Okay guys, bottom line [s]

Where's your evidence which undoes years and years of counter evidence and experience?

Where's your manufacturers Specification and Certification?

Where's the thermal modelling which justifies your position?

Where's the laboratory tested evidence?

Oh, and where's the LABC Guidance Note?

Back in the 80's when council grants were available. The standard specification was to rip out timber floors and concrete them instead. Good luck to you if you are determined to take the risky approach. I would guess though that because you are deliberately contravening building regulations (which is an offence) that should you sell your property in the future you would be responsible for any remedial works

First I have not said I intend to do this I am only interested in the possibility. Second I have no intention to ever sell my house.

I don't know how to do a percentage of a U-value (65% would make it better with my maths skills ) but in my head that would make it quite easy to achieve a minimal risk situation at the junction with even modest insulation. 200mm platinum EPS on the wall and about 400mm EPS bead UFI would seem more than adequate!

Hi pmcgowan, do a percentage of the thickness then, if you put 200mm on the walls and 400mm in the floor then 2/3's of the average of 200+400 = 200, so to get a cold bridge free junction wrap the junction with 200mm of insulation. But you can't do that because its an existing house so your options are; drop the external insulation down as far as you can or heat up the cold bridge by using it as a solar dump where you dump excess solar heat. If you can heat the ground around your foundations from 10 degrees to 15 degrees then the cold bridge is reduced to 50%.

What does this mean in the overall context of your house? A cold bridge free junction should read 17 degrees when its minus 10 outside, if you stop your insulation at finish floor level then the temperature at the skirting will be 13 degrees, dropping the insulation 500mm below finish floor level improves the temperature at the junction to 16 degrees which is satisfactory for a renovation. Check the bottom of this page http://www.viking-house.co.uk/passive-house-renovation.html

Posted By: fostertom On my forthcoming newbuild project, Leca is blown in (or placed in bags + loose) to enclose the floor joists soild and to come tight (in thermal contact) up to the 25mm CPB (cement particle board) deck and tile flooring thereon. Below, the Leca is on DPM direct on the subsoil (and the CPB is an inboard VCL). Towards the centre of the floorplan, the Leca is as thin as can be, just under the joist soffit. Going outboard, it thickens, just enough so the DPM will drain outward any water or condensation that might get into the void. Further outboard, the joists bear on railway sleepers (up to 6m long 'crossing (points) sleepers') on edge as ground beams and the dpm dips to go under them. So outboard of the ground beams, the Leca is in considerable thickness, around the floorplan perimeter. Then at the perimeter the wall EWI is continued down about 1m in narrow-bucket trench (i.e. 'free-floating', not attached to any wall) and the Leca comes out to the inner face of that.
So the block of subsoil under the floorplan is enclosed in 200 EPS downstanding 1m, and that subsoil block is in thermal contact with the interior, via Leca of varying thickness under the CPB and tiled finish. This is an attempt to make an effectively heavy solar-absorbing floor (because tall wide glass area to SE) in contact with its subsoil (protected from edge-loss) but with a modicum of insulation to ensure that floor surface keeps cosy.

Hi Tom, I've been doing some calculations on this by changing the ground temperatures in the PHPP, It seems the benefit of dumping heat into the ground is substantially reduced every-time 100mm of insulation is added to the floor, the benefit is reduced to nearly zero when we put 300mm of EPS in the floor. Changing the ground temperature beneath one Passive House from 10 degrees to 15 degrees only reduces the December heating bill from €4.30 to €2.70, this is probably because the large 40m2 Solar Roof on this particular Passive House reduces the heating demand from 15kWh/m2.annum to 2kWh/m2.annum.

Posted By: pmagowanFirst I have not said I intend to do this I am only interested in the possibility. Second I have no intention to ever sell my house.

Like I said, good luck if you choose to go with it. I genuinely would like this to be an approach for upgrading the existing stock.

The sad fact is though that in the quest to superinsulate existing houses many have lost sight of the reasons they were built the way they were. Superinsulation is becoming more important than maintaining the durability of the existing fabric.

Great link Mike. I have seen this happen only 4 times in about 450 insulated floors of which i am aware. It has always seemed counter-intuitive, but when I get more chance to read the article I reckon it may be more clear. Reverse the Q: Why does it sso often *not* go wrong? Nick

Posted By: Mike GeorgeTom, Can I ask why you are specifying timber ground floor joists in a new build? Why not a ground bearing slab with all the insulation you like?

First, on a campaign to eliminate the concrete industry from my buildings (so i can still fly to Goa with clear conscience!). Briiliant prefered carpenter/joiner/builder an02ew likes this method but the other tenderer prefers conc, so have designed an alternative pumped conc reinf found/floor incl the downstand perim insul. Will be interesting to compare costs when both tenders are in.

Second, don't want lots of insulation underfloor - just a little, to keep surface temp cosy. Moving the insulation to downstand around the edge of the block of subsoil, so the latter lends its thermal massiveness to the interior, esp for temp stability allowing bigger south glazing.

I agree with pmagowan that lots of very drafty cold floors are being kept for fear that we may cause moisture problems when those moisture problems are already there in many houses. When I am talking about my own house I am considering taking the risk that my already damp underfloor void won't get much worse. I can leave a couple of loose floor boards and inspect too. I am also thinking of monitoring moisture at key points. However I am more worried about fire risk and cabling.
I can see Mike's issue with Building Regulations - perhaps it'll become a standard refurb technique in the future. We certainly need it.

I too have been looking at the Building Science site: http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

What's interesting here is that the standard Energy Saving Trust type approach of hanging insulation between joists causes condensation on the underside of joists. See photograph with drips on underside of insulation. At least the pumped solution would be an improvement on this.

Posted By: Ed Davies“CPB”?

Cement Particle Board - chipboard but cement-bound

Patrick. The article I linked to (the scary photo) also discusses joists completely encased in PUR. If you read it you will see encasing joists is not the solution

Posted By: Mike Georgeyou will see encasing joists is not the solution

Does it say that? It wants high vapour resistance under the joists + encasement.

However, this article is mid-US based, where hot summer air carries lots of moisture - high RH - which enters crawlspaces that never see the sun so condensation happens all over its cold surfaces. The more ventilation, the more condensation. Not the UK situation. Only in spring and autumn will this happen here, drying out in summer and with cold winter air, which is actually warmed as it enters the crawl space so reduces RH. The upward 'vapour drive' will happen much less in UK.

The problem is the ventilation - the source of the condensing moisture. It's not (or hardly) the subsoil endlessly evaporating. Of course if you have liquid water ingress that needs fixing.

So stop the ventilation, enclose all the timber solid in breatheable, non-wicking, free-draining insulation. How can the timber then get wet? Any dewpoint will be within the insulation well below timber level. If timber does get wet, e.g. by spillage from above, why shouldn't the water drop off, down and away?

This antiquated Bldg Reg really needs arguing out - doesn't need research to prove the easy basic physics of it. As Mike's article says, the whole Reg is based on the days before susp floors were uninsulated, and the whole timber as exposed to the crawlspace was warmed by the interior, the warmest thing in the crawlspace therefore the last place that wd attract condensation. Methods have changed but this Reg perpetuates one of the myths - 'ventilate to prevent condensation' - that have long since been recognised and abandoned for walls, roofs etc,

Tom. Yes. It is in the US. I acknowledged the climatic difference. The physics is the same though. Also yes, it says about high vapour resistance in the 'correct' location. This is the variable which is different in our climate. Here it is specified on the warm side. Their problem is they can get high levels of diffusion both ways. Some argue only a breathing construction is acceptable

They are not talking about UK regulations. You cannot argue that their experience is different and also argue that our regs should be the same

The problem with fully filling voids is that it does not apply either UK or US best practice. My reason for posting the link is to illustrate what can happen when it all goes pear shaped.

From a look at that link I would say the best treatment is to not have a crawlspace. It seams the ventilation is cooling the timber and causing condensation particularly when the timber is isolated from the heat of the house by insulation. Their phrase of importance is 'warm wood is happy wood'. I would say this applies. Stop letting cold and damp air under the house, Insulate beneath the wood and problem solved. The timber is open to the house and can ventilate here the same way as your kitchen worktop can. Bring the wood in from the cold!