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.
I'd suggest monitoring the humidity.
Possibly even within the insulation.
Not much point at the moment if the great outdoors is around 90% humidity, but you don't want long periods of high humidity in the loft.
A BlueTooth hygrometer is about a tenner or less on ebay.
If the loft is cold, you need to take some some care over sources of water vapour.
Ideally the ceiling would be a vapour barrier, but ordinary plasterboard with a few coats of emulsion has reasonable vapour resistance.
I'd suggest checking for vapour pathways in the ceiling, light fittings (specially downlighters), extractors, random gaps, the loft trap.
Also, don't tolerate any rain leak however small, without warmth in the loft to dry it out, it doesn't take many drips to cause a problem.
Possibly even within the insulation.
Not much point at the moment if the great outdoors is around 90% humidity, but you don't want long periods of high humidity in the loft.
A BlueTooth hygrometer is about a tenner or less on ebay.
If the loft is cold, you need to take some some care over sources of water vapour.
Ideally the ceiling would be a vapour barrier, but ordinary plasterboard with a few coats of emulsion has reasonable vapour resistance.
I'd suggest checking for vapour pathways in the ceiling, light fittings (specially downlighters), extractors, random gaps, the loft trap.
Also, don't tolerate any rain leak however small, without warmth in the loft to dry it out, it doesn't take many drips to cause a problem.
Well I’ve not done it yet.
The point is if I start to do over 25% of my roof then I invoke the blessed ‘building regs’ and they say I’d need to upgrade to 270mm insulation and bury my rafters.
Despite 75% of it still using bitumen felt and the whole lot being humidity ‘leaky’ plasterboard and so on into the loft space.
The BR make sense if your entire build is also to modern standards, but to just throw insulation into an old loft seems to me like a recipe for disaster.
In theory I could take the entire roof off in a rain-free world, and moisture from in the house would get through the ceiling, into the rafters, condense, and be packed in unable to dry out.
I’d assume I’m missing something?
But the BR guidance on insulation for doing large roof updates seems pretty terrible and best avoided.
The ‘best’ I’ve thought is to use 150mm foil PIR to gain moisture blocking performance through ceilings, get a good U value, and only occlude rafter tops by 50mm rather than burying them… so not great but infinitely better than no airflow.
Plus you’d enhance flow from eaves, or reduce risk of occluding it.
Ie, all gains. Rather than a better U value only with a lot of potential downsides/risks!
The point is if I start to do over 25% of my roof then I invoke the blessed ‘building regs’ and they say I’d need to upgrade to 270mm insulation and bury my rafters.
Despite 75% of it still using bitumen felt and the whole lot being humidity ‘leaky’ plasterboard and so on into the loft space.
The BR make sense if your entire build is also to modern standards, but to just throw insulation into an old loft seems to me like a recipe for disaster.
In theory I could take the entire roof off in a rain-free world, and moisture from in the house would get through the ceiling, into the rafters, condense, and be packed in unable to dry out.
I’d assume I’m missing something?
But the BR guidance on insulation for doing large roof updates seems pretty terrible and best avoided.
The ‘best’ I’ve thought is to use 150mm foil PIR to gain moisture blocking performance through ceilings, get a good U value, and only occlude rafter tops by 50mm rather than burying them… so not great but infinitely better than no airflow.
Plus you’d enhance flow from eaves, or reduce risk of occluding it.
Ie, all gains. Rather than a better U value only with a lot of potential downsides/risks!
I’m no expert but do you need counterbattens? Aiui the membrane should sag sufficiently so water runs centrally and down and under battens mid rafter gap?
It’s when it’s pulled tight you get issues as water migrates to the rafter (or counterbatten) and then through the elongated nail holes due to being too tight?
I’m not sure what advantage there is to non-breathing membrane? Except maybe putting it on upside down
It’s all amazingly bonkers the lengths we go to on ceilings.
From my calcs some insulation is better than none, and stuff like MVHR would be where my money went over going crazy on a loft to the detriment of longevity or failure mode risks from lapsed or immediate maintenance (ie, real life not idealised perfect future world I expect some architects live in)
Still, I’ve yet to see anyone say why rafters buried in rock wool or similar insulation across a condensing temp gradient won’t result in soggy and rotting rafters in short order!
It’s when it’s pulled tight you get issues as water migrates to the rafter (or counterbatten) and then through the elongated nail holes due to being too tight?
I’m not sure what advantage there is to non-breathing membrane? Except maybe putting it on upside down
It’s all amazingly bonkers the lengths we go to on ceilings.
From my calcs some insulation is better than none, and stuff like MVHR would be where my money went over going crazy on a loft to the detriment of longevity or failure mode risks from lapsed or immediate maintenance (ie, real life not idealised perfect future world I expect some architects live in)
Still, I’ve yet to see anyone say why rafters buried in rock wool or similar insulation across a condensing temp gradient won’t result in soggy and rotting rafters in short order!
Mr Whippy said:
....
Still, I ve yet to see anyone say why rafters buried in rock wool or similar insulation across a condensing temp gradient won t result in soggy and rotting rafters in short order!
If the rafters are inside the insulation, they are warm and there won't be condensation on them.Still, I ve yet to see anyone say why rafters buried in rock wool or similar insulation across a condensing temp gradient won t result in soggy and rotting rafters in short order!
(but you could get condensation on the inside of your felt/membrane, dripping liquid water through the insulation onto the rafters.
That's bad, warm and wet promotes rot.
Cue 'Good Morning Vietnam' reference:
"It's going to be hot and wet! That's nice if you're with a lady, but it ain't no good if you're in the jungle". )
Condensation occurs when a high concentration of water vapour reaches a cold surface.
You avoid condensation by reducing the density of water vapour or increasing the temperature.
That means managing the indoor humidity, controlling the passage of water vapour from indoors to the loft, and allowing the escape of water vapour from the loft to outside the building.
Sorry I was under the impression if you have room temp or 21degC on your ceiling, between there and say -4degC on the top of your insulation, there will be a point between that water vapour condenses.
Surely that point can lay inside buried wooden components that in the case of upgraded loft insulation have no airflow over them?
In any case it sounds like there are mitigations for having to go chasing U value for loft/ceiling insulation.
Also yes the “now colder loft which will be more condensationy” because it’s getting less warmth from the rooms below, but still getting its moisture… is a big risk too!
Definitely not just a ‘follow the rules’ task, but one that needs staging and nuanced consideration.
The main issue for me was 1st task is possibly chunks of old bitumen felt knackered, and think the ridge felt has likely cracked too.
So replacing with quality breathable felt and replacing tiles and battens a no brainer, in phases.
But it’s completely the wrong time to also stuff in a ton of extra insulation until it’s all completed, moisture control tightened up, ventilation improved, and all felt swapped etc.
It sounds like BR won’t be invoked stupidly and sensible consideration will be given.
I’m still very weary of the general advice on blindly topping up insulation though.
With the rush for ASHP and stuff I think a much more holistic approach is needed to finding efficiency AND value for money AND material longevity.
Surely that point can lay inside buried wooden components that in the case of upgraded loft insulation have no airflow over them?
In any case it sounds like there are mitigations for having to go chasing U value for loft/ceiling insulation.
Also yes the “now colder loft which will be more condensationy” because it’s getting less warmth from the rooms below, but still getting its moisture… is a big risk too!
Definitely not just a ‘follow the rules’ task, but one that needs staging and nuanced consideration.
The main issue for me was 1st task is possibly chunks of old bitumen felt knackered, and think the ridge felt has likely cracked too.
So replacing with quality breathable felt and replacing tiles and battens a no brainer, in phases.
But it’s completely the wrong time to also stuff in a ton of extra insulation until it’s all completed, moisture control tightened up, ventilation improved, and all felt swapped etc.
It sounds like BR won’t be invoked stupidly and sensible consideration will be given.
I’m still very weary of the general advice on blindly topping up insulation though.
With the rush for ASHP and stuff I think a much more holistic approach is needed to finding efficiency AND value for money AND material longevity.
Mr Whippy said:
Well I ve not done it yet.
The point is if I start to do over 25% of my roof then I invoke the blessed building regs and they say I d need to upgrade to 270mm insulation and bury my rafters.
We didn’t bother with building regs when we replaced our 1970s bungalow roof. Neighbours x 2 haven’t either.The point is if I start to do over 25% of my roof then I invoke the blessed building regs and they say I d need to upgrade to 270mm insulation and bury my rafters.
We’ve gone from bitumen felt to breathable membrane plus a vented continuous ridge system - adding this seasons fashion of 270mm insulation as well as vented soffits and ridge while it’s all in bits could well keep overall condensation down compared to a sweaty bitumen covered loft but I too would worry about the timbers buried under the new insulation.
I like the idea of a Bluetooth moisture meter.
Top tip : sprayable insulation at rafter level is bad m’kay. Sister in law has just had her
Mr Whippy said:
Sorry I was under the impression if you have room temp or 21degC on your ceiling, between there and say -4degC on the top of your insulation, there will be a point between that water vapour condenses.
Surely that point can lay inside buried wooden components that in the case of upgraded loft insulation have no airflow over them?
......
Reality is that the loft spends a lot of the Winter day significantly warmer than 'the greater outdoors'.Surely that point can lay inside buried wooden components that in the case of upgraded loft insulation have no airflow over them?
......
If the loft has any breathability, then those few degrees of warmth remove water at a rate.
So the concentration, vapour pressure, of water decreases across the insulation layer, because vapor is leaving the cold side.
The concentration of water in the air can also be expressed as dew point.
So you can draw a graph of the temperature varying across the insulation, and on the same graph the dewpoint.
If the dewpoint is ever above the temperature, condensation you will have.
Water vapour is a gas, will behave as a gas and needs to be understood as such.
The best approach is to reduce the vapour pressure at the hot side of the insulation by:
Not having the house too humid indoors
Avoiding gross holes in the ceiling such as cheap downlighters etc
Ideally a vapour barrier on the plaster board, but a few coats of paint has a fair vapour resistance.
Personally I think too much ventilation can be a mistake in edge cases.
You loft should be more like a breathable shell holding fairly still air, like a windproof goretex coat.
Don't push that analogy too far though.
I think it's worth remembering that rock wool or similar is at least somewhat vapour permeable so any moisture making it through your plastic based emulsion coated ceiling can still be driven up through the insulation to the loft space.
One benefit of using rock wool or similar is that it's pretty easy to go up, pull some out and have a look from time to time. I was a bit worried about overdoing things but figured if I had a problem then throwing away some excess rock wool wasn't the end of the world.
All seems fine a few years later with everything bone dry any time I remember to look.
One benefit of using rock wool or similar is that it's pretty easy to go up, pull some out and have a look from time to time. I was a bit worried about overdoing things but figured if I had a problem then throwing away some excess rock wool wasn't the end of the world.
All seems fine a few years later with everything bone dry any time I remember to look.
I’ve ordered three of these…
https://www.amazon.co.uk/gp/aw/d/B0BVZC9Q31
I can have one in the hall, one in the loft, and one outside, and plot the curves to see what is what over time.
Hopefully it’ll serve as a good point of reference for changes made.
Ie, is the loft responding in a good way or a bad way.
I’ll probably stay away from rock wool depth increases for now and just make sure what is there is perfect and runs nicely to the eaves.
If I risk invoking BR because of roof work I’ve at least got reference material!
https://www.amazon.co.uk/gp/aw/d/B0BVZC9Q31
I can have one in the hall, one in the loft, and one outside, and plot the curves to see what is what over time.
Hopefully it’ll serve as a good point of reference for changes made.
Ie, is the loft responding in a good way or a bad way.
I’ll probably stay away from rock wool depth increases for now and just make sure what is there is perfect and runs nicely to the eaves.
If I risk invoking BR because of roof work I’ve at least got reference material!
One thought is that one thing you'd see if moisture levels really rocketed in the timber would be movement in the ceilings or around the wall plates, as in joints between boards cracking/rucking or cracks in the plaster/ceiling in the outside corners depending on what the levels did over time. You can get this anyway just from seasonal variation.
If rot was really a possibility then you could preempt that with suitable wood treatment? Wouldn't help anything else that might get soggy though.
Ultimately if you really needed to you could fix the vapour permeability, it's not the most difficult or expensive job ever just painful/messy/disruptive!
If rot was really a possibility then you could preempt that with suitable wood treatment? Wouldn't help anything else that might get soggy though.
Ultimately if you really needed to you could fix the vapour permeability, it's not the most difficult or expensive job ever just painful/messy/disruptive!
Don't forget there are diminishing returns as you increase thickness. My 70s house had 50mm.
This article suggests that at 100mm you already have most of the benefit.
Obviously with a new build it makes sense installing 270mm as the extra cost is minimal and the house is designed for it.
If you vary concerned about condensation you are not losing much sticking at 100mm.
Or perhaps top up existing stuff by 50mm between rafters leaving the rafters able to breath.
https://www.theloftboys.co.uk/lofts/loft-insulatio...
This article suggests that at 100mm you already have most of the benefit.
Obviously with a new build it makes sense installing 270mm as the extra cost is minimal and the house is designed for it.
If you vary concerned about condensation you are not losing much sticking at 100mm.
Or perhaps top up existing stuff by 50mm between rafters leaving the rafters able to breath.
https://www.theloftboys.co.uk/lofts/loft-insulatio...
irc said:
Don't forget there are diminishing returns as you increase thickness.
Most definitely, after a certain point you're chasing marginal gains vs the obvious improvement you get early on and you'd be better chasing something different to improve overall performance - often improving detailing. But ever more is seen as better and it's easy to do even if you'll quite possibly never get close to a return on the extra cost of it.
Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff