Options for under patio door threshold

[Kaymo]

Hi,
I am replacing a window with a patio door of the same width. The cut out has been done and I am now preparing the opening for the new triple glazed door. As it will be very heavy and with a low level aluminium threshold, I want to have the threshold substrate prepared and completely level and flat in advance. The positioning of the door is defined by the corner post and the depth of the door (145mm), which means it will sit directly above the original cavity. What I have done is taken the wall down to ground level, filled the bottom of the cavity with concrete to get a sold base, then built back up with one course of aerated 3.6N block a little deeper than the door.

I need to go up about 50mm more and so far considered two options. One is compacfoam, which would give me insulation under the threshold to avoid a cold bridge. This is v expensive, and the other issue is that I can't get it in the right size, so I would have to cut it and this is probably not going to help with a flat and stable base. The other is a strip of treated timber, which would be cheap and easy to source at the right width and depth, but wouldn't have the insulating qualities of compacfoam.

Are there any alternative options I have not considered?

Thanks.

 

[Notch7]

It is a bit difficult to see whats going on here from your expaination.


Typicall the internal blockwork is cut down to concrete slab level, then floor insulation and floor screed runs across to the back of the external skin.(with dpm between)

If your door goes inline with the cavity then you could put in a strip of dpm and fill with concrete, if it needs to be higher, add cut blocks to suit.
The floor insulation then runs across to that (which would be inline with internal face of door frame. There is no thermal bridging because its below floor level.

Externally you need to infill with a split course if face brickwork.

 

[Kaymo]

Thanks Notch, hopefully below will make it clearer, sorry for the childlike drawing, had to do it with the mouse! I could have poured concrete to get it level instead of using block, but I was trying to avoid a thermal bridge at the edge of the door, so the combination of block with compacfoam would have met the structural and insulating requirements. I am just questioning the compacfoam now because of cost/benefit as it will run into the £hundreds and not sure it is worth it and wondered if there were any other options. If using timber then DPC would go under it, if compacfoam DPC not needed, although I might need to use one anyway to keep BCO happy.

 

[foxhole]

Internal insulation between internal slab and block prevent cold bridge, not at door frame.

 

[Kaymo]

Internal insulation between internal slab and block prevent cold bridge, not at door frame.

Thanks foxhole, I plan to put insulation in that gap, but there is also a cold bridge through the threshold as per the blue arrows below if there is block directly underneath. Its not hugely significant but the less conductive the material directly underneath the aluminium threshold the better.

 

[foxhole]

The frame itself is a thermal bridge so no advantage in separating it from base .

 

[Kaymo]

The frame itself is a thermal bridge so no advantage in separating it from base .

Not sure what you mean by this? If you mean the door frame then it is 145mm depth timber, so the frame has a u value of around 0.7, even at the shallower points on the windows its probably about 1.2, neither of which would normally be called a thermal bridge. The aluminium threshold will offer no resistance and conduct directly into whatever is underneath it, so if it is block then it would only be a path a few cm of block between the outside and inside.