Insulating Floor Slab?

[Jim Chisholm]

We've a roughly 8mx12m detached early '80s house.

It is retrofitted with cavity wall insulation, modern high performance double glazing, and c 300mm loft insulation (except for some sloping roof that is on the list for attention).

I don't believe anyone insulated beneath floor slabs in typical housing of that date.

I've been told that in a typical detached house 20% of energy is lost through an uninsulated floor slab. With the other measures in place this must now be a more significant consumer of percentage of our energy.

I'd thought of digging down to the footings, which a test suggests at nearly one metre (yes one whole metre) from DPC, and inserting 50 or

100mm of calcium silicate faced closed cell foam board.

I can't find any site that details likely reduction in heat loss or that details the detail required.

Of course the floor slab and the ground beneath will then act as a better heat store.

I'd thought of fixing a tanalised timber just above DPC and using sheet aluminium or similar to protect the first 300mm or so from damage when digging flower borders etc. This might also reduce the 'cold bridge' effect at DPC level, as I assume the cavity will be filled to DPC level.

Doing this by hand should keep me fit in my retirement!

Any suggestions for sources of information, or will you just send the men in white coats!

{We've not flown for years, and the car has only been filled up twice since Christmas, so this seems to be a good way of yet further reducing our 'Carbon Footprint'}

Jim Chisholm

[Chris J Dixon]

Not quite sure I fully understand your plan. Are you just wrapping the footings in insulation?

What percentage of the 20% heat loss you attribute to the floor slab are you calculating is conducted laterally, rather than straight down?

Chris

[Ben Blaukopf]

Doesn't matter does it? If he Walls: area 360sqm. U-value 0.45 (cavity wall with insulation). AU =3D

162 Ceiling: area 60sqm. U-value 0.3 (loft space with insulation). AU =3D 18 Windows: area 20sqm?. U-value 2 (if the windows are decent). AU=3D40. Floor: area 60sqm. U-value 0.8 (uninsulated slab). AU=3D48

sigmaAU =3D 268/ 48/268 =3D=3D 18% of the heat is going through the floor.

[The Natural Philosopher]

yes. You should do it the standard way..go down about a foot, hardcore, level and blind, DPM then insulation under and UP THE SIDES a bit, then screed (with UFH if you want). Its a fairly expensive job so don't skimp. Id go at least 3" of celotex or whatever.

[harryagain]

I have already done this sort of thing. An 80's house will likely have 50mm expanded polystyrene under it. Drill a hole in the slab and watch for white flakes coming up when you break through.

The heat loss is hard to establish, it depends on the subsoil and what aggregates were placed under the floor as well as any insulation.. The U value 0.8 mentioned above is a worst case & you are likely to have some insulation. If you have 50mm, 0.2 will be nearer the mark. You need to remember that the coldest air in the house is next to the slab so the heat loss is not as great as you might think. Thick carpets and underlay also help.

have 150mm foam insulation under most of my floor = U value of around

0.05. I think. Yes you need vertical perimeter insulation too, normally 25mm.

[The Natural Philosopher]

Blue. Or pink

EXTRUDED polystyrene, not EXPANDED.

[Phil L]

That's what I thought at first, til I noticed his proposals WRT flowerbeds.

That's right - he's on about digging a metre deep trench around his house and insulating it from outside.

[Phil L]

That's because there won't be any

No, it will be just as cold, and you'll be hundreds of pounds worse off, having buried your insulation in the garden.

For this to have any hope of making even the tiniest amount of difference, the insulation would have to be bonded to the exterior brickwork - no gaps for sludge or soil / sand to wash in otherwise it's a pointless exercise. If it was bonded to the brickwork, the difference would by next to nothing.

Why not look at raising the levels of the downstairs floors? - put in sheets of insulation and have floating floors

[The Natural Philosopher]

relative bollocks without an underfloor slab

[Rick Hughes]

2" of Celotex would meet all Building Regs ... floor plus perimeter insulation is worthwhile .... but it is a big job to do ... Run a check on savings vs cost of job ... may not stack up.

[harryagain]

Your dementia troubling you again TurNiP?