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Old 09-25-14, 04:44 AM   #29
TimSmall
Geek about the house
 
Join Date: Dec 2012
Location: Brighton, Great Britain
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I don't think there's any problem with using fibre glass, so long as it's kept pretty warm. i.e. the lowest PIR board needs to provide (rule of thumb) approx 50% of the insulation value, so that the timber is warmer+dryer.

Given your crawl space, I think I would put EPS underneath the polythene, and also stick it to the walls, and leave the timbers open. Air seal between the polythene and the floor. No ventilation. Cut out the bottom of the timber where it would protrude through the EPS, and replace with AAC block.

The "airspace" in the image which AC quoted from the building science article has two functions.

1. To keep the timber warmer i.e. insulation is towards the lower side. Filling the gap would make the timber cooler, and thus damper.

2. Any points with increased moisture (e.g. spill, moisture ingress from walls etc.) will readily spread out along the airspace and exit the structure over a much wider area (so quicker) than if it were full filled.
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