sliding door threshold detail with external wall insulation?

[Ed_]

I'm struggling to find a good way of mounting my sliding doors, given I will have 100mm of external wall insulation as the outermost part of my wall buildup:

 

I think the door track needs to go over the concrete slab, for strength, which leaves me struggling to protect the 100mm of external insulation. An extended sill seams like the best idea, but I can't really find any of these being offered, perhaps i'm not looking in the right places?

 

As drawn i'm envisaging a wide sill that bridges the concrete to over the insulation, sat on compacfoam for a bit of strength to spread point loads, with the sliding door track set ontop of that. However I can't find suitable sills and i'm not sure if this arrangement would still put undesirable forces through the external insulation, for example when someone steps on the sill.

 

Are there any other ways that don't introduce big thermal bridges?

 

[Redbeard]

Any metal fabricator should be able to make you what you need. Try EWIStore: https://ewistore.co.uk/shop/external-wall-insulation/cms-810-120/

[craig]

That detail for the sliding door sitting on compacfoam over the insulation and partly over the concrete slab isn't substantial enough, I'd be concerned with the point load over time and potential operational issues occurring. Sliding doors need a solid base to sit on over the entire depth and needs to be level. It can sit on structural insulation but it needs to be done right, part of it is sitting on insulation and that's asking for problems. How's the threshold being fixed in place, how is it be packed (if needed)?

 

[Nickfromwales]

2 hours ago, craig said:

That detail for the sliding door sitting on compacfoam over the insulation and partly over the concrete slab isn't substantial enough, I'd be concerned with the point load over time and potential operational issues occurring. Sliding doors need a solid base to sit on over the entire depth and needs to be level. It can sit on structural insulation but it needs to be done right, part of it is sitting on insulation and that's asking for problems. How's the threshold being fixed in place, how is it be packed (if needed)?

 

Current MBC PH TF project is all Norrsken, and they are all going to be completely off the slab and atop the EPS with 20mm of (Compacfoam) CF200 set in situ to take the loads (biggest slider is <4m iirc).

 

Same on last few with raft founds (Rational and Velfac) with zero issues tbh.

 

I just asked the installers to go all-in on the side and head brackets, and made sure the CF or Bosig is set down very robustly. Threshold can be strapped too if needed, but by the time these are set down onto foam and CT1/other, the sheer weight stops these things from moving about at all.

 

[Alan Ambrose]

Won’t the eps / compactfoam compress with the load tho?

 

i should say I have a similar detail coming up, so I’m v interested in a robust solution.

Edited Thursday at 18:24 by Alan Ambrose

[mjc55]

yes, I will have a similar construction and was considering Thermablock as a solution.

[craig]

Compacfoam and or thermoblock are fine but the weight needs to be distributed. On the detail above I don’t think the compacfoam is substantial enough and is sitting on slab and insulation.

[Nickfromwales]

On 05/03/2026 at 18:23, Alan Ambrose said:

Won’t the eps / compactfoam compress with the load tho?

 

i should say I have a similar detail coming up, so I’m v interested in a robust solution.

Will the EPS that's holding the whole house up compress? Erm, "no".

 

Once the weight of the static (not dynamic) load of the window / door is spread out over the 3/4/5m lengths, and is sat onto ridiculously strong CF200, then you can literally park a car (or a window) on it. Bear in mind it is higher load capacity EPS too, not the stuff that stops your new washing machine getting damaged in the delivery truck.  

 

If both Norrsken and MBC are doing this routinely, I think the pill of chill can be taken.

 

As principal consultant I have to go to actual peoples actual builds, and actually do these things. I sleep well, and my phone stays quiet at night, which comes from nearly a decade of working with high-end fenestration and all types of foundations and insulation systems. Nobody's rung me up to say they've had an issue.........yet.

 

I am quite meticulous in the execution and methodology of these installs, (batshit crazy levels of OCD actually), so it is accepted that a crap fitter using the same materials would have a poor outcome with reduced longevity. Do the job right, do it once.

[Nickfromwales]

15 hours ago, craig said:

On the detail above I don’t think the compacfoam is substantial enough and is sitting on slab and insulation

The entire 2 storey outer leaf of the MBC TF twin wall system sits on it (the EPS upstand), and it's rated to also take the feck knows how many kg/m2 of cement board + adhesive + 30mm thick stone slips that hang of that outer leaf.

[Russdl]

@Ed_

 

Not read all of the above, apologies but it is Friday night. 🍷
 

If it’s of any use we used GRP angles to support the sliding doors and negate any thermal bridge. 
 

MBC, timber frame. 
 

[craig]

33 minutes ago, Nickfromwales said:

The entire 2 storey outer leaf of the MBC TF twin wall system sits on it (the EPS upstand), and it's rated to also take the feck knows how many kg/m2 of cement board + adhesive + 30mm thick stone slips that hang of that outer leaf.

it does but if you look at the two details in comparison, they are different. The detail from MBC is spreading the load between EPS  & slab. The above detail is compac foam sitting 60% approx. on slab, 40% on insulation. 
 

It needs supported full depth and down, not on 100mm of insulation.

 

[Ed_]

Thanks everyone for your thoughts, only just catching up.

 

@craig whilst the compacfoam does rest on the EPS and concrete, the track of the sliding door is above only the concrete so the load path will be directly down through the compacfoam Into the concrete, the compacfoam will see no bending and the EPS will see essentially zero load from the slider. I could adjust the compacfoam so it is only on the concrete but I see no advantage to doing thay- the compacfoam should help spread any point loads from the sill. Or at least that's what I'm thinking, maybe I'm wrong?

 

@Nickfromwales is the EPS the frame rests on integral with the slab Insulation? My concern is that mine probably will not be, and therefore may move somewhat Independently of the slab. And then I worry about the DPC, probably because I don't understand the detailing.

 

@Russdl I'd love to see any more details of the GRP and it's fitting if possible.

[Nickfromwales]

2 minutes ago, Ed_ said:

@Nickfromwales is the EPS the frame rests on integral with the slab Insulation? My concern is that mine probably will not be, and therefore may move somewhat Independently of the slab. And then I worry about the DPC, probably because I don't understand the detailing.

 

That particular one is a KORE raft foundation system, so yes, it's a big 'L'-shaped profile so cannot 'float' or detach. Your detail is perfectly 'doable' with the correct fixings and methodology to keep the insulation in place.

 

Looking again at your detail, why the 300mm slab design with UFH in screed over just 50mm of insulation? Who's design is this, and what is the rationale for the spec?

[Ed_]

It is not finalised, but it is a combination of me and my structural engineer.

 

The rationale is that it is a walkout basement and the detail between the basement and the timber frame that makes up the top two floors means I can only have 100mm of external insulation on my basement walls (unless I step it out below the plinth, which I don't like the idea of), which is probably not quite enough, so I'm bulking it up a bit with 50mm internal Insulation (it would be enough if it was all basement, but it's not enough for the sections which are above ground- about 50% of the wall area). Under the slab will be 150mm, just because, so there is probably no need for the 50mm internal insulation there except I'd rather thermally isolate the screed a bit more from the concrete slab as the walls will be colder than the floor. Does that make sense? All thoughts appreciated, it seems like the best compromise of quite a few aspects to me but I'm not an expert...

[craig]

1 hour ago, Ed_ said:

whilst the compacfoam does rest on the EPS and concrete, the track of the sliding door is above only the concrete so the load path will be directly down through the compacfoam Into the concrete, the compacfoam will see no bending and the EPS will see essentially zero load from the slider.

Has that detail been checked by the supplier/manufacturer? As with experience, the fixed pane/sliding section are over the slab but the front section which people will walk on is not. It has resulted in problems with operation in the past and warranties. All I’m doing is giving my experience and knowledge, you may find if issues occur that the warranty is invalid as it has not been installed as per manufacturers recommendations (I recommend that the manufacturer has checked it and confirmed it’s within their recommended details and warrantied).

[Ed_]

7 minutes ago, craig said:

Has that detail been checked by the supplier/manufacturer? 

No, I haven't selected a manufacturer yet, but thanks for the tip.

[craig]

9 hours ago, Ed_ said:

but thanks for the tip.

No problem, as supplier I would be asking for it to be changed as would my manufacturers.