Underfloor heating, damp and finally able to ask PigletJohn an actual question!

[bajgal]

Dh and I are just about to have our crumbling victorian mid-terraced house refurbished, so no more wires dangling out of the walls or cold ground floor bathroom we hope! I want to have underfloor heating on the ground floor to save space, but Dh has said that he's worried about it possibly causing problems with damp. He says we have an airbrick at the front of the house, but not at the rear where there's a concrete extension and that this might cause problems with airflow and damp, particularly with the underfloor heating. I've no idea whether he's right and so I'd really appreciate some help. I've been a reader of PigletJohn's advice and I thought I'd reach for the stars and see whether they might be able to help!! Thanks!

I doubt it will be viable to add UFH

However, if you are going to dig up the floors anyway, especially in what is probably an old rear extension, you should add a DPM, and insulation, and you can add a couple of ducts to ventilate the subfloor so you can get a throughflow of air under the floors from front to back. And if you are going to dig up and relay the floors, that is about the only time it makes sense to add UFH. And renew all underfloor cables and pipes, including the incoming water supply, which is probably leaking and may be lead.

That will take care of the money you were going to spend on your next two cars or ten years holidays in Marbella.

Sorry.

I have not been on MN very long but have heard about the now legendary Piglet John on other threads. I feel like I've just seen a celebrity!

PigletJohn is an Urbane Myth.

PigletJohn thank you! Urbane (😀) myth or not, your advice is hugely appreciated and I feel rather magical that you responded! We do have to take up the floorboards and break up some of the old concrete foundation in the old extension to level the floors, so I thought we could just lay the underfloor heating at the same time. Were your concerns about laying the underfloor heating the same as DH’s, as the quotes I had were perhaps a holiday, but not the cars too?! What would you suggest we use for ducts which we can put in the foundation of the new extension we’re building and DH (he’s become an admirer too!) asked if you think having a front air brick in the middle and the rear at the side will matter? He thanked you too and also asked where you’d lay the dpm? Thanks again.

The damp risk is due to obstructed ventilation of the void under the old floor, and the probable absence of dpm under the later solid floor, and probable leaks from old lead or steel pipes, and broken clay drains and gullies. These are all common, if not ubiquitous, faults in older houses.

Looking on the bright side, the solid floor is probably quite thin concrete, and no match for a modern breaker.

You'd have to dig it out to about a foot below FFL (your intended future finished floor level). A sturdy woman or two can dig it out and barrow it to a skip. Then dig out and renew the old pipes and clay gullies and drain, back filling with crushed stone, bashed down with a vibrating plate. Then sand to blind any sharp fragments, then a plastic DPM, folded up the wall to reach as high as the DPC in the walls like a tray; then rigid foam insulation slabs; then your heating pipes; then your concrete floor (and possibly a smooth screed, or maybe the concrete ground and polished).

This is a good time to lay a new 25mm or 32mm plastic water pipe right out to the stopcock or meter on the supply under the pavement. It will later enable you to get excellent flow for your showers and baths.
The plastic ventilation ducts would probably be embedded in the concrete, depends on the height of the old airbricks and the ground level (which will probably need to be reduced by digging by the sturdy woman).

Fired clay gullies and drains in old houses are always cracked, broken and leaking (I am prepared to believe that there is a old house with perfect gullies somewhere in the country, but I have never seen it, nor met the owner).

Local builders will be familiar with this work, provided they are diligent and experienced. Poor quality work is difficult and expensive to rectify once concrete has been laid on top. In your area the standards required will be copied from templates into Building Regs plans so you can look at them by finding the online plans attached to applications and approvals recorded and searchable on your local council website for extensions. They're a good source of background info .

AFAIK BR's are not required for relaying an existing solid floor but one of the BCOs on here, or your council's enquiries office will know for sure.

Goggle Periscope Airbricks and the ducts will usually be listed beside them on the merchants' or makers' sites

uk.search.yahoo.com/search?p=periscope+airbricks&ei=UTF-8&fr=crmas&fr2=intlr&ei=UTF-8

Enormous thanks again PigletJohn for your comprehensive answer. I can certainly see why your fabulous myth perpetuates! Everything you've said all seems to make sense, even to me! Luckily I consider myself to be a sturdy woman too, although I suspect some (sturdy) men will actually be being doing the work.
The tip about changing the water pipe was something DH hadn't thought of, so he's really grateful for this advice. He asked if you have a course he could take to help him to increase his knowledge! He also wondered whether you had any views on a breathable - he's mentioned sheep's wool (!) - vs solid insulation for under the timber floor? He favours the former due to its 'breathability'. I'm just smiling at the thought! Once again, I'm ever so grateful for your erudite advice.

You use rigid foam insulation slabs under a concrete floor because it is weight-bearing and will support the concrete.

Wooden floors aren't very cold but bare boards are draughty. Mineral wool is cheaper than foam and can be stuffed into irregular gaps round the edges of the room where cold air and dust blows under the skirting, and can be laid between the joists, supported by mesh, if you are going to lift the floorboards or have access from beneath. It muffles noise slightly. Use a variety treated with Ecose that doesn't shed dust and fibres.

I don't weave lentils and don't know about sheeps' wool. No doubt it is treated to resist moths and remove the sheepy smell.

Thanks again PigletJohn, your advice is once again very gratefully received. Hopefully the sheep's wool insulation won't smell of sheep, although I have to admit to not actually being sure what that would smell like!