New external door and exposed cavity- to fill or not to fill

[smarriott]

Morning

I have had a set of french doors put in where previously existed a window.

The house is about 17 years old and has a good size cavity. Trouble is the cavity appears on the inside of the house and I am not sure what to do with it.

I am having the floor tiled in a couple of weeks and the cavity needs filling with something. The tiler suggested filling with 75mm polystyrene and he would tile on top, which sound like trouble to me.

I was going to line the cavity with thick blue builders polythene and fill with concrete.

Can anyone offer any advice on the best approach, obviously I don't want to be breaching the damp proofing in anyway.

Thanks


After removing the windows and brickwork beneath

 

[churchilldog]

Expanding foam to "fill" cut down then tile over. Depending on size of cavity depends on what expansion foam you need.

 

[smarriott]

Thanks for the suggestion, I am just a little concerned the expanding foam filler will not provide the stability to tile over.

The cavity is 4". My preferred option would be to fill with blue membrane and concrete.

Can anyone advise if this might cause damp problems?

Thanks

 

[Deluks]

If the tiles are thick enough, then going over insulation board shouldn't be a problem if the tile actually bridges over it.

 

[smarriott]

The tiles are 10mm porcelain so fairly weightly, trouble is there is nothing exposed of the outer blue brick to rest them upon.

So 4" of tile would be resting on expanding foam filler in a farly high traffic doorway. I forsee movement and cracking issues.

 

[Deluks]

Tricky one, but insulation foam board should be fairly stable against thermal or moisture related movement, unlike say timber or metal.

The problem with mass filling with concrete is that of a thermal bridge, granted it's right next to glass which in itself is 'lossy' but these things are best avoided if possible.

What about, say, 50mm of insulation board (room side) wedged in with an off cut of 50mm thick paving slab, mortared or glued into place (decent cartridge adhesive, not copydex!)


A pic would clear things up a little.

 

[freddiemercurystwin]

If there are any spacers fitted below the cill (ie a gap), is there any way to slide one of these

under the cill to sit on top of the brickwork? Really, this should have been fitted before the door was inserted. Various sizes are available.

 

[smarriott]

I am not so much bothered about thermal loss, it's more the risk of water and damp if I fill with polythene and concrete

 

[Deluks]

Then your plan should work. With regards to bridging the dpc the devil is in the detail so it's tricky to advise on here without looking at it.
You could mix polystyrene beads into the concrete to reduce thermal loss, or cut some celcon blocks to go in the gap.

 

[noseall]

Fill the floor trough with screed and line it with polythene. Don't worry about thermals as the french door is one great big cold spot.

 

[freddiemercurystwin]

I am not so much bothered about thermal loss, it's more the risk of water and damp if I fill with polythene and concrete

I realised that, I assumed you wanted a little extra support for the tiles which the closer I suggested would offer if you can get support beneath the frame.