An idea for wing insulation plus french drain, principally around the perimeter of an extg building, as an alternative maybe to the disruption of pulling up floors for underfloor insulation. Maybe even a solution to suspended timber ground floors with void benesth:

Dig trench down to bottom of found wall or strip found - but abs no deeper, except trench bottom sloped 15o down outward then up a little near the trench wall, to form a shallow gutter-bottom to the trench. Perforated pipe land drain surrounded with clean no-fines aggregate - the usual french drain - but the difference being, use largest-size (10-20mm diam) Leca aggregate http://www.netweber.co.uk/flooring-systems/weber-products/products/lightweight-aggregate/leca-insulation-fill.html instead of sharp stone. Just an inch of Leca under the pipe, fill the trench with Leca and top with topsoil. Wrap the entire chunk of Leca with geotextile (Terram) to prevent silt infiltration, and maybe a sheet of polythene under the topsoil, to prevent rainwater ingress.

Leca is Lightweight Expanded Clay Aggregate and has a Lamda value of 0.011W/mK - so one third to one quarter as insulative as EPS, as wing insulation. But we're putting it in at least three to four times as deep and wide as any EPS wing insulation wd be. It's cheap, slightly more than the sharp stone it replaces, light and easy to install (see the pic in the above link - doesn't even need to be unbagged!) and a sounder underlay for topsoil, which seems a bit unhappy on top of EPS.

As a solution for an exceptionally shallow extg found, the drain with minimal Leca surround can still be done, then EPS wing on top of that, topped with topsoil.

Thanks Dave. Weber Technical say it's available either plain or silicone sprayed to retard water uptake, but even if plain, and in saturated conditions, water will get into the pores but just as soon drain out again. In fact I'd prefer non-coated so it drains out as easily as possible. If full of water, then obviously little insulation value, but by definition a french drain is something with air in its spaces, albeit maybe vapour-saturated. Only if groundwater (or in this case flood water) rises will it become liquid-saturated, and that hopefully will be v rare, and will be a matter of drying out the whole house ground floor! In the literature it's shown installed under the floor dpm, and the quoted lamda value 0.11W/mK (figure edited 3.3.11) takes into account an allowance for capillary damping - but not saturation.

I'm suggesting Leca instead of any other EWI below GL, because in its depth + width it's equally insulative as EWI, is a lot cheaper and easier, and also serves as french drain.

Also awaiting info from Weber Scandinavia about the practice there of using Leca backfill to reduce reqd depth of pipes and founds by removing frost/freeze risk. Something the housebuilders shd be into, I'd have thought.

Would foamed glass act as a French drain?

Posted By: fostertomthe quoted lamda value 0.011W/mK

I've only just noticed this. The decimal place is in the wrong place there of course; it should read 0.11W/mK. See http://maxitleca.co.uk/33722 for confirmation. 0.011 is aerogel territory!

Posted By: pmagowanWould foamed glass act as a French drain?

If it's loosefill aggregate rather than blocks. Something like this http://www.lime.org.uk/products/limecrete-floor/components/hasopor-recycled-foamed-glass/

Seems to be about twice the price - but then it should insulate better in the wet.

Leca would seem to be a simpler solution, re availability etc. I don't know much about the foamed glass stuff and I am sure my local yard won't either. Also if efficiency is the problem and foamed glass is twice the price you can put in twice as much leca!

Paddy

Would Activated Alumina be suitable for this purpose. I ask because I have been using on the farm as a french drain material and found it excellent. I get it very cheap as recycled material which also helps.

Saw Weber demonstrating Leca bag infill prior to slab at ecobuild. Looked easy, including concrete pour!

The only thing I wondered about your wing insulation was if the insulation value would be compromised by the gaps caused by the shape of the bags. I guess with careful packing/orientation this could be minimised.

One of my problems is I've inherited an uninsulated ground bearing concrete raft (engineer spec) which includes a south facing apron for uninhabited glass conservatory which extends beyond building line. The development has 'commenced' because of raft/sewer/groundworks. I need to make major changes, but need to keep slab if possible.

Grappling with what to do for the best, insulation/moneywise before approaching planners, so perimeter/ wing insulation threads are a great help to me.

<blockquote><cite>Posted By: fostertom</cite>The whole point about a french drain is that it stays drained dry.</blockquote>

But rainwater is still travelling through it isn't it? Is that really dry enough? I'd have thought any bag or impediment to free draining would make things worse. Digging through our loose stone backfill it's certainly pretty wet even though it's very free drainin.

I like the idea though and wish you'd posted it a year ago when we were backfilling the 3m deep trench behind our hillside house!

Posted By: RobinBrainwater is still travelling through it isn't it? Is that really dry enough?

Wd be interesting to know what the RH is, in the airspaces of a french drain - that's what will determine the long term moisture content of anything passively (i.e. not oozing groundwater) bounding the airspace. Remember the fill is not a soggy poultice - it's clean lumps making point-contact with the EPS.

Posted By: ChrisEnglandleca - isn't there a risk of spalling during frosty weather

Have you heard that, Chris, or are you just wondering? Another great use for Leca is as backfill over very shallow strip founds, where the Leca provides the insulation against ground-freeze heave, instead of 1m or more of soil. This is approved and widespread in Scandinavia. In that use, I guess Leca must be considered immune to frost.

Tom,

Further investigation indicates my worries about frost are probably unfounded - see, e.g.

http://www.maxitleca.co.uk/33722

Also suggests attainable u-values.

We used foam glass for our French drain. I have no idea if it is making a difference thermally, but the large chunks do help with drainage. The perforated pipes are around two sides of the hours so far and come together in a junction. When I first fitted it, I used to run out in rain storms to check and the amount of water that flowed through was surprisingly large. It then made it's way into the stream that runs on the boundary of our garden.

I'm not sure I'd use smaller leca beads as they are quite small. I read the info below regarding aggregate size in a French drain:

"The larger the aggregate, the less likely blockages are to occur and the water will be able to flow through more quickly. The ideal size of aggregate used is around 10-20mm wide"

Fantastic - bizzy, will dig in asap