Heated slab external edge insulation question

[Spinny]

My rear extension has a 130mm concrete slab with heating pipes within, all connected up and functioning. The slab has 150mm PIR insulation under and 25mm PIR edge insulation around the perimeter just inside the blockwork walls. 

 

There is a 3.5m wide opening with bi-folds installed at one end and the inner leaf of the cavity wall was left out so the concrete slab runs out to the outer leaf of blockwork under the bifold. The bifold track is installed on this outer leaf of blockwork. The original specification had a course of celcon insulating blocks across the bottom of the opening and under the bifold track and a 25mm upstand of edge insulation inside this, then the concrete slab. However the builder had 'construction issues' and to resolve these it was eventually agreed to allow concrete blocks to be used instead of the celcon insulating blocks. Furthermore the front of the concrete slab and the course of blockwork was found to have a bowed outwards curve under the bifold opening when they came to fit the bifolds. To fix this the blockwork had to be removed and later relaid while the front bow on the concrete slab was cut off to straighten it up with a big circular cutter - this also led to some damage and reduction in the effective width of the 25mm edge insulation.

 

So the designed insulation at the bifold opening was reduced from 25mm PIR + 100mm celotex block to 15-25mm PIR only. To provide some compensation for the loss of insulation the architect advised adding 25mm XPS insulation on the outer side of the concrete blockwork under the bifold. The ground level outside is going to be made up with hardcore and a patio to be level with the bifold track, so the XPS insulation will be underground when complete. I am in the UK at about the same latitude as London. My questions are:

 

1/ How long will this XPS insulation last in that position ?

2/ Am I best to add a cavity drain membrane over the XPS to help to keep it dry ? (or is there a waterproof coated XPS product out there)

3/ How do I adhere the XPS insulation to the concrete blocks ?

4/ Given the XPS insulation will be to a depth of 300mm (maybe 400mm) and this insulation will complicate the patio construction - will this give me a worthwhile heating benefit or should I just forget the XPS altogether ?

 

many thanks...

[kandgmitchell]

Well I hope XPS will last a long time in the ground as I have 150mm of it across the entire underside of my insulated raft. It's closed cell structure so keeps itself dry.

I had to remove the perimeter insulation and then put it back (long story), when I did I glued it to the side of the concrete raft with CT1. In you case the patio makeup will keep it in place anyway?

[ab12]

To provide some compensation for the loss of insulation the architect advised adding 25mm XPS insulation on the outer side of the concrete blockwork under the bifold. The ground level outside is going to be made up with hardcore and a patio to be level with the bifold track, so the XPS insulation will be underground when complete. I am in the UK at about the same latitude as London. My questions are:

4/ Given the XPS insulation will be to a depth of 300mm (maybe 400mm) and this insulation will complicate the patio construction - will this give me a worthwhile heating benefit or should I just forget the XPS altogether ?

 

Am I reading this correct. XPS upstands will will be added to the external blockwork i.ef those facing garden/patio rather than the internal leaf of blockwork. That is definitiely a new one on my (although I am a novice) just never heard of external insulation especially beneath ground level although there is no reason why it cant be used I suppose.

 

Let wait for the experienced members to respond.

 

 

[Spinny]

Yes you are reading it right. Photo attached.

So would be putting 25mm XPS externally against the blockwork and it would end up below ground when the patio is made up to level with the threshold.

 

I don't want to create a problem 10-20 years down the line by having it there or getting ants in it etc.

 

Clearly there will be still be cold coming up into the blockwork under the bifold through the footings, but I guess the external XPS would inhibit losses out towards the ground under the patio which is nearer the surface and therefore gets colder in winter.

[Nickfromwales]

Lose the external blocks that run under the threshold and replace with Marmox blocks, if worried. You can do this in a day with the bifolds fully retracted as there will be no weight on the threshold then. 
 

Adding the XPS to the outside of an already cold block is just some nonsense solution to get you signed off and out of their hair imho. 

[Spinny]

Thank you Nick, I have never come across Marmox blocks before, an interesting alternative to celcon blocks which seem to be essentially reinforced XPS blocks. Not something my architect or my structural engineer ever mentioned.

 

Problem is the bifold doors are installed by the bifold company (not local), and I have already had 4 install visits to get to this point with the bifolds finally installed and working properly.The concrete blocks under the threshold were originally celcon. It is a long story...

When the builders poured the heated concrete slab they decided to 'tack mortar' the celcon blocks into place under the bifold opening. Their plan being to avoid having to cut all the blocks manually down to threshold level later in situ, because they would be able to remove them and machine cut them and re-lay them. However unknown for many many months (until the bifold fitters arrived) when they did this the blocks bowed forward under the weight of the poured concrete hence the rework to cut the front of the slab square again. In addition when they relaid the celcon blocks the bifold company found some to be loose. The builders tried to fix by re-laying them with 'quick setting mortar' but they still had movement 36 hrs later. The builder insisted the celcon blocks were the problem because they didn't take mortar well. The architect said any competent builder shouldn't have a problem, the structural engineer said sometimes they could suck moisture out of the mortar too fast and it was important to wet the blocks. I said I am not playing trial and error building at my expense when the bifold company are charging me money for every failed visit. In the end the architect agreed that concrete blocks could be used with XPS on the outside. The whole shebang ran over many months. We have since had to sack the builders after a negotiated settlement and are now 2.5 years into an extension project that was supposed to be done in 6 months.

 

So unfortunately paying the bifold company to remove and refit the threshold (2 visits from 5 hours drive away), plus finding a competent builder to re-lay marmox blocks or anything else, plus paying more architect fees at £100 per/hr and more structural engineer fees to sign it off, plus waiting months to achieve this, just isn't a viable option anymore. It is also unknown whether the bifold company would guarantee a fitting onto marmox blocks as presumably any fixings have to be carefully positioned.

 

I think I am stuck with either putting a layer of XPS on the outside down to an accessible depth (architects recommendation), or leaving it out(another visiting builders suggestion). So still interested in answers and thoughts on my questions above.

[Nickfromwales]

On 08/03/2025 at 11:08, Spinny said:

Thank you Nick, I have never come across Marmox blocks before, an interesting alternative to celcon blocks which seem to be essentially reinforced XPS blocks. Not something my architect or my structural engineer ever mentioned.

 

Problem is the bifold doors are installed by the bifold company (not local), and I have already had 4 install visits to get to this point with the bifolds finally installed and working properly.The concrete blocks under the threshold were originally celcon. It is a long story...

When the builders poured the heated concrete slab they decided to 'tack mortar' the celcon blocks into place under the bifold opening. Their plan being to avoid having to cut all the blocks manually down to threshold level later in situ, because they would be able to remove them and machine cut them and re-lay them. However unknown for many many months (until the bifold fitters arrived) when they did this the blocks bowed forward under the weight of the poured concrete hence the rework to cut the front of the slab square again. In addition when they relaid the celcon blocks the bifold company found some to be loose. The builders tried to fix by re-laying them with 'quick setting mortar' but they still had movement 36 hrs later. The builder insisted the celcon blocks were the problem because they didn't take mortar well. The architect said any competent builder shouldn't have a problem, the structural engineer said sometimes they could suck moisture out of the mortar too fast and it was important to wet the blocks. I said I am not playing trial and error building at my expense when the bifold company are charging me money for every failed visit. In the end the architect agreed that concrete blocks could be used with XPS on the outside. The whole shebang ran over many months. We have since had to sack the builders after a negotiated settlement and are now 2.5 years into an extension project that was supposed to be done in 6 months.

 

So unfortunately paying the bifold company to remove and refit the threshold (2 visits from 5 hours drive away), plus finding a competent builder to re-lay marmox blocks or anything else, plus paying more architect fees at £100 per/hr and more structural engineer fees to sign it off, plus waiting months to achieve this, just isn't a viable option anymore. It is also unknown whether the bifold company would guarantee a fitting onto marmox blocks as presumably any fixings have to be carefully positioned.

 

I think I am stuck with either putting a layer of XPS on the outside down to an accessible depth (architects recommendation), or leaving it out(another visiting builders suggestion). So still interested in answers and thoughts on my questions above.

Why do you need to pay the architect anything? Also, your structural engineer should have no interest in such a minor and inconsequential detail?

 

I assume you have no desire to DIY this and have been 'hands-off'?

 

This would be a multi-visit, in-situ repair tbh, but the work is very simple to do. It's down to how much sanity you have left of course, and easy for me to preach. 2.5 years for a 6 month program is just about the amount of WTF that would break some people, so good to see you still seem in control and not have not yet confined yourself to a padded cell . Just for safety sake, prob best to remove at least the most easily accessible sharp objects though, eh? lol . If you don't laugh, what other options are left?

 

Nobody's going to be deprived of life if you go with the architects solution, but there is a better way and I thought I'd mention it. If you can find someone to facilitate the works, I, (and the other helpful folks on here), will be more than happy to help as much as is possible.

[Nickfromwales]

Oh, and the suggestion by whomever that quick set mortar and the most porous block known in the universe should ever meet, needs the aforementioned padded cell. Just plain fecking ridiculous tbf!!!  

[ETC]

Why are the doors set that far forward?

[Spinny]

16 hours ago, ETC said:

Why are the doors set that far forward?

I don't think it is that unusual these days is it ? The frame needs to be fixed into something and the cavity is PIR insulation and a cavity closer, so they fix the frame to the inside edge of the outer block. Not ideal as there can be some thermal bridging through the block and round the thermal break in the alu frame (although they end up with packers and foam around the frame too). We used two different window suppliers and they both fit like that.

[Spinny]

16 hours ago, Nickfromwales said:

Why do you need to pay the architect anything? Also, your structural engineer should have no interest in such a minor and inconsequential detail?

 

I assume you have no desire to DIY this and have been 'hands-off'?

My thoughts are generally to build what the architect specified so that in the event of any future problem, you can point the finger at them, and they don't have the ready made get out that you didn't build what they specified. Any changes have therefore been bought off with the architect first, and using Marmox blocks would need to go around that loop. (Unfortunately the original architect left the practice early in our build and so the senior partner had to take over but we were then charged £100/hr instead of £70/hr!)

 

I have done various bits and pieces as has sometimes been unavoidable (protecting the works & keeping the rain out, removing lathe and plaster ceiling and hacking plaster off the walls, putting preservative on the joists, putting in conduit and cables under the floor, driving across the country in a hired van to collect a rooflight from a company in liquidation. etc). I live on site so have kept an eye on the works and been a kind of makeshift project manager, come tea maker, and 'that can't be right can it' person, and having to source, order and take delivery of anything that can't be bought off the shelf at any builders merchant from underfloor heating parts, to LED strip controllers and profile, to smart home modules, to aerogel, to roof straps. I am in my 60's now having been a young man when work began of course 🤪👴, so not up to much any more.

[Nickfromwales]

On 08/03/2025 at 11:08, Spinny said:

So unfortunately paying the bifold company to remove and refit the threshold (2 visits from 5 hours drive away)

Sorry if I wasn’t clear, the threshold stays where it is and you work the solution around it remaining in situ.

 

You open the door 100% to do removals, then use the Marmox blocks and Illbruck 330FM foam to affix back into position. No cement work just cut / stick / wait / do next one.
 

You close the door 10% of the way, install the first block, foam under it and pack it down to cure for 20 mins. Then pack properly and foam the top. 
 

Then close the door another 10% and do the same, repeating until you’ve done 100% of the work.

 

Go back then and fit more packers and foam these into place. Some of these may already be screwed into place through the threshold, just remove those for the key fixing points, and add some more for intermediate fixing through as you see fit; just until the threshold isn’t deflecting noticeably under foot.
 

Then seal up for weathertight and job done.✔️ 

[ETC]

1 hour ago, Spinny said:

I don't think it is that unusual these days is it ? The frame needs to be fixed into something and the cavity is PIR insulation and a cavity closer, so they fix the frame to the inside edge of the outer block. Not ideal as there can be some thermal bridging through the block and round the thermal break in the alu frame (although they end up with packers and foam around the frame too). We used two different window suppliers and they both fit like that.

BCNI_CONSTRUCTION_DETAILS_-_November_2023.pdf