In the early days of wall insulation building control would not let us put it in below dpc.

I think they were wrong, did at the time, still do now.

On a recent SIPs build the insulation in the panels started 125mm above finished floor, after a massive fight with the SIPs technical department they allowed us to do something to mitigate this thermal bridge by adding insulation on the outside across the sole plate area.

On the majority of masonry builds insulation still stops at dpc leaving a thermal bridge at the edge of the floor just where you don't want one.

With timber frame insulation starts 97mm above dpc, again a thermal bridge leaving the sole plate prone to pick up condensation. It is the most vulnerable piece of wood in the home apart from any near a shower.

So I ask how far down should wall insulation go? To dpc? Below dpc? To footings? To bottom of foundations?

Way back, with cavity wall construction (in those 65-75mm cavity days) I used to take the cav insulation right down to top of the strip found, instead of filling it with conc below DPC. Must have helped.

Trouble with the question, is that it assumes an insulation filled stud frame is OK on its own. The uninsulated sole plate is replicated all over the stud frame, at every stud, nogging and head plate.

The insulation-between-studs bit of the sandwich should be as thin as structurally possible - say 95 - and it should be sheathed externally with 150-200 EWI, which kills all the stud thermal bridges, and is easily taken down as deep as you like - pref to foundation level.

They would only allow additional insulation internally, until we started to explain to the facts of life, a year later and they now accept additional insulation outboard so sips panels at least at sole plate level.

Certification and guarantee/warranty, I reckon early SIPs could find problems down the line because the technical department clearly didn't understand the issues, on comment was that there could not be any damp as it was above a dpc!

I plan to carry cavity insulation down to strip foundation as per the Golcar passive house, yet to get it pass building control but the inspector is open to new idea,s.

I recall being told by an ICF vendor some time ago, that using ICF instead of “trench fill” costs about the same, as the cost of the ICF is offset by using less concrete. Why are ICF not used below ground for new build, even if it is timber frame above ground?

Posted By: fostertomWhat a studio-perfect pic of a messy process! Can I use it?

He must have had my wife there, hoovering out the trench

Posted By: djhHe must have had my wife there, hoovering out the trench

In fact, is that a Dyson-like cylinder cleaner I can see at the far end of the trench?

Ha, thanks guys. Feel free to use the pic Tom. That is indeed a Dyson vacuum cleaner you can see.

You would laugh if you saw how we use the hoover when working with EPS (it's not far behind any rasping and held directly below anyone doing any sawing). I've also been seen hoovering our garden and the neighbour's driveway.

The white bit below the door is 250 or 300 kPa EPS. We were originally going to butt the EWI up to the underside of the existing brick sill but this really bugged me (as it would be the only very cold thermal bridge in the house). Fortunately at the last minute it occurred to me to simply cut off the brick overhang and do the whole section in a different grade of EPS (of which we had some left over from elsewhere). We're going to fit an aluminium sill over that section.

We backfilled with 200 mm thick of leca, partly because it has some insulation value but also because it absorbs some moisture so I was thinking perhaps that will help prevent the trench from filling up with water.

Despite the garden sloping slightly towards the building we didn't put a french drain in. We've done a lot of things the right way on this project but I just couldn't be assed with this one. My main concern is that it would eventually silt up without maintenance and be useless anyway. We dug down 100-150 below the top of footing so there's a little channel down there to fill up with water before it reaches the insulation. We're also going to put a line of paving along the wall there and fall it towards the garden slightly.

I thought the Dyson was R2-D2.

I have not created a new thread due to the excellent photo above. Is insulation on external footings only a "reasonable" retrofit for solid floor property.
We have solid floor with no insulation on 2/3 of downstairs in 1950s semi ... brick cavity wall insulation. Side extension is insulated slab 2003 regs.
Back of house is being refurbed soon
Is external insulation on footings on outside a sensible option to reduce heat leak through floor?
Floors are mainly original parquet and no chance of me being allowed to lift them ...
Realise need to do something about dpc details .. .may be an aco drain above the new insulation?

PinH - thanks for these. Cavity insulation was blown in as retrofit so should be mostly down to bottom? I would have to check if we do work and have some plan to sort it out if not. Noted tip on XPS!

It is looking interesting thermally, if the following is anywhere near correct.
Potentially says U value drops from ball park about 0.45 to 0.26 W/m2/K with 8m of perimeter P/A=0.13
(with P= 2 m. P/A 0.03 it drops to U=0.1 which seems unlikely.)

U = 0.05 + 1.65(P/A) – 0.6(P/A)² simplified formula found in previous posts about this from an old BRE paper.
This is simplified saying only significant heat leak to front perimeter after works completed.

(I have a middle slab on original house say 8x7.6 m. On 7.6 m sides, on side we 2003 4m wide extension with insulated slab (0.25w/m2K 75 mm poly and on the other side it is neighbour/party wall So overall downstairs is 12 m by 7.6 m).

Noted Tony point in one of posts saying insulation under floor near to the walls is the most important thing and this should be thickest. Also read large commercial buildings only insulate near walls.

Found this - For very large floors with no edge effects and no insulation the U values seems to be as low as 0.15 !! see page fig b1 on page 39 . WOW !
https://www.bre.co.uk/filelibrary/pdf/rpts/BR_443_(2006_Edition).pdf

Posted By: Peter_in_Hungarybut first make sure that the cavity wall insulation is good and was installed down to the bottom of the cavity and is without voids otherwise EWI is a waste of time/money

That's a bit strong isn't it? It's good advice in general, and very important if the airtightness is on the inside, but if, for example the airtightness layer is moved to outside the cavity then it doesn't matter much what's in the cavity - it's just a gap in the wall structure, but being inside the airtightness and insulation it's not really doing any harm. Obviously big vertical air circulation voids in the wall are not what you want as they can move heat around and connect leaks, but having some voids in a largely filled cavity is not the end of the world.

I agree that just EWIing over an unfilled, unsealed, cavity without airtightness measures is not going to work.