I'm a little new to this so bear with me if I've missed the blindingly obvious.

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

Do the Building Regulations not require a vapour barrier?

Thanks for replying.

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.

Had a word with the building inspector.

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.

Thanks for the calcs. Couldn't see the wood for the trees.

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!

PS I don't think thinsulex is vapour permeable. So perhaps better to put that on the inside of any other insulation?

"Sheeps wool is only a bit green"

When compared to what david?

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 escape

The 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?

Mick 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.

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.

Posted By: CWattersI'd still be nervous about relying on this in a modern timber framed house

but

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.

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 humidity

A 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?

I asked Excel to look at an EVT type wall of cedar cladding / batten / panelvent / 200mm timber stud with warmcel / paneline / 25mm service void. I was going to use sasmox or fermacell to line it. Excel wouldn't do the calculations because there was "no vapour check". I revised the design, lining with vapour check plasterboard and they were quite happy. There seems to be confusion within Excel itself about EVT.

Posted By: JulianThere seems to be confusion within Excel itself about EVT

So 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!

That's if vou go in for all that hygroscopic spin - I dont, at least not for all buildings, though I agree it is often very important in terms of the conservation of historic building fabric

Its not that I disagree with 'breathing construction' its just that I do not think it necessary in most situations. I fail to see the advantage where there is any form of rain screen cladding. Where is the moisture that you are so worried about going to come form? If from the inside then it can be just as effectively dealt with by means of a simple VB bhind the plasterboard. This also makes for better airtightness in my view.