Floor to ceiling window, closing cavity (at floor level)

SodiumCooled

Hi, I am just looking for some suggestions on how I might close the cavity at floor level across the length of the window in an upstairs bedroom where he window extends to floor level. The cavity is 250mm and I need to insulate and allow the floor screed to run out to the window frame but with such a big gap I am not sure how best to do it. I have asked my engineer to do a detail for it but he has yet to come back.

Using plywood has been suggested fixed to the hollow core and cantilevering over the cavity out to the frame or steel plate in a similar way. I would be concerned on the strength of the plywood as it will be supporting the screed and a persons could potentially put their full weight on it at any point in the future. The steel would be strong but also a possible cold bridge. It also can't be too thick as if it's fixed to the hollow core it will mean either the insulation or screed thickness will have to reduced in this area to maintain level with the rest of the room.

I would welcome any suggestions.

dathi

google "insulated thresholds"

10-10-20

https://www.boards.ie/discussion/comment/120963586#Comment_120963586

Just googling out of interest, I found some EPS types on an Irish site which are interesting to see installed...

I would have thought that the EPS should be supported along the whole length to avoid spot-loading at the wooden shims! Plus, what prevents the threshold falling if there is water ingress at the junction and causes the wood to rot? Also I would have thought that they would have been better recessing the threshold into the cavity as there is no bonding or mechanical linkage between the threshold and the concrete slab, potentially allowing shifting... I get that there is potentially going to be a block/step in front of the EPS... but it all looks like a very poor product demonstration.

SodiumCooled

Hi, thanks for the response - I am probably not clear enough on what I mean. It's an upstairs window sitting on the cill which is at floor level. Its is the gap between the edge of the hollowcore and the window frame that I am talking about. I don't have a picture which would help but if you look at the attached image from google which shows a downstair window in the image but if you can imagine it was upstairs the area between the red line and the window is the cavity and is just basically a hole currently of 250mm deep and 2m wide along the length of the window.

Finished it will look like in the image but there needs to be something to cover the cavity to allow the floor insulation and screed to run to the frame.

10-10-20

https://www.boards.ie/discussion/comment/120964761#Comment_120964761

So if this is a 1st floor window, is the blockwork just incomplete at that point and you need to backfill it and then install a thermally-broken cill? If not, why was the 250mm void left there?

SodiumCooled

https://www.boards.ie/discussion/comment/120964783#Comment_120964783

Hi, not really sure what you mean by blockwork incomplete? The void is the cavity between the inner and outer leaf of block work, it has to be there.

I may still be confusing people, I will try to get a photo later.

10-10-20

https://www.boards.ie/discussion/comment/120964830#Comment_120964830

Maybe it's me, but on the basis of the position red line in the example photo, surely that's the position of the inner side of the inner leaf, no?

SodiumCooled

https://www.boards.ie/discussion/comment/120964844#Comment_120964844

I thought that image was a better way to explain on first look, but looking again its not the best drawing.

10-10-20

Ok, so 250mm wide and not "250mm deep"? right? I read 250mm deep and then saw 250mm along the width in the image, so I assumed it was both 250mm deep and wide. I think that's the confusion.

SodiumCooled

Yes, that was a bit confusing apologies for that - should have said wide. I have actually found photo, I didn't think I had any so that should make things much clearer.

The white is strip is the protective tape on the window frame.

10-10-20

https://www.boards.ie/discussion/comment/120964923#Comment_120964923

👍️ Ok, understood!

I'll leave it to the true experts on here for an answer as I'm only a hack!

sydthebeat

wonderful (as in, these things should have been detailed out preconstruction)

the gap will have to be made up with timber and fully insulated. i assume your pumping the 250mm cavity afterwards?

id fix a batten to the bottom of the hollow core, and one on the same level to the outer leaf, infill with joists which would be notched over these battens, and bridge as necessary

is there another window head under the first floor window? DPCs are vital here

SodiumCooled

https://www.boards.ie/discussion/comment/120965073#Comment_120965073

Thanks very much for the response - in fairness on the preconstruction detail this was not in the original plans, there was just standard windows. I made this change on-site with the blocklayer directly during block laying, the architect only saw it after it was done (and had no problem with the change).

I think I understand your suggestion but just wondering how it will work with the DPC, if we fix a batton wouldn't it puncture the DPC? The battons and mini joists is a very nice idea. Would you then insulate between these and cover with ply to enable floor insulation be put on top (and the floor screed run out over it all?). Yes cavity will be pumped.

There is another window head from a downstairs window overlapping going about 1/3 of the way below this ope, what impact does this have?

sydthebeat

https://www.boards.ie/discussion/comment/120965454#Comment_120965454

i asked about the window underneath because you could then run a continuous dpc from under the first floor window to join onto the ground floor window... but as its not directly below, along the full length of the window, then thats out as an option.

is the window cill wrapped in DPC? its hard to tell from the photo, if so, how far below this is the bottom of the concrete floor slab? is it level? ie a 150 floor slab and a 150 window cill?

SodiumCooled

https://www.boards.ie/discussion/comment/120965927#Comment_120965927

So I went over this evening and as usual I didn't get things exactly correct working of memory. I have done a little block diagram looking from the side which might help explaining - it also helps answer some of your questions.

Both cill and hollowcore are 150 but as you can see the cill is well above the hollow core there is only about 30mm or so of overlap between them - this allows for floor insulation and screed to finish at the window frame. The gap between the hc and the cill is actually about 160ish since the cill overhangs the cavity (a fact I keep forgetting). The DPC covers the cill plus a little extra with the mortar joint etc so from the top of the block work just below the DPC is approx opposite the middle of the hc slab. The hc slab is also partially sitting on an rsj which is there to carry the hollowcore over the ope below - probably not overly relevant but I mentioned anyway.

It will be somewhat awkward to implement your suggestion I think with the over hang of the cill. I wonder with only a 160mm gap and adding maybe a 100mm baton on the HC to close the gap to 60mm would that be a small enough gap to just cantilever ply out as far as the cill? Getting that 100mm baton onto the HC would be tricky enough though I feel.

sydthebeat

Ah ok, 160mm over hang is much easier to resolve than 250mm.

Personally I would:

Use 18mm ply cut 450mm wide and fix to top of HC slabs. Insulate over, using 20mm thinner insulation in this section.

Fix UFH pipes (which is what I assume your doing) and lay B283 mesh over in this section, 500mm wide (confirm with certifying engineer)

Pour screed.

dathi

https://www.boards.ie/discussion/comment/120964760#Comment_120964760

they are not wood shims its a steel bracket with pvc shims

hesker

I’m only a hack too but I think this could be closed using high density EPS.

You could cut the EPS so that you have two triangular sections that fit together in a French cleat type arrangement.

One section can be glued and screwed to one face and the other can sit down on top of the first.

Or alternatively just drop in a box length of it and screw through at an angle

SodiumCooled

Thanks all for the suggestions, I think the simplest solution is that suggested by syd above - I can jam some PIR insulation and secure with expanding foam between the cill and HC (just level with the top of the HC). Then cut the section of ply as suggested above to fit in and be secured to the HC.

One questions would be is cold bridging a concern with timber? i.e. can I push the ply right up right again the DPC/cill or should a small gap be left which could be filled with foam or similar?

I will also confirm my architect is happy with the soultion as suggested also.

sydthebeat

https://www.boards.ie/discussion/comment/120977127#Comment_120977127

160mm wide x 18mm plywood, insulated above and below, will have a completely negligible thermal bridge.

Dont worry about it.

FJMC

Sorry - tried to post on here before and for some reason it didn't load.

Ideally you want an insulated cavity closer that can be load bearing.

Cavalok might be worth looking at / or speaking to.

It might also be easier to only have a floor finish over the cavity closer rather than the screed - subject to what your floor finish is.

F

Mellor

https://www.boards.ie/discussion/comment/120977178#Comment_120977178

Agreed. The ply is not a concern. The screed needs to to be considered too. As it now interfaces with the outer leaf, rather than inner as normal.

sydthebeat

https://www.boards.ie/discussion/comment/120985919#Comment_120985919

the standard spec here would be 25mm edge insulation, which would help with expansion as much as thermal bridging. Not much different than a standard cill detail to be honest.

Mellor

https://www.boards.ie/discussion/comment/120985951#Comment_120985951

Its definitely the better detail. But I’ve seen edge insulation omitted at first floor. Presumably as the hollow core is not a cold bridge they think it’s not required.

I was detailing a floor level window sill (pre construction). It would be exactly that, a standard cill detail dropped to FFL

SodiumCooled

https://www.boards.ie/discussion/comment/120985951#Comment_120985951

https://www.boards.ie/discussion/comment/120986246#Comment_120986246

Thanks for the additional comments, I was planning to just use the 8mm foam edging strip along the cill to help with expansion but maybe since it's in contact with the outer leaf this isn't sufficient.

I am doing the floor insulation myself but on the suggestion/spec of my architect I am not using edge insulation anywhere either on the ground or first floor. He has said he has moved away detailing edge insulation in recent times as it leaves an awkward edge for finishing since you can't plaster to the floor. To allow for expansion of the concrete he has specified the use of an 8mm foam strip instead which is stuck to the wall after insulation is down and also has a strip the folds out and sticks to the polythene covering the insulation to stop any screed getting underneath the strip.

Mellor

https://www.boards.ie/discussion/comment/120986369#Comment_120986369

that does not sound very good to me, and precisely the shortcuts I was think.

Ground floor slab needs to be thermally broken (as much as possible) from the could foundations.

First floor screed should be thermally broken from blockwork to enable faster response in the heating. Where the screed approaches the outer leaf, this break is critical

SodiumCooled

https://www.boards.ie/discussion/comment/120988890#Comment_120988890

There is a mannok aircrete thermal blocks as the first block on all ground floor walls which should significantly reduce the thermal bridging to the walls from the slab and while only 8mm my understanding is that the foam edge strip does give a thermal break also. I'd be surprised if my architect was not confident it will perform well, he is a bit of a stickler in other things he has detailed on reducing thermal bridging.

On 1st floor there are no thermal blocks on the hollowcore but I would have thought this is it is not seen as necessary off the hollowcore and that the 8mm foam must be seen as sufficient as a thermal break.

At the cill is possibly a little different though I will need to see if an edge insulation is needed at this location though as it is in contact with the outer leaf.

Mellor

https://www.boards.ie/discussion/comment/120989839#Comment_120989839

The thermal blocks are better than standard concrete, but I would not consider them adequate to replace edge insulation. Any insulation is a thermal break to a degree, but 8mm is not a significant thermal break imo. At first floor the insulation is required for the screed not the hollowcore. Should be decoupled everywhere and especially at the outerleaf

SodiumCooled

https://www.boards.ie/discussion/comment/120993605#Comment_120993605

Apologies I misunderstood your comment on the first floor - you are referring to floor insulation not edge insulation. We are putting 50mm of floor insulation throughout the 1st floor.

edit: actually ignore the above I think you are referring to edge insulation, what I meant is that I did not think the hollow core would be transferring much cold onto the blocks on the inner leaf and in turn onto the screed. The outer leaf is possibly a different story. If I were to place 25mm edge insulation at the cill how is this gap finished? Just run the laminate floor our over the insulation to the cill to cover the gap? This gap appears to be problematic hence why the foam is being specified throughout rather than edge insulation.

sydthebeat

https://www.boards.ie/discussion/comment/120994118#Comment_120994118

the poster IS referring to edge insulation.

8mm is not really sufficient thermal break at all at this window detail, if there is no other insulation between the cill and the poured screed.

16mm may be enough to counter expansion (though personally i wouldn't be happy with that) but 8mm along a 50-75mm deep screed wouldn't be acceptable in my view.

As a technical matter, it wouldn't comply with the 'acceptable construction details' which allows you to assume a thermal bridging factor of 0.08 in your Part L assessments. Instead youd have to use a 0.15 factor which may impact your Part L compliance.

see here:

interestingly, in order for you to achieve that 2.9 m2K/W resistance, youd need to use a 65mm strip of PIR insulation (at 0.022 thermal conductivity value)

SodiumCooled

https://www.boards.ie/discussion/comment/120994173#Comment_120994173

Thanks, the attached image makes sense as this is how all other windows are finished except with an approx 160mm section of PIR tightly fixed between the block work and cill. Of course this is not possible in this instance since the cill sits higher. If I was to add 65mm of PR the screed would finish 65mm back from the cill - the insulation would also need to actually go up the window frame a little as finished floor level is above the cill and into the window frame. I am not sure how easy it will be to deal with this 65mm strip of insulation with no screed at finished floor level. At that thickness I would think it would be difficult to simply run the laminate out over it.

As for the 8mm strip, aside from my architect specifying it I have been looking online this seems to be a fairly common expansion gap product, the product (from multiple suppliers) is marketed specifically as a UFH screed expansion foam so is it really that insufficient in your opinion?