Building Regulations and Standards: Insulating a warm (hybrid) pitched roof.

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- CommentAuthorJimbo7
- CommentTimeFeb 3rd 2012
We're just at the stage of spacing out the roof trusses on our extension/refurb project. The roof is being completely replaced and we'll be using the loft for habitable space. Internal height is marginal, so we'd really rather not put any insulation below the rafters.

So we have 172mm of rafter depth in which to insulate, and could manage about 25mm over the top before ridge height becomes an issue. The plan was for 150mm of rigid foam between, flush with the top of the rafters, 25mm over the top, followed by counter battens, breathable membrane (draped between battens for drainage), tile battens and then a mixture of reused and new concrete tiles. Below the 150mm foam would be a small void, taped and sealed vapour control layer, and/or 12.5mm foil backed plasterboard.

However, the BCO is pushing back on this, and the insulation companies aren't helping. They're all saying that between and over needs to have at least the same depth over the rafters as in between. None of them can explain why though.

It's stupid and irritating because several of the insulation companies have between & over examples in their product brochures with relatively thin "over" components. (eg. Xtratherm and Quinn both do).

I'm assuming the reason they're not keen is interstitial condensation risk. I've downloaded the BuildDesk demo, and it doesn't seem to have a problem with the method we were planning to use, and reckons we'd have a U-value of 0.14W/m2K. (We specified a maximum of 0.16W/m2K, so we'd be pretty happy with that)

Has anyone here built a roof with similar construction and method of insulation? Any opinions on whether the proposed method is sensible?

Jim
- CommentAuthordavidfreeborough
- CommentTimeFeb 3rd 2012 edited
The problem with the thin over rafter insulation is that the inner face of the over rafter component, & therefore the rafter tops, will be in the dew point zone. You can get away with this if the over rafter insulation is hygrospcopic & breathable, e.g. woodfibre, but its not a good idea with foil-faced PIR/PUR.

However, a solid sarking board is good for keeping the wind out of the roof & the insulation. Its also a good idea to put the sarking membrane flat against this to further improve windtightness. So how about?

Tiles;
Tile battens;
Counter battens, 25mm vent to eaves & 5mm vent to ridge;
Taped breathable sarking membrane;
22mm T&G woodfibre sarking board;
170mm PIR/PUR between rafters;
Taped vapour control layer;
Plasterboard.

David
- CommentAuthorJimbo7
- CommentTimeFeb 3rd 2012
Yes, our Project Manager has been speaking with the senior BCO, and it is interstitial condesation risk that is putting the kibosh on our current plan.

Our PM asked one of the insulation manufacturers why they have a scheme in their brochure that can't be used, they said the thinnest "over" figure was based on "old data".

It seems to be a very fluid situation, with the recommendations changing every 6 months or so.

So whatever solution we choose, it's apparent that any over-rafter insulation plan will mean having to have at least 50-60mm (which would probably not work for us due to ridge height related planning issues), with no chance of bettering the 0.16W/m2K specified.

We would also have to have a 50mm ventilated gap below the breatheable membrane (they're absolutely insistent on this), with the membrane draped over the gap.

We could probably have just about managed 50mm with 25mm counter battens, so at the moment it's looking more likely that we'll have to have at least 12mm of insulation below the rafters. It may not sound like much, but could make the difference between a near miss and a bruised bonce

Thanks for the suggestion. I'm a little wary of using woodfibre though - AIUI it's rather flammable!

Jim
- CommentAuthorpmagowan
- CommentTimeFeb 3rd 2012
How can you get interstitial condensation if you have a vapour control layer? Surely if this is done right no moisture will be able to get into the roof structure. Or am I missing something?
- CommentAuthordavidfreeborough
- CommentTimeFeb 3rd 2012 edited
Posted By: Jimbo7So whatever solution we choose, it's apparent that any over-rafter insulation plan will mean having to have at least 50-60mm (which would probably not work for us due to ridge height related planning issues)
It depends how you ask the question & whether you have a BBA certificate showing your proposed solution. Its worth downloading the BBA certificates for all the insulants & breather membrane's you're considering using. Woodfibre sarking boards are covered by the NBT Isolair BBA. Timber sarking boards are covered by most PIR/PUR insulation & breather membrane BBA certificates.

Posted By: Jimbo7We would also have to have a 50mm ventilated gap below the breatheable membrane (they're absolutely insistent on this), with the membrane draped over the gap.
Someone is misunderstanding or mixing up differing requirements.

Bituminous roofing felt needs to be ventilated from below to limit condensation. The whole point of a breathable membrane is that you ventilate above it, not below it, thereby reducing wind washing & thermal bypass. Some breather membrane BBA certificates don't even require ventilation above & rely on the gaps between tiles to dissipate water vapour.

As long as the breather membrane's BBA certificate allows use in contact with insulation (i.e. it is what is often described as "non-tenting"), its best fitted flat to the insulation with counter battens & ventilation above. If you don't use counter battens then you need to let it drape over the rafters so that water can drain below the tiles battens, but that isn't relevant in this case.

In some cases you'll see it recommended that the breather membrane is draped over the counter battens so that water drains aways from the tile battens fixings. However, for anything apart from low pitch roofs, this is unecessary & may actually create more problems than putting the membrane below the counter battens because there are often a lot more tile batten fixings than counter batten fixings. And even in this case the ventilation should be above, not below the membrane.

Posted By: Jimbo7Thanks for the suggestion. I'm a little wary of using woodfibre though - AIUI it's rather flammable!
I've been through the roof build-up selection process a number of times & only woodfibre board ticks all the boxes as a sarking board. Its below the tiles, so flammability is not a major factor, its no more flammable than the rafters/battens & its less flammable than many oil based plastic insulants.

David
- CommentAuthordavidfreeborough
- CommentTimeFeb 4th 2012
Posted By: Jimbo7We could probably have just about managed 50mm with 25mm counter battens, so at the moment it's looking more likely that we'll have to have at least 12mm of insulation below the rafters. It may not sound like much, but could make the difference between a near miss and a bruised bonce
If you don't have a rigid sarking board you'll need to fit either 12mm plywood or 22mm timber longitudinal binders & diagonal braces below the rafters for racking strength. Perhaps you could use 25mm timber for this & in-fill with 25mm PIR/PUR?

David
- CommentAuthortony
- CommentTimeFeb 4th 2012
or both ways diagonally fixed builders galvanised or ss strapping 0.2mm thick 30mm wide
- CommentAuthorJimbo7
- CommentTimeFeb 4th 2012 edited
Posted By: davidfreeborough
Posted By: Jimbo7We would also have to have a 50mm ventilated gap below the breatheable membrane (they're absolutely insistent on this), with the membrane draped over the gap.
Someone is misunderstanding or mixing up differing requirements.

Bituminous roofing felt needs to be ventilated from below to limit condensation. The whole point of a breathable membrane is that you ventilate above it, not below it, thereby reducing wind washing & thermal bypass. Some breather membrane BBA certificates don't even require ventilation above & rely on the gaps between tiles to dissipate water vapour.

I think that's the thing. They've been finding some instances of roofs installed without a gap below the breathable membrane that have been suffering from interstitial condensation problems. Thus they currently won't sign off on a roof without it.

Posted By: davidfreeborough
Posted By: Jimbo7Thanks for the suggestion. I'm a little wary of using woodfibre though - AIUI it's rather flammable!
I've been through the roof build-up selection process a number of times & only woodfibre board ticks all the boxes as a sarking board. Its below the tiles, so flammability is not a major factor, its no more flammable than the rafters/battens & its less flammable than many oil based plastic insulants.

Fair enough. Our Project Manager actually suggested it, but when I looked at the details for the stuff he recommended, I saw that it had a "Class E" reaction to fire classification rating, which when I looked it up suggested it could go from ignition to flashover in within 2 minutes, which sounded fairly alarming

Jim
- CommentAuthorJimbo7
- CommentTimeFeb 4th 2012 edited
Posted By: davidfreeborough
Posted By: Jimbo7We could probably have just about managed 50mm with 25mm counter battens, so at the moment it's looking more likely that we'll have to have at least 12mm of insulation below the rafters. It may not sound like much, but could make the difference between a near miss and a bruised bonce
If you don't have a rigid sarking board you'll need to fit either 12mm plywood or 22mm timber longitudinal binders & diagonal braces below the rafters for racking strength. Perhaps you could use 25mm timber for this & in-fill with 25mm PIR/PUR?
David

The longitudinal binders are behind the ashlar walls. There doesn't seem to be any diagonal bracing marked on the truss plan - we probably need to ask the designer about that...

Jim
- CommentAuthorJimbo7
- CommentTimeFeb 4th 2012
Posted By: pmagowanHow can you get interstitial condensation if you have a vapour control layer? Surely if this is done right no moisture will be able to get into the roof structure. Or am I missing something?

Indeed, good question. Could it be from an external source, e.g. if conditions change from high humidity to cold?

Or perhaps they've just found that even well sealed ceilings ultimately leak moisture eventually...

Jim
- CommentAuthortony
- CommentTimeFeb 5th 2012
It can also come from air that was in the void during the day cooling and dumping its moisture onto the coldest place.

A vapour barrier will not stop condensation forming, but without one loads more can form especially on a cold night.
- CommentAuthorborpin
- CommentTimeFeb 5th 2012
Posted By: Jimbo7I think that's the thing. They've been finding some instances of roofs installed without a gap below the breathable membrane that have been suffering from interstitial condensation problems. Thus they currently won't sign off on a roof without it.
This probably is not the membrane's fault though. Too little insulation and too much bulk air movement through the structure. That is part of the problem; just looking at one thing in isolation!
- CommentAuthordavidfreeborough
- CommentTimeFeb 5th 2012 edited
Posted By: tonyIt can also come from air that was in the void during the day cooling and dumping its moisture onto the coldest place.

A vapour barrier will not stop condensation forming, but without one loads more can form especially on a cold night.
I think these are key points to understanding condensation in roof spaces.

Many of the breather membrane BBA certificates are quite explicit about not allowing any voids or loft spaces between the insulation & sarking boards. For example, Tyvek Supro's BBA allows sheet sarking board materials on a (hybrid) warm roof, but does not allow them on a cold roof. This is because the air in the void or loft will carry moisture which can condense when temperatures change. Another reason why I think its best to keep the breather membrane flat against the insulation.

A vapour barrier will never be perfect, but when conditions are right for condensation to form, without a vapour barrier you have a source of water vapour which will keep pumping water vapour into the roof for as long as it keeps condensing into water. Well sealed OSB is all that is required to keep this under control, which is why woodfibre manufacturers recommend putting the OSB on the inside & woodfibre on the outside.

David
- CommentAuthorborpin
- CommentTimeFeb 5th 2012
The BBA for Tyvek Supro is quite interesting in how it phrases it

"6.5 For roofs with an insulated inclined ceiling, ventilation above or below the underlay will not be required provided that the passage of moisture by diffusion and by convection is controlled, eg, by a vapour control layer (eg TYVEK SD2) or a continuous envelope of insulation with a high vapour resistance."

It is the "continuous envelope" that is the interesting way of phrasing it. That pretty well sums up how I am going to do it.
- CommentAuthorCWatters
- CommentTimeFeb 6th 2012
<blockquote><cite>Posted By: pmagowan</cite>How can you get interstitial condensation if you have a vapour control layer? Surely if this is done right no moisture will be able to get into the roof structure. Or am I missing something?</blockquote>

My belief is they are so worried about the problen that a belt and braces (VPC and ventilation) approach is required. Possibly because they don't trust builders to install the VPC very well.
- CommentAuthorrichardh
- CommentTimeNov 21st 2012
Revisiting this older thread in the context of a listed building renovation of our farmhouse in Perthshire. Revised roof construction (outside in) is:

original rough slate tiles
remains of felting
original sarking boards (about 20mm)
50mm air gap between sarking and insulation
90mm Kingspan K7 between rafters (these are about 165mm deep and spaced at roughly 440mmm centres)
approx. 25mm air gap
Superquilt insulation blanket immediately below rafters
counter battens
plasterboard

So far, so good, I hope. Now for ventilation. The ridge is no problem - we're just putting in a ventilated ridge. For the eaves into the air gap below the sarking we were originally going to use soffit mesh where the rafters project beyond the building (no soffits). That was until we discovered that the rafters and sarking boards are hard down onto the stone wall head (itself 800-1500mm thick).

I don't want to use hooded ventilators or flush slate ventilators for both aesthetic reasons and to avoid having to put in another consent application. What I'm thinking of doing is simply drilling a 70mm hole in the sarking board just above the eaves, putting something like an HD7000 circular soffit ventilator in each and laying a membrane fixed at the top only below the specific tiles that the ventilator would be under. We'd then allow percolation under the rough tiles and the venturi from the ridge vent to do the rest. This would give approx 5000mm2/linear metre of roof and a completely invisible fix.

Any thoughts or alternative suggestions here?
- CommentAuthorEd Davies
- CommentTimeNov 21st 2012
Not exactly relevant to your question but what's the purpose of the 25 mm air gap between the Superquilt and the Kingspan? I'd have thought it would be better to avoid circulation here - particularly to reduce the effect of any leakage between the Kingspan and the rafters.
- CommentAuthortony
- CommentTimeNov 21st 2012
Probably because it is multifoil
- CommentAuthorGraeme Berry
- CommentTimeNov 21st 2012
richardh, i'd be careful about those vents, might be worth checking with your BCO as i'm pretty sure the ventilation openings need to be unobstructed, and they may argue that your proposed arrangment means the tiles and membrane obstructs the openings. something like the HD1200m over fascia might be better, if you have enough wiggle room in the slates?

be interested in hearing how you get on though, i'll shortly have the exact same problem (& i'm in perthshire too :) )
- CommentAuthorEd Davies
- CommentTimeNov 21st 2012
Thanks Tony. Yes, sorry, should have looked at what Superquilt is. Was thinking of the breathable bagged rockwool stuff.
- CommentAuthorViking House
- CommentTimeNov 21st 2012 edited
"I'm a little wary of using woodfibre though - it's rather flammable!"
I wrapped my brothers Passive House in woodfibre board http://viking-house.co.uk/1st-irish-factory-built-certified-passive-house.html My dad collected the woodfibre offcuts and said he'd burn them in his stove, months later he was cursing the stuff saying it was brilliant stuff for quenching fires.
- CommentAuthorrichardh
- CommentTimeNov 21st 2012
Yep, the 25mm airgap is indeed for the multifoil - the combo of K7/multifoil was the sweet spot for the thickness/cost/performance/space trade-off. As for the BCO view of vent blocking, I suspect you're absolutely right, but what I'm most concerned about is having something that works and doesn't overly screw with the appearance of the building. In my various set-tos with the National Park, I've not notably found an enlightened view of conservation and renovation - when we moved in the house had 1980s Wickes outside doors and the Park wanted us to replace them with custom copies of 1980s Wickes doors... Attitudes seem to have improved of late but I'm much more into doing what I believe (given appropriate research and advice) to be right - the slight punt I'm taking here is that there will be sufficient percolation under the rough slates to provide the necessary airflow. If I'm wrong it's my problem!

Had looked at the HD1200M but I don't see how that would work with our roof, where the rafters simply project 40-50cm beyond the outside face of the wall and the sarking board overlies that with tiles atop. Of course, being Highland Scotland, the tiles are pinned directly to the sarking boards (no battens), so any continuous ventilation panels would simply prevent the tiles being fixed. Which would be sub-optimal ;-) Minimising disruption to the roof is also something of a priority - after 200 years, it's in fabulous condition and the LWA principle definitely applies!

@Graeme - I'll certainly let you know how it goes - feel free if you'd like to wander over and see what we've done (and learn from our mistakes!).

Richard
- CommentAuthordavidfreeborough
- CommentTimeNov 22nd 2012
Do you mean 40-50cm or 40-50mm?

Is it not possible to fit an over-fascia type vent between the top of the masonry & the underside of sarking board? See page 16 of the following:

http://www.glidevale.com/downloads/glidevale_roof_ventilation.pdf

David
- CommentAuthorrichardh
- CommentTimeNov 22nd 2012
<blockquote><cite>Posted By: davidfreeborough</cite>Do you mean 40-50cm or 40-50mm?

Is it not possible to fit an over-fascia type vent between the top of the masonry & the underside of sarking board? See page 16 of the following:

<a href="http://www.glidevale.com/downloads/glidevale_roof_ventilation.pdf" rel="nofollow">http://www.glidevale.com/downloads/glidevale_roof_ventilation.pdf</a>

David</blockquote>

Hi David - I did mean 40-50cm (or maybe a little less)- large roof overhangs hereabouts! I did look at the glidevale but the problems there would be: we'd need to strip off the tiles all the way along the roof (about 40 linear metres); remove a couple of sections of sarking board; grind back the masonry of the wallhead to create a space (we did experiment with that); fit the vents; refit everything else. Slow, expensive, disruptive and in an environment where we've had about two dry days since March!

Richard
- CommentAuthordjh
- CommentTimeNov 22nd 2012
Posted By: richardhthe house had 1980s Wickes outside doors and the Park wanted us to replace them with custom copies of 1980s Wickes doors

Priceless!
- CommentAuthordjh
- CommentTimeNov 22nd 2012
Posted By: richardhWhat I'm thinking of doing is simply drilling a 70mm hole in the sarking board just above the eaves, putting something like an HD7000 circular soffit ventilator in each and laying a membrane fixed at the top only below the specific tiles that the ventilator would be under. We'd then allow percolation under the rough tiles and the venturi from the ridge vent to do the rest.

Just a thought but your ridge vent will be pulling air out of the roof, yes? So air will be wanting to come in through the vents at the eaves. And that in turn will tend to suck the membrane down over the vents. So will you get enough ventilation?
- CommentAuthorrichardh
- CommentTimeNov 22nd 2012
<blockquote>Just a thought but your ridge vent will be pulling air out of the roof, yes? So air will be wanting to come in through the vents at the eaves. And that in turn will tend to suck the membrane down over the vents. So will you get enough ventilation?</blockquote>

Definitely a concern and I'm hoping the membrane will prove to be unnecessary 'belt-n-braces'. I think what we'll do is keep an eye on the humidity/condensation in the air gap where these are accessible and adjust to suit. This is definitely an experiment: We're trying to balance the visual considerations of a historic property with as close as we can get to modern levels of insulation and energy use - something which building regs and planners don't really take into account. Regs tend to work on clearly defined structures that have prescriptive solutions and considerable margins to allow for 'worst case' scenarios- what they're not good at is allowing a more nuanced approach which uses the more 'chaotic' features of an old building, each of which has to be a separate case.

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