Help with insulating timber suspended floor


Jun 21, 2022
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United Kingdom
Hi all.

I have recently had a small side extension completed to my house, and the builder has wrapped up and left. I have an issue, whereby the new front door (the opening of which the builder created) is one brick course higher than the side door leading out of the utility on the opposite side of the house. The builder says that the DPC steps at some point around the house, so he put the door in at the level at which the DPC was (seems a sensible thing to do). What this means though, is that when he installed new joists to tie into the existing in this area of the house, there is a gap of approximately 125mm between the top of the timber joists and the underside of the new front door frame (cill).

I have already installed 120mm PIR insulation between the floor joists, to provide a good level of insulation to the area. The floor is receiving water UFH, and so if there were not this level issue, it would have just received 18mm routed UFH chipboard, and then tile adhesive and tiles. Given that the make up of these will be approximately 40mm, I would still have an 85mm gap between top of finished tiled floor and the underside of the door frame, and this would leave the top course of brickwork below the door showing.

The best solution to get around this I can think of is as follows:

There is a partition creating the utility which incorporates the lower existing side door, and so I will probably just leave this area with no UFH, and there will be a step down into this room at the threshold. For the other areas outside of the utility (incorporating the higher front door), the only way I can think to bring this level up is to install yet more PIR insulation (80mm) OVER the joists, prior to the routed chipboard and my tiles.

So the make up would be:

5x2" joists with 120mm (full fill) PIR insulation between joists
80mm PIR insulation
Tile adhesive

Is this acceptable? Are there any strength issues that may arise? I can't see why the tiles and chipboard would not adequately spread any loads imparted on the floor across the layer of PIR below.


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