What goes under the door sill?

[MortarThePoint]

Does anyone have a door sill threshold detail I can have a look at? It's not for a level threshold, but I am finding it difficult to find a drawing.

 

I am unclear what goes under the door sill (see red arrow). Should there be an insulated cavity closer there underneath the door sill? I am using blown bead insulation, so more broadly need to understand if insulated cavity closers are needed elsewhere anyway or if I can just use uninsulated cavity closers.

 

I've used a sill height of 25mm. I am not sure if that is standard, but I saw one that was. It will be a timber sill. I have up to 50mm for screed and floor finish.  I have probably put the door in the wrong place relative to the wall face, as I just popped it in roughly. The structural floor element is a precast concrete floor slab (Thermabeam) with insulation prebonded on its underside.

 

 

[nod]

4 hours ago, MortarThePoint said:

Does anyone have a door sill threshold detail I can have a look at? It's not for a level threshold, but I am finding it difficult to find a drawing.

 

I am unclear what goes under the door sill (see red arrow). Should there be an insulated cavity closer there underneath the door sill? I am using blown bead insulation, so more broadly need to understand if insulated cavity closers are needed elsewhere anyway or if I can just use uninsulated cavity closers.

 

I've used a sill height of 25mm. I am not sure if that is standard, but I saw one that was. It will be a timber sill. I have up to 50mm for screed and floor finish.  I have probably put the door in the wrong place relative to the wall face, as I just popped it in roughly. The structural floor element is a precast concrete floor slab (Thermabeam) with insulation prebonded on its underside.

 

 

This is what I did 

[Mr Punter]

If this is new build you can lose the inner leaf under the door and run the eps and screed up to the brick outer leaf.  Doing as you are you risk differential movement where you have blockwork v eps

[PeterW]

5 hours ago, MortarThePoint said:

Does anyone have a door sill threshold detail I can have a look at? It's not for a level threshold, but I am finding it difficult to find a drawing.

 

I am unclear what goes under the door sill (see red arrow). Should there be an insulated cavity closer there underneath the door sill? I am using blown bead insulation, so more broadly need to understand if insulated cavity closers are needed elsewhere anyway or if I can just use uninsulated cavity closers.

 

I've used a sill height of 25mm. I am not sure if that is standard, but I saw one that was. It will be a timber sill. I have up to 50mm for screed and floor finish.  I have probably put the door in the wrong place relative to the wall face, as I just popped it in roughly. The structural floor element is a precast concrete floor slab (Thermabeam) with insulation prebonded on its underside.

 

 

 

Door stands on the edge of the brickwork. What is the gap filled with currently behind the bricks..? I would use EPS or PIR to reduce any cold bridging and then screed straight over the top of it

[MortarThePoint]

1 hour ago, nod said:

This is what I did 

 

Thanks no, looks good. If I understand correctly, that is some membrane taped (on both sides) to the membrane that is coming up from under the beam&block? That membrane is then passed over the PIR insulation.

[MortarThePoint]

39 minutes ago, Mr Punter said:

If this is new build you can lose the inner leaf under the door and run the eps and screed up to the brick outer leaf.  Doing as you are you risk differential movement where you have blockwork v eps

 

I should have explained, the shaded grey part is the concrete element of the Thermabeam floor and the EPS is prebonded to that. You can see the EPS in the photo of the end of a Thermabeam slab below which is a darker grey than the concrete. The EPS extends all the way to the edge of the cavity and some concrete pillars cut through it to support the slab and the wall above it.

 

[MortarThePoint]

23 minutes ago, PeterW said:

Door stands on the edge of the brickwork. What is the gap filled with currently behind the bricks..? I would use EPS or PIR to reduce any cold bridging and then screed straight over the top of it

 

@PeterW and @Mr Punter , I think you're both suggesting I put EPS in the cavity below the membrane and then screed over the top of that. That makes the screed the only structural part of that and it's only 35mm thick cantilevering over a 100mm cavity. That makes me a bit nervous it could crack under load.

[PeterW]

5 minutes ago, MortarThePoint said:

 

@PeterW and @Mr Punter , I think you're both suggesting I put EPS in the cavity below the membrane and then screed over the top of that. That makes the screed the only structural part of that and it's only 35mm thick cantilevering over a 100mm cavity. That makes me a bit nervous it could crack under load.

 

Why is it cantilevered..? The EPS/PIR is supporting it. The insulation is structural

[MortarThePoint]

20 minutes ago, PeterW said:

 

Why is it cantilevered..? The EPS/PIR is supporting it. The insulation is structural

 

The EPS or PIR would be a vertical sheet in the cavity. Remember that the EPS of the floor is prefab into the floor so ends at the edge of the cavity. I guess the EPS or PIR could be cut to the height of the cavity at that point so it is bearing on the cavity fill concrete at the base of the cavity, but I wouldn't have thought that is very strong. EPS squishes pretty easily. That leaves the screed as the only truly structural part doesn't it?

[nod]

54 minutes ago, MortarThePoint said:

 

I should have explained, the shaded grey part is the concrete element of the Thermabeam floor and the EPS is prebonded to that. You can see the EPS in the photo of the end of a Thermabeam slab below which is a darker grey than the concrete. The EPS extends all the way to the edge of the cavity and some concrete pillars cut through it to support the slab and the wall above it.

 

Yes it’s normal to knock the internal skin out in doorways and add insulation in its place and run the floor insulation into it 

[MortarThePoint]

Below is what I now understand would be the normal situation for a beam & block floor with insulation and screed carrying on across the cavity.

 

 

If I carried on the screed across the cavity:

 

With EPS sheet insulation added in to void below:

[Mr Punter]

Ask Thermabeam to  provide you with a typical door threshold detail.

[MortarThePoint]

3 minutes ago, Mr Punter said:

Ask Thermabeam to  provide you with a typical door threshold detail.

 

Good thought, I've just emailed them so fingers crossed. They haven't been hugely forthcoming we details so far though.

Edited June 21, 2020 by MortarThePoint

[Mr Punter]

If you have not signed up with them yet, why not just normal beam and block?  Could save a fair bit.

[MortarThePoint]

I asked and never got an answer about this from the Thermabeam guys

 

Seems like everyone does some form of bodge where they rely on the screed to cantilever over the cavity a bit. LABC drawings call for a structural cavity closer or a reinforced cavity closer, but talking to cavity closer manufacturers there doesn't appear to be such a thing. This drawing shows a way of doing it, but still relies on the flexural strength of the screed as the cavity closer isn't structural:

 

 

https://www.slideandfold.co.uk/alumina-low-threshold-with-cill.pdf

 

Good luck drilling the 'External Brick Work' that close to its edge.

[cwr]

 This what I intend. Our cavities only go two courses below the floor level so not really much to build up. The 100mm of structural insulation below the door will be foamglas.

Edited July 31, 2021 by cwr

[MortarThePoint]

13 minutes ago, cwr said:

 This what I intend. Our cavities only go two courses below the floor level so not really much to build up. The 100mm of structural insulation below the door will be foamglas.

 

Thanks, looks like a good arrangement. Is that a timber frame above ground level? Looks like very thick walls.

[Makeitstop]

I remember reading this some time ago which kind of tackles a similar issue...

 

 

[MortarThePoint]

Purenit (link) looks like good stuff.

 

It's a bit late for me as I'm not sure I'd have space for anything over about 10mm thick sat as a bridge over the cavity and then masticed under the sill. Mocked up below. The Purenit would probably need to be bedded on something which would add an additional ~5mm. I don't know whether 10mm of Purenit would be stiff enough.

 

 

I think the back of my door sill will be approximately 30mm behind the cavity edge and so the sill would cantilever 30mm and so the screed cantilever 70mm (100mm cavity). I'm looking at having a 50mm screed now, so maybe adding a small strip of mesh would be good. If there is a product like bed joint reinforcement that adds strength in two dimensions, not just one, that would be good to embed in the top of the screed to help with tensile strength.

[cwr]

"Is that a timber frame above ground level? Looks like very thick walls"

 

It's block cavity walls, with 240mm cavity. Yes, nice and thick.

[MortarThePoint]

I think I may have enough height for something sensible to bridge the cavity. Not sure what, but the image below shows the structural material in RED bridging the cavity. Some candidates:

• Purenit, ideally 20mm or less bedded on mortar on the Subfloor and outer leaf brickwork.
• Slate, plenty of space for slate. It forms a cold bridge but not much of one in comparison to all the people that just bridge the cavity with screed.
• Hardie backer type cement board
• A metal sheet ~3mm thick

It would be great to be able to use something like wood that could also be cut to form the cill, but I think the rot risk with wood is way too high. Is there a nice structural, hydrophobic but paintable, easily machinable and cheap material that I could use here?

 

[Mr Punter]

I lose the inner leaf and take the insulation up to the outer brickwork.  You could run the hollowcore closer for this area.  A 25mm thick upstand of Celotex so as not to cold bridge the screed.

[MortarThePoint]

58 minutes ago, Mr Punter said:

I lose the inner leaf and take the insulation up to the outer brickwork.  You could run the hollowcore closer for this area.  A 25mm thick upstand of Celotex so as not to cold bridge the screed.

 

I can't do that unfortunately as my insulation is underneath the concrete subfloor (precast Thermabeam).

 

Looking at the datasheet, HardieBacker looks like a good option. Below is the table relevant to the HardieBacker 500 Cement Board which is 12mm thick. It has pretty high flexural strength, 2x or 3x that or cured screed. Purenit (datasheet) has a compressive strength which is only 4% of HardieBacker, so it's flexural strength will be poorer as well. Halving the thickness of a bending member reduces is stiffness by a factor of 8 and its strength by a factor or 4, so even though the HardieBacker is half the thickness of 25mm Purenit, it would have about 6x the strength.

 

Rearranging the equation for Flexural Strength, the fracture force is F = (2 * width * thickness^2 * flexural_strength) / (3 * span). Popping some numbers in:

• thickness = 12mm *(100%  - 6%) = 0.01128m
• width = 100mm = 0.1m (complicated this as there is obviously more to the side of the loaded the area, but if there is load spreading under a point load...)
• span = 100mm = 0.1m
• flexural strength = 10MPa (this was the stated requirement and the result was higher)

This gives a fracture force of F = 850N = 85kgf. Great, that should strong enough! A bloke in high heels can stand on the centre of the cavity and not crack it.

 

Thermally, it has half the thickness of 25mm Purenit and about twice the thermal conductivity, so same result. The Psi-value would probably be something like 0.19W/mK * 0.012m / 0.030m = 0.08W/mK. That's better than most lintels (0.3 to 0.5 W/mK). So if 2.4m long and a 20C temperature difference, it will conduct 0.08W/mK * 2.4m * 20K = 3.8W of heat. I used 30mm as the length of the bridge as that is the distance between the warm screed and opposite cavity edge.

 

It's even thin enough that I can get a nice bed of mortar under it on both sides. Would be cool if you could get it in ~225mm wide strips or ~225mm square tiles, but it should cut easily.

 

 

Edited August 16, 2021 by MortarThePoint

[Mr Punter]

15 minutes ago, MortarThePoint said:

A bloke in high heels can stand on the centre of the cavity and not crack it.

 

Are you thinking of inviting @pocster round?

[markc]

12mm hardie backer will do nicely, it is incredibly strong and easily resists the twisting forces likely in this application.

bugger to cut, scoring with diamond blade is the best but the dust is horrendous.