Green Building Forum - roof ventilation and insulation Wed, 10 Jun 2026 11:15:56 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51376#Comment_51376 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51376#Comment_51376 Mon, 06 Apr 2009 13:21:19 +0100 miltwin
We are re-building a small granite barn (6m x 3m) for 'ancillary accomodation' and, despite reading and reading, I am a little concerned over the roof ventilation between slate and batten.

The barn is being re-built as a breathable building, i.e. lime used throughout, etc. The pitched roof will have 150mm sheepswool insulation (no air gap), a klober perma-forte membrane, battens and then wet-laid slates.

The issue I can't resolve is whether I should be using counter-battens:

- to allow run-off of vapour condensing on the back of the tiles/slates?
- to provide adequate ventilation?

I realise that there is probably not enough insulation in the roof but this is a necessary compromise for headroom. We are restricted on overall height as we live in a conservation area.

What boards to people recommend for supporting the wool/underside? Pavatherm Floor NK? I'd like to avoid plasterboard.

Thanks for any advice given.

Robin]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51391#Comment_51391 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51391#Comment_51391 Mon, 06 Apr 2009 19:00:04 +0100 tychwarel
You will need over verge ventilators that will sit on top of the fascias and over the membrane,

pavatherm would be good as an underlayer as it will increase your insulation value, however your inner layer needs to be at least 5 times less vapour permeable than the perma forte and for that pavatherm nk on its own wouldn't be good enough.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51398#Comment_51398 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51398#Comment_51398 Mon, 06 Apr 2009 21:06:56 +0100 CWatters roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51400#Comment_51400 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51400#Comment_51400 Mon, 06 Apr 2009 21:45:47 +0100 Mike George
The so-called breathing construction methods would not require a VB Sheepswool for example has a BBA http://www.secondnatureuk.com/files/pdf/BBA_Brochure.pdf Detail pg2]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51406#Comment_51406 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51406#Comment_51406 Mon, 06 Apr 2009 23:22:40 +0100 miltwin
I'll run it past the building inspector asap. Hadn't occurred to me to call them. So much to do. I hope they are sympathetic and agree that a vapour barrier is not required.

The passed plans had foil-backed plasterboard / 100mm celotex / thinsulex foil over rafters / 50mm air gap / tyvek breathable membrane / battens / tiles. I can't help think that this would've stored up problems for our granite, lime rendered barn and would seem more suitable for a new build.

The granite walls are uninsulated by the way.

tychwarel, do you mean something like these over fascia vents - but on top of the membrane?

http://www.klober.co.uk/product2.php?id=12

And why does the inner layer need to be at least 5 times less vapour permeable than the perma forte? I'm a bit confused on that one. Isn't the idea to help, not hinder, the water vapour through the roof?

I've much to learn.

Thanks again for your comments.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51409#Comment_51409 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51409#Comment_51409 Tue, 07 Apr 2009 08:48:07 +0100 tychwarel
The inner layer has to be less vapour permeable because the flow of water vapour is from the warm wet inside to the relatively cold dryer outside. hence it is important that any vapour that gets into the roof structure has a free flow to the outside. the easiest way of doing this is to put any vapour restriction on the inside.

Also it is highly unlikely that building a vapour tight roof (one with a vapour membrane) would have any effect on the breathability of a mass granite wall particularly as the only possible vapour or liquid water transfer through such a wall will be through the mortar joints.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51413#Comment_51413 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51413#Comment_51413 Tue, 07 Apr 2009 11:02:08 +0100 miltwin
He was adamant that using such a thickness of wool will not pass building regs. My partner will not compromise on headroom so that's possibly the end of a breathable roof.

The inspector also thought that we'd been fortunate as it is to have had regs pass the insulation in the plans (see above)... probably something to do with the age and size of the barn.

In the hope of rescuing the situation - or now really for my information I suppose, can anyone tell me what the equivalent thickness of fleece is that I would require. I'm lost in the figures given by celotex/thermafleece and the like but am still researching - it's got to be somewhere out there or in another forum post.

It's a struggle all of this. I don't want to use petro-chemical stuff but it looks like I have no choice.

tychwarel, thanks for the vent tips. I understood granite to be porous, especially, as in our case, where poor quality granite was used.

Thanks all.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51415#Comment_51415 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51415#Comment_51415 Tue, 07 Apr 2009 11:28:45 +0100 Nick Parsons
Thickness in metres (so 150mm = 0.15) / thermal conductivity (0.039, I think, for Thermafleece) = the R value (3.84)
Plasterboard 10mm or so I think has an R value of 0.19. Combined internal and external surface resistances = c 0.15.

Total R in this instance = 4.18

1/R = U (= 0.24 in this case - it won't pass even on this 'bald' figure, and I have allowed nothing for thermal bridging). Come to think of it, I was just about to quote part L1B (refurb) when I looked back and saw 'rebuilding'. Is this refurb or new-build on an old footprint? (If refurb, sloping ceilings require U = 0.2. Flat ceilings, U = 0.16).

Manufs/suppliers will calc U value for you anyway, but it's useful to have the above at your fingertips for quick checks.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51423#Comment_51423 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51423#Comment_51423 Tue, 07 Apr 2009 13:18:57 +0100 miltwin
It's a completely new roof on an old building. The previous roof was aging asbestos sheets (and that was a nice disposal job!) with decaying rafters/beams.

A friend said we should've demolished the barn and had an allotment instead!]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51437#Comment_51437 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51437#Comment_51437 Tue, 07 Apr 2009 19:12:28 +0100 CWatters Posted By: miltwin And why does the inner layer need to be at least 5 times less vapour permeable than the perma forte? I'm a bit confused on that one. Isn't the idea to help, not hinder, the water vapour through the roof?

No these days the concern is to prevent water vapour from inside the house condensing in the structure of the roof. To avoid this a belt and braces approace is normally adopted:

A vapour barrier installed on the warm side of the insulation stops any vapour getting through the insulation to the cold side where it might condense.
AND
The timbers on the cold side are ventilated to allow any that does get through to escape.

The latter can be done one of two ways. You can either use

1) an impermeable membrane AND a 50mm ventilated cavity under it.
2) a vapour permeable membrane and no ventilated cavity.

However if you decide to go with option 2 you have to check with the insulation manufacturer that this is acceptable. I believe it is for Kingspan. Not sure about other makes. Some require a reduced cavity of 25mm to allow the membrane to drape between the rafters and not contact the insulation etc.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51438#Comment_51438 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51438#Comment_51438 Tue, 07 Apr 2009 19:15:08 +0100 CWatters roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51440#Comment_51440 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51440#Comment_51440 Tue, 07 Apr 2009 20:02:53 +0100 david Sheep farming consumes lots of petrochemicals.
Transport, cultivation, fertilisers, veterinary products etc.
Sheep farming damages upland ecology and pollutes rivers and lakes.
Sheeps wool insulation is only a bit green, and as you've discovered it's not as effective as some other insulations which are less expensive.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51441#Comment_51441 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51441#Comment_51441 Tue, 07 Apr 2009 20:11:47 +0100 Mike George
When compared to what david?]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51446#Comment_51446 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51446#Comment_51446 Tue, 07 Apr 2009 21:42:28 +0100 rogerwhit roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51468#Comment_51468 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51468#Comment_51468 Wed, 08 Apr 2009 11:39:44 +0100 fostertom Posted By: CWattersthe concern is to prevent water vapour from inside the house condensing in the structure of the roof. To avoid this a belt and braces approace is normally adopted:

A vapour barrier installed on the warm side of the insulation stops any vapour getting through the insulation to the cold side where it might condense.
AND
The timbers on the cold side are ventilated to allow any that does get through to escapeThe point is, there's an alternative to this belt-and-braces approach - that is, the so-called breathing wall concept.

The breathing wall concept dipenses with any inboard vapour barrier and instead relies upon a gradient of vapour resistances, so that the inbopard layer's resistance is about five times (some say three times) more water vapour resistant than the outboard layer, and all these resistances are low. The claimed advantage is that it's more natural and healthy for the internal water vapour regime to be in contact with the external, with a controlled interchange ('breathing' in and out) between, rather than hermetically isolating the internal water vapour regime from the external with a vapour barrier, which being just a sheet of plastic film is unlikely to remain fully intact though the life of the building (pinholes and hairline cracks are quite enough to seriously breach a vapour barrier).

Also, absence of a vapour barrier allows any hygroscopic properties that the insulation may have, to be available to the interior, to moderate balance and stabilise its humidity. This is claimed to provide very comfortable conditions, like a cob farmhouse - I can vouch for the latter. Such hygroscopic insulations are basically the plant/animal ones - sheeps wool, Warmcel etc. Hemcrete similarly. Artificial insulations have negligible hygroscopicity.

Excel, the Warmcel co, are the main champions of 'breathing wall'. However, I've so far found no authoritative research/papers to back the concept. Where does the 'five times' rule-of-thumb come from? Does anyone have anything?]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51483#Comment_51483 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51483#Comment_51483 Wed, 08 Apr 2009 15:17:21 +0100 CWatters Posted By: fostertom Where does the 'five times' rule-of-thumb come from? Does anyone have anything?

Probably done to ensure that water vapour allways evaporates away on the outside faster than it can enter the wall on the inside. I can see it working if done right. I'd still be nervous about relying on this in a modern timber framed house or roof but can't see a problem for a brick wall. I guess the the question is do breathable walls and roof eliminate the need for room ventilation? If trickle vents or MHRV is still required then why not let that deal with the whole issue of controling humidity?

Excel seem to recommend Paneline on the inside and this is an OSB like produce made without glue.. They say....

"Natural waxes are added to the Paneline formulation to achieve the required sheathing performance. Paneline’s formulation has been developed to provide the required vapour resistance to control the migration of water vapour from the inside of a building, through the EVT/ TRADIS panel, to the exterior."

I believe it's made by..
http://www.panelagency.com/paneline.html

but their site is short on technical information about Paneline and I can't tell how permeable it is.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51601#Comment_51601 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51601#Comment_51601 Thu, 09 Apr 2009 19:49:22 +0100 david "Sheeps wool is only a bit green" When compared to what david?
Rockwool for instance.
If you claim sheepwool is "greener" please quote some evidence.

Rogerwit
"In defence of sheep farming, it modifies upland ecology"
This modification has changed moorland and bogs into grassland, removed hundreds of miles of hedges, created a monoculture of non- native grasses, poisoned watercourses etc. How is this a defence of sheep farming? Our taxes paid for this destruction and now we are paying again to undo some of the damage.
Why does all land have to have an economic use? The uses you cite are both uneconomic. Sheep farming depends on subsidy and upland sheep farming is disappearing. There are very few sheep left in N WScotland, the huge flocks of the 1970's have disappeared and large areas of Sutherland & Caithness conifer forest has been restored to bog and moor.
Forestry has always been subsidised and most of the profits made are in the form of tax breaks.
"Without sheep much upland would be recolonised by scrub and trees".
And why not? R. Ponticum is only a problem in a relatively small area and Sycamores should be tolerated.
I don't understand the point about commecial veg. growing. Are you saying that growing veg. is "green" because the same use of petro chemicals is involved? Or are you saying sheeps' wool is a good insulation because it uses the same quantity of petrochemicals as growing veg. We have to eat vegetables. We don't have to use sheeps' wool to insulate houses.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51604#Comment_51604 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51604#Comment_51604 Thu, 09 Apr 2009 19:57:44 +0100 Mike George Posted By: davidMick George
"Sheeps wool is only a bit green" When compared to what david?
Rockwool for instance.
If you claim sheepwool is "greener" please quote some evidence.

I think its for you to quote the evidence. You made the asssertion after all, not me.

I would have thought though that the energy used to spin stone at a high temperature into mineral wool is significantly greater than the process for turning sheeps wool into....well... sheeps wool.

I have to say that I have not read 'evidence' either way so am interested in why you claim the opposite is the case.

I am very cynical of so called green products in general as often they often do not live up to the 'spin'

I think we should start a thread on this one so as not to hyjack this one - will do so.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51609#Comment_51609 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51609#Comment_51609 Thu, 09 Apr 2009 20:20:48 +0100 fostertom Posted By: CWattersI'd still be nervous about relying on this in a modern timber framed housebut
Posted By: CWattersExcel seem to recommend Paneline on the inside and this is an OSB like produce made without glue.. They say....

"Natural waxes are added to the Paneline formulation to achieve the required sheathing performance. Paneline’s formulation has been developed to provide the required vapour resistance to control the migration of water vapour from the inside of a building, through the EVT/ TRADIS panel, to the exterior."
so they seem happy enough with it in their timber framed solution. It's frustrating that Excel must have done the research to be that confident, but nothing seems to be available, and their tech guys don't seem to know.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51611#Comment_51611 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51611#Comment_51611 Thu, 09 Apr 2009 20:27:28 +0100 CWatters roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51613#Comment_51613 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51613#Comment_51613 Thu, 09 Apr 2009 20:31:14 +0100 fostertom Posted By: fostertomabsence of a vapour barrier allows any hygroscopic properties that the insulation may have, to be available to the interior, to moderate balance and stabilise its humidityA current project is an upgrade of a 1985 bungalow with tall 45o pitch raftered room-in-roof. It has vapour check plasterboard and skim and I'm considering blown-in Warmcel.

I'm wondering just how much deliberate scratching of the existing vapour control layer on the topside of the plasterboard (accessible once the felt and battens are off) will convert it into a weak-to-moderate vapour resistor, about twice as resistant as the OSB that will go over the rafters and five times as resistant as the outboard breather felt.

And will that weak-to-moderate vapour resistance leave the internal water vapour regime effectively/sufficiently connected to the hygroscopic Warmcel?

Anyone hazard a comment?]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51615#Comment_51615 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51615#Comment_51615 Thu, 09 Apr 2009 20:42:01 +0100 Mike George
Just because an insulation is 'breathing' it doesn't mean you cannot restrict the flow of vapour through it. Just that if you do allow it into the insualtion then you must also make sure it evaporates outboard.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51617#Comment_51617 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51617#Comment_51617 Thu, 09 Apr 2009 20:51:02 +0100 Julian roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51618#Comment_51618 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51618#Comment_51618 Thu, 09 Apr 2009 20:53:36 +0100 fostertom Posted By: CWattersI believe it's made by..
http://www.panelagency.com/paneline.html

but their site is short on technical information about Paneline and I can't tell how permeable it is.
Posted By: CWattersIt depends how permeable the paneline is. "Control" could mean block :-)
Here y'are: www.ribaproductselector.com/Document.aspx?ac=&t=8&f=COL811530.pdf&ci=11530&from=4

6.4mm Paneline has water vapour resistance 1.02MNs/g, so a bit more resistant than 12.5 plasterboard 0.75MNs/g and half as resistant as 9mm OSB 1.95MNs/g, according to http://www.greensteps.co.uk/tmp/assets/1163178050906.pdf . In other words, very unresistant! 100mm of blown cellulose insulation (Warmcel) has 4.5MNs/g - more resistant than the inner Paneline, which is wax-modified supposedly to have five times the resistance of the outboard insulation, breather felt etc.

It just doesn't add up!]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51626#Comment_51626 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51626#Comment_51626 Thu, 09 Apr 2009 21:19:30 +0100 fostertom Posted By: JulianThere seems to be confusion within Excel itself about EVTSo it seems. The latter part of this article is quite good http://www.forevergreen.org.uk/articles/articlesthebreat.html
but it's also puzzling, which is strange considering its author: "At Architype we have built more than 100 houses and other buildings using breathing construction"

"the materials in the wall are carefully specified, so that sufficient vapour resistance is provided on the inside of the wall to prevent any risk of interstitial condensation, whilst the materials in the middle and outside of the wall are of a low enough vapour resistance to allow natural moisture migration. The general rule is that the vapour resistance of the inside layer should be at least five times that of the outside, although calculations to BS5250 indicate that a 2:1 ratio works."

but then:

"The most typical breathing wall arrangement an internal lining such as Gyproc Duplex (a plasterboard with a high vapour resistance), “Warmcel” insulation between studs, and bitumen softboard sheathing on the outside"

That's a hell of a lot more than 5:1 in fact it amounts to a standard wall with inboard plastic VCL (vapour control layer).

"In addition, the hygroscopic properties of Warmcel enable it to absorb surplus moisture at times of high internal humidity and to release it when conditions allow, effectively giving a “security” reservoir."

Not with a VCL in between!

Later on, a 'breathing' building using Gyprox duplex board is contrasted with a normal housebuilder's one using 'plastic vapour barrier plasterboard'. It's the same thing!]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51627#Comment_51627 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51627#Comment_51627 Thu, 09 Apr 2009 21:27:37 +0100 fostertom Posted By: Mike Georgejust because an insulation is 'breathing' it doesn't mean you cannot restrict the flow of vapour through itThe whole point of breathing construction is to provide just enough vapour resistance but no more. That way it does actually 'breathe' water vapour in and out, and it leaves the interior in beneficial contact with the hygroscopic capabilities of the Warmcel (or sheepwool/hemp etc organic) insulation, which stabilises internal humidity levels, like a cob farmhouse, and apparently is good for air quality too - with minimised need for ventilation.]]> roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51630#Comment_51630 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51630#Comment_51630 Thu, 09 Apr 2009 21:33:34 +0100 Mike George roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51632#Comment_51632 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51632#Comment_51632 Thu, 09 Apr 2009 21:35:18 +0100 fostertom roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51655#Comment_51655 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51655#Comment_51655 Fri, 10 Apr 2009 00:37:13 +0100 Mike George roof ventilation and insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51663#Comment_51663 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3834&Focus=51663#Comment_51663 Fri, 10 Apr 2009 09:32:38 +0100 fostertom