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We have French windows on the first floor of our three storey house (and no, we don't get a lot of snow). The door-step of which is a nice wooden step, which looked ok at first. After a few years though the skirting board in the corner of the window started to get mouldy and wet. So I pulled up the step and underneath is a big gap which exposes the bare metal lintel for the doorway underneath, and guessing some other opening somewhere because there is a wicked draught blowing through, a small hairdryer's worth on a windy day. The floor has always been cold by the doorways which presumably has caused the condensation in the corners. Here's a pic from my garage adventures of the one in the front - a bit less of a gap but still the same problem:
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The joists run parallel to the window, and in this case there is insulation in the rest of the floor but not between the wall and the last joist, a gap of about 6-8". I was considering blocking the joist space either side of the window with expanding foam to cut out the draught, and then filling the gap itself with polyester insulation from a duvet on account of it being much cheaper and easy to work with than rockwool.
So the question is, can I do this? Is the gap required for ventilation or something? Will I have further damp problems if I block this airway?
Is it a cavity wall and, if so, is the cavity insulated? If the answer to either of those is "no" then you will still have a cold bridge, ie those bricks(?) exposed on the inside will be cold and prone to condensation. What you are proposing might help, and certainly shouldn't make things any worse. IMHO.
Those are thermalite blocks, they have a pretty high insulation value I think. There's a cavity (it's a 10 year old house) but no insulation, afaik. In any case, the house is toasty warm everywhere else - it's just around these doorways. Probably because of the draught getting in.
A 10 year old house certainly should have insulation, no if's no buts.
I can't quite work out which gap you are considering filling, is it the one between the first joist and internal leaf (therms) of the house?
personally I'd be getting busy with the bodgers foam. Someone who really knows about this stuff could say a lot more about the risks of damp and so on but you can always dig it out. If wind is blowing thru there then it can drive water thru
If its a new built house how long is the guarantee - cos that is clearly shoddy workmanship
Might be worth looking under the trim around the door too. That's probably not sealed.
Not picturing exactly where the Gap is but the wet skirting board is probably wicking condensation up from the cold parts. Expanding foam (low expansion and as little as possible if you want the door to open again) should solve the problem.