Insulation

Loft Condenstation Insulation Help [no felt - 75mm rafters - existing floor insulation - draughty]

Anonymous user 23/02/2024 - 3.52 PM

Hi, I've a ~1920s terrace with a very old roof (apparently possibly original timber), however over this third winter I've noticed a fair issue with damp/wet/mould on the timbers - in partricular the battens, rafters, & ridge. It has been pretty solidly dry since moving in and has already had insulation laid above the ceiling, along with boarding installed accross the majority of the "floor" space. Around mid Jan I went up for something and it really was pretty damp in the air up there - a lot of the wood was wet with mould growing. After my initial panic of thinking there was some major leak a local roofer had a look and confirmed actually there didn't appear to be anything coming in, but that it was essentially a condensation issue. That fitting insulation between & accross the rafters should resolve it after treating the wood to kill the mould/rot. It's pretty draughty up there (and you can see through to the sky here and there) as the slates are present directly - there is no felting or anything above the battens. It does not leak, and is very good at keeping the heat in the rooms below (I'd need to measure the depth but the roofer was under the impression no more would be needed - and the house is indeed very warm). The advice I recieved was to fill the depth of the rafters between them with kingspan & then insulated plasterboards underneath. After looking up what to do I seem to have hit a snag. It seems that I need to have a 50mm gap for air flow - but the rafters are only 75mm themselves - which would only leave 25mm depth between the rafters. Is it even worth installing this 25mm layer? The rafters are about a foot/30cm apart which makes it a lot of cutting for what seems like minimal return. Have I got it right that there needs to be the 50mm gap - and if so, could the later that is meant to be "between" the rafters actually be under them, with the insulated plasterboards underneath them again. And will this actually solve the issue as it seems that all would be happening would be a reduction of the ventilation which by everything I've read would actually make the condensation issue worse somehow. I'm hoping I've miunderstood in my ignorance, and any advice here would be appreciated. Thanks Matt