversite

chris

[geraldthehamster]

Blimey, talk about putting the OP off. Yes, if you wanted to rip up your entire floor and get underneath it, and had an unlimited budget for PU boards, you could do all that. Theoretically you'd eliminate cold bridging across the joists, but really I don't think you'd feel the benefit under floor of 6 inches of PU board (which is closer to the standard for rafters, not floors), as opposed to 2 inches of board or a roll of space blanket between the joists.

Cheers Richard