Thermal bridging in a fireplace

[Countrypaul]

We are putting down UFH on most of the ground floor of our renovation, this will be on 120mm of PUR and covered with a 70mm screed. We will also have a Wood Burning Stove with back boiler located in the lounge. The building regs require that we have 250mm of non combustable material beneath the stove so we cannot use PIR insulation there, wat other choices are there to avoid cold bridging?

Rockwool would meet some of the requirement, but is no good due to lack of rigidity.

The only soluiton I have come up with to date is to use Foamglas slabs in the fireplace instead of PIR.  What have others used? Any other suggestions?

[Warble]

Vermiculite concrete under the hearth slab or Thermalite blocks supporting the edges of the slab with an air space under.

[Nickel2]

If you are digging out the hearth/fireplace below floor level, why not just dig the hole deeper, insulate the bottom, then put in 250mm of concrete to screed level. It may be time-consuming, but if it is for your own purpose and it meets specs, why not?

[Countrypaul]

The floor is laid (Beam and block) and the fireplace concreted to the same level. There is 190mm to build up from there, the original assumption being 120mm insulation and 70mm screed for the main floor but the fireplace cold bridging was not really considered. The architect assumed that the PIR could include the fireplace as it would be enclosed in concrete (above and below and round 3 sides) on the basis that building regs tend to assume older designs and often not allow for more modern construction. I could follow his lead and discuss it with BC, but it seems simpler and safer to find a proper solution without digging up the existing concrete.

Putting insulation just under the area would still also leave a 250mm cold bridge round the perimeter.

Thermalite blocks may be a possibility, though they would probably cover the entire area in order to support the concrete/screed (no point in leaving an air gap in the fireplace anyway just makes construction more difficult), but could be used as upstand agains the walls of the fireplace.

I'll look into the vermiculite concrete idea.

[Warble]

Unless you have some unusual point loads I would have thought that a sheet of Rockwool ROCKFLOOR would have been adequate for your purpose.

[Countrypaul]

Unless you have some unusual point loads I would have thought that a sheet of Rockwool ROCKFLOOR would have been adequate for your purpose.

Seems like a good option until...

I can't imagine the point loadings are that unusual, a Wood Burning stove weighing in at 200Kg (maybe 250Kg with flue and full of water & wood) standing on 4 feet and assuming a 70mm screed to spead the load. The main problem wth the Rockfloor seems to be fnding somewhere that sells it in small quantities - I need roughly 1 m2 at 120mm thick (could work with 100mm to 150mm if pushed I guess). The only option I've found so far is 25mm packs working out at around £70+VAT + delivery which looks like it will be more than the Foamglas!

[Warble]

You could try:- insulationcart.com

[Countrypaul]

Thanks Warble,

I had found that site that's where I got the prices from, unfortunately the price still works out at £112 for 120mm - though I guess I would get a £10 discount as a new customer.
Still £100 for 1000mm x 1200mm seems a bit steep to me, but might be better than the Foamglas (who may want £150 for delivery!!).

[TwoHorsePower]

large bag of vermiculate, bag of fondu  cement (this is an aluminate or 'high alumina' cement, available in most builders yards)  and sand  - Makes a lightweight insulating refractory concrete fit for 1000c. Cheap too, perhaps  <£30?

i have used it to cast a registry 'plate' above a woodburning stove  - I used some lightweight rebar (same guage as a 6" nail)  to reinforce as it was unsupported from above and below. Works great and would not hesitate to use the same for underneath a stove, though may perhaps need tiled for a harder wearing surface. Some folk use the same recipe for burn chambers & insulated risers in rocket mass heaters.

Hope this helps

2HP

[eabadger]

sounds like building regs is a bit ott.
is the distance not from the fire base, stove on feet?
given that double insulated flues can run a 50mm or 60mm gap to combustibles i would have though screed wood be a good enough barrier and perfectly acceptable.
where i have installed a wood burner in a bedroom, i made a hearth of dyed concrete with colored pebbles as aggregate, when it was hardening off, washed off and brushed back, now looks pretty we think.
could you if you want to comply to the letter, just cast a slab under the stove and tile it same as floor?

[Bodidly]

The building regs require that we have 250mm of non combustable material beneath the stove so we cannot use PIR insulation there, wat other choices are there to avoid cold bridging?

250mm is that right? When I did our place some years ago pretty sure it was nothing like that much but it might have changed. If it is can you just have a thick hearth?

[Nickel2]

The fir would only be used if it was cold, so little valuable/useful heat would be lost when burning. If the fireplace was not in use, but it was chilly enough to put the heating on, a suitably decorated insulated panel could be placed across the fireplace to prevent draughts.

[Countrypaul]

Here is a simple explanation of hearth sizing - the building regs say the same thing. http://www.stovesonline.co.uk/stove-hearth-size.html

There should not be too much problem with draughts, the air supply is ducted to the stove (from under the house) and the house itself is (hopefully) fairly tight in terms of air leakage - can't test it yet as the floor insulation/UFH/screed is not yet in place.

It seems daft to have gone to all the trouble of minimising cold bridging elsewhere, taping up all joints to minimise air leakage but then make no attempt to insulate/seal the hearth.

The vermiculite concrete option seems to be one with little concrete (forgive the pun) information about how good an inulator it is, everything I have read is very much anecdotal - this may be because it potentially varies so much, or noone has actually carried out a proper measurement of how it performs.

[eabadger]

that link seems to say that the height is from the hearth bed, have i read that correctly? if so depending on stove you can use existing floor, but it states it must be defined? we just have the stove (a big beast) on the tiled floor which is on the screed over the ufh, no issues.
have you chosen the stove an asked them?
i wished we had put more insulation in, not less, anywhere.

steve

[MR GUS]

Please do recall: when I removed our 70's nightmare brick fireplace complete with terracotta tiles & lots of brick I dug down to see what else was going on, (it was a standard inefficient concrete jobby open chimney ..duff & cold as hell.

I dug down beneath the hearth & for some godforsaken reason were a few pieces of wood, burnt /charred  which really made me question ..

A. who in their right mind would place blackened burnt wood in there & fill it up with cement / concrete (the vicar was a chancer mind).

B. combustibles within the concrete / cement ..which had against all odds got hot enough in a void to char (burn),..we will never know, this is supposition.

BUT as EA says, & lots to back it up over the years here, the fire regs to joe public aren't necessarily well written to be made clear sense of till you really, look at it.
& therefore as EA states, if you are buying a wood burning stove on legs, then it is height from the burning chamber that matters, as well as the protrusion of the opening from combustibles & hearth distance  accordingly.