Top up loft insulation and interstitial condensation?
Discussion
My loft has ~ 100mm loft insulation and ~ 100mm deep rafters.
It’s a late 70s bungalow so no fancy DP membranes on ceilings etc.
If BR is 270mm then that’s 170mm or half a foot of insulation burying your rafters and bottoms of trusses.
Surely this pushes the interstitial condensation point into the depths of buried timbers that can’t then get airflow around them?
Surely rafters are at risk of absorbing condensation and being unable to offload it?
Or can you run the insulation in line with existing rafters so they’re still exposed? (albeit heavily occluded)
It’s a late 70s bungalow so no fancy DP membranes on ceilings etc.
If BR is 270mm then that’s 170mm or half a foot of insulation burying your rafters and bottoms of trusses.
Surely this pushes the interstitial condensation point into the depths of buried timbers that can’t then get airflow around them?
Surely rafters are at risk of absorbing condensation and being unable to offload it?
Or can you run the insulation in line with existing rafters so they’re still exposed? (albeit heavily occluded)
It’s ventilated via the underside of the bottom tiles with those vents that lap over into gutter tops and to the front face of the soffits.
The soffits are solid.
A bit like this:
Bitumen felt lapped up over ridge so no ridge venting.
I assume the paradigm is cross-flow.
100mm insulation largely leaves eaves gaps down to vents clear but some look occluded or bunged up with insulation touching the felt.
I plan to use those spacer vent things to allow me to push insulate right down to the edge of ceilings but keep the ventilation clear.
But the point still stands, if there is ~ 150mm of rock wool over rafters, how can they air out?
The U gains are ok but not worth impacting the lifespan of your roof over.
I appreciate if you re-roof with modern felt and do all the associated re-roofing to invoke bringing to modern standards then fine update stuff, but I don’t get how buried timber sitting across an interstitial condensation gradient is at all allowed?
The soffits are solid.
A bit like this:
Bitumen felt lapped up over ridge so no ridge venting.
I assume the paradigm is cross-flow.
100mm insulation largely leaves eaves gaps down to vents clear but some look occluded or bunged up with insulation touching the felt.
I plan to use those spacer vent things to allow me to push insulate right down to the edge of ceilings but keep the ventilation clear.
But the point still stands, if there is ~ 150mm of rock wool over rafters, how can they air out?
The U gains are ok but not worth impacting the lifespan of your roof over.
I appreciate if you re-roof with modern felt and do all the associated re-roofing to invoke bringing to modern standards then fine update stuff, but I don’t get how buried timber sitting across an interstitial condensation gradient is at all allowed?
Wouldn't be a huge surprise for it to be a genuine issue, but not enough that people would notice consequences any time soon? Wonder if there's any research?
Wouldn't be the first time a retrofit insulation 'improvement' caused an issue because the overall construction wasn't designed for it.
Wouldn't be the first time a retrofit insulation 'improvement' caused an issue because the overall construction wasn't designed for it.
The last time I topped up some roof space insulation, I suggested to the customer to have some felt lap vents installed to try and head off any problems, this particular roof not having any existing ventilation as far as I could tell. Maybe install some and use the eaves baffles to retain the airflow over the facia.
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