Green Building Forum - How far down should wall insulation go? Sun, 31 May 2026 05:04:10 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231905#Comment_231905 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231905#Comment_231905 Tue, 10 Nov 2015 22:20:47 +0000 tony
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?]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231908#Comment_231908 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231908#Comment_231908 Tue, 10 Nov 2015 22:58:46 +0000 gyrogear
(if there's a basement, it is easier and cheaper to insulate inside; and if there's a crawlspace, it should be insulated inside anyhow...)

gg]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231917#Comment_231917 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231917#Comment_231917 Wed, 11 Nov 2015 09:12:55 +0000 fostertom
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.]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231923#Comment_231923 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231923#Comment_231923 Wed, 11 Nov 2015 11:26:08 +0000 Ed Davies Posted By: tonyOn 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.What was their objection? I can see they might think it unnecessary (if meeting building regs is the only requirement) but what harm could it do? Concern it'd set a precedent? Worry about water running down the wall and getting trapped behind the extra insulation? Something else?]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231925#Comment_231925 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231925#Comment_231925 Wed, 11 Nov 2015 11:41:07 +0000 tony How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231928#Comment_231928 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231928#Comment_231928 Wed, 11 Nov 2015 12:55:07 +0000 Ed Davies How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231936#Comment_231936 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231936#Comment_231936 Wed, 11 Nov 2015 14:37:14 +0000 tony How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231940#Comment_231940 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231940#Comment_231940 Wed, 11 Nov 2015 15:30:14 +0000 Triassic

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.
I've often wondered why the twin or insulated stud approach to timber frame construction never caught on?]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231942#Comment_231942 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231942#Comment_231942 Wed, 11 Nov 2015 16:46:21 +0000 joe90 How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231948#Comment_231948 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231948#Comment_231948 Wed, 11 Nov 2015 18:22:40 +0000 Shevek How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231954#Comment_231954 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231954#Comment_231954 Wed, 11 Nov 2015 21:35:44 +0000 ringi How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231960#Comment_231960 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231960#Comment_231960 Thu, 12 Nov 2015 09:25:51 +0000 fostertom How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231961#Comment_231961 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231961#Comment_231961 Thu, 12 Nov 2015 09:35:43 +0000 djh 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 :devil::bigsmile:]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231977#Comment_231977 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231977#Comment_231977 Thu, 12 Nov 2015 13:09:13 +0000 davidfreeborough
David]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231981#Comment_231981 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231981#Comment_231981 Thu, 12 Nov 2015 13:42:29 +0000 djh 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?]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231998#Comment_231998 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=231998#Comment_231998 Thu, 12 Nov 2015 18:28:45 +0000 Phil.Chaddah-Duke How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232150#Comment_232150 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232150#Comment_232150 Sun, 15 Nov 2015 21:03:46 +0000 Shevek
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.]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232154#Comment_232154 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232154#Comment_232154 Sun, 15 Nov 2015 22:36:05 +0000 Triassic How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232164#Comment_232164 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232164#Comment_232164 Mon, 16 Nov 2015 10:05:38 +0000 gravelld How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232166#Comment_232166 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=232166#Comment_232166 Mon, 16 Nov 2015 10:12:35 +0000 Shevek How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280392#Comment_280392 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280392#Comment_280392 Tue, 12 May 2020 09:49:56 +0100 LF 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?]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280394#Comment_280394 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280394#Comment_280394 Tue, 12 May 2020 10:43:51 +0100 Peter_in_Hungary Posted By: LFIs external insulation on footings on outside a sensible option to reduce heat leak through floor?
Yes - but 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.

Posted By: LFRealise need to do something about dpc details .. .may be an aco drain above the new insulation?

The general advice if using EPS EWI the DPC is not an issue i.e. you can insulate over it because EPS does not wick water so there will be no bridge created over the DPC.]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280400#Comment_280400 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280400#Comment_280400 Tue, 12 May 2020 12:41:02 +0100 LF
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]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280415#Comment_280415 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280415#Comment_280415 Tue, 12 May 2020 16:36:11 +0100 Peter_in_Hungary Posted By: LFNoted tip on XPS!
Err No, - I was talking about EPS
EPS and XPS are not the same each with differing properties.]]> How far down should wall insulation go? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280479#Comment_280479 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13794&Focus=280479#Comment_280479 Fri, 15 May 2020 02:29:15 +0100 wookey 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.]]>