Internal sliding door threshold - fill cavity with foam?

21 Dec 2005
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United Kingdom
House built in 1979. I believe it was built with the PVC sliding door which we've recently replaced for an aluminium door (as part of a much bigger project). As per the photos, there is a gap between floorboards and rear of door (threshold?) and I can see down the cavity. The DPC is two courses down. Internal floor is hollow / timber floorboards. We get the occasional nighttime slug visiting which is minging and I assume they're coming up through the gap. I can feel a cold draft coming up. I can see water at the very bottom of the cavity (we seemingly have a high water table). We're getting condensation collecting in the sliding door tray which I assuming is damp air rising through the gap, hitting the door and turning into water droplets (correct me if anyone disagrees????). In the pics you can see damp chipboard. I think this is from condensation running off the door, onto the carpet and getting the wood wet (again, feel free to correct).

Long bit of background, short question - should I just fill the void (being careful not do go below DPC) and this could solve all my problems? I've seen advice saying people can use foam (but multiple layers to build it up), or I've got lots of PIR spare so could cut some (tight fitting) to put into the gap and then foam and remaining gaps.

Am I well off the mark? Something else I need to consider? Are both my approaches OK and is there a preference?



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