Green Building Forum - Woodwool board or plasterboard over rafters? Sun, 19 Apr 2026 01:32:35 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277948#Comment_277948 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277948#Comment_277948 Mon, 13 Jan 2020 12:58:34 +0000 bardo Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277951#Comment_277951 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277951#Comment_277951 Mon, 13 Jan 2020 16:10:09 +0000 djh Posted By: bardoI was going to use wood wool e.g., Savolit with 5mm clay render to match the effect on the straw /clay rendered walls below.
That sounds like a reasonable idea, we have similar but with lime plaster instead of clay plaster.

However this would mean many hours fixing noggins to carry the woodwool. So I'm wondering if plasterboard will work, though I wonder if the less permeable performance will cause any issue.

I don't understand why the woodwool will need noggins but the plasterboard won't? I don't think the permeability of plasterboard will affect anything, given a VCL behind it even if the VCL is variable.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277952#Comment_277952 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277952#Comment_277952 Mon, 13 Jan 2020 16:11:26 +0000 fostertom Posted By: bardoslate onto battens, then Pro-Clima solitex membrane laid directly onto 11mm OSBYou'll need dowslope battens between the slating battens and the Intello/OSB surface, otherwise ea batten will be a dam collecting any water.
Posted By: bardoI would have used a less permeable product in retrospect
Why? the more permeability the better. Or do you mean 'more' permeable? OSB (make sure ir's OSB3) is v adequately breatheable, as is plasterboard or woodwool. The OSB can be the airtight layer, if systematically glued and screwed at all joints (over noggins where nec to support joints) and sealed at the perimeter. Then no need for a fragile expensive sheet memrane for airtightness.

Your buildup is an excellent 'breathing' construction, provided no vapour resisting layer is introduced. Robust because it can re-dry (after extreme conditions, or at winter-end) both outward and inward. The plasterboard or woodwool needn't have noggings, from any air/vapour-tight point of view; either can be punctured at will e.g. by electricans.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277953#Comment_277953 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277953#Comment_277953 Mon, 13 Jan 2020 16:26:12 +0000 bardo Posted By: fostertom
Posted By: bardoslate onto battens, then Pro-Clima solitex membrane laid directly onto 11mm OSB
You'll need dowslope battens between the slating battens and the Intello/OSB surface, otherwise ea batten will be a dam collecting any water.
Posted By: bardoI would have used a less permeable product in retrospect
Why? the more permeability the better. Or do you mean 'more' permeable? OSB (make sure ir's OSB3) is v adequately breatheable, as is plasterboard or woodwool. The OSB can be the airtight layer, if systematically glued and screwed at all joints (over noggins where nec to support joints) and sealed at the perimeter. Then no need for a fragile expensive sheet memrane for airtightness.

Your buildup is an excellent 'breathing' construction, provided no vapour resisting layer is introduced. Robust because it can re-dry (after extreme conditions, or at winter-end) both outward and inward. The plasterboard or woodwool needn't have noggings, from any air/vapour-tight point of view; either can be punctured at will e.g. by electricans.

Hi Tom, and thanks! We used OSB3 and I forgot to mention that we used counter battens so no damming. Yes, I meant to say more permeable than OSB, for example panelvent.

We taped the roof membrane though didn't tape over or glue the OSB so an opportunity missed.

The thinking for the inner membrane is due to a concern over condensation forming within the cold part of the OSB due to the material's low permeability. The calcs done by a friend suggested that in the most humid time of winter there could possibly be excess moisture without the membrane. I presume also from an air tightness point of view. We are using 300mm of insulation btw.

Re noggins]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277954#Comment_277954 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277954#Comment_277954 Mon, 13 Jan 2020 16:45:30 +0000 bardo Posted By: djh
Posted By: bardoI was going to use wood wool e.g., Savolit with 5mm clay render to match the effect on the straw /clay rendered walls below.

That sounds like a reasonable idea, we have similar but with lime plaster instead of clay plaster.

Cheers

However this would mean many hours fixing noggins to carry the woodwool. So I'm wondering if plasterboard will work, though I wonder if the less permeable performance will cause any issue.

I don't understand why the woodwool will need noggins but the plasterboard won't? I don't think the permeability of plasterboard will affect anything, given a VCL behind it even if the VCL is variable.

The ijoists run at 600mm centres. Looking at the woodwool tech sheet it suggests noggins to reduce the span when there's nothing solid behind. Perhaps as it's on the roof and will only have 5mm of clay plaster there's no need for this if plasterboard?]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277956#Comment_277956 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277956#Comment_277956 Mon, 13 Jan 2020 17:53:12 +0000 fostertom
Pity, that. My tinkering with WUFI some time ago showed that a breathing construction able to re-dry both inward and outward, robustly trumped reliance on any inboard expensive fragile 'designed to fail' vapour barrier.

As far as wetting of OSB sheathing, if min 40% of the insulation value is placed outboard of the OSB, it never wets, in a breathing construction.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277957#Comment_277957 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277957#Comment_277957 Mon, 13 Jan 2020 19:28:57 +0000 bardo Posted By: fostertomYou described the inner membrane as 'an intello or Thermofloc VSD airtight membrane', not as a vapour barrier - check?

They are both airtight and vapour check membranes. My understanding was that the OSB3 doesn't breathe so well which could lead to excess moisture held in the sheepswool insulation. I was also under the impression that plasterboard doesn't breath so well either - hence the development of breathboard as a replacement.

Pity, that. My tinkering with WUFI some time ago showed that a breathing construction able to re-dry both inward and outward, robustly trumped reliance on any inboard expensive fragile 'designed to fail' vapour barrier.

I like the idea of this however I was told by an eco building company that the relatively low permeability of OSB3 could cause issues with moisture retention. I have heard the same from two different 'expert' sources.

As far as wetting of OSB sheathing, if min 40% of the insulation value is placed outboard of the OSB, it never wets, in a breathing construction.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277958#Comment_277958 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277958#Comment_277958 Mon, 13 Jan 2020 20:10:10 +0000 djh
Which data sheet are you looking at? https://www.mikewye.co.uk/wp-content/uploads/2017/05/Savolit-Plus-application-guidance-website doesn't mention noggins and specifically says the ends do not need to be supported if sheets are glued together.

But since the ijoists are at 600 mm centres and the boards are 600 x 1200 mm, there are no unsupported ends anyway are there?

Plasterboard does need noggins, AFAIK.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277959#Comment_277959 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277959#Comment_277959 Mon, 13 Jan 2020 20:11:10 +0000 djh Posted By: fostertomAs far as wetting of OSB sheathing, if min 40% of the insulation value is placed outboard of the OSB, it never wets, in a breathing construction.
But in Bardo's case, the sheathing is exactly that - sheathing and it is external, not covered by any insulation.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277981#Comment_277981 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=277981#Comment_277981 Tue, 14 Jan 2020 09:52:51 +0000 fostertom Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278002#Comment_278002 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278002#Comment_278002 Tue, 14 Jan 2020 19:54:53 +0000 bardo Posted By: fostertomYeah I know - that's the trouble with using deep I-joists and is why EWI-like solutions are best (what's to be learned from this case, for anyone else reading and planning).

Why are deep ijoists a problem in this case? The OSB webbing is only 9mm which I'm told means no thermal loss across the insulation layer. Or did you mean something else?]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278004#Comment_278004 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278004#Comment_278004 Tue, 14 Jan 2020 20:15:48 +0000 bardo Posted By: djhbardo wrote: "The ijoists run at 600mm centres. Looking at the woodwool tech sheet it suggests noggins to reduce the span when there's nothing solid behind."

Which data sheet are you looking at?https://www.mikewye.co.uk/wp-content/uploads/2017/05/Savolit-Plus-application-guidance-website" >https://www.mikewye.co.uk/wp-content/uploads/2017/05/Savolit-Plus-application-guidance-websitedoesn't mention noggins and specifically says the ends do not need to be supported if sheets are glued together.

But since the ijoists are at 600 mm centres and the boards are 600 x 1200 mm, there are no unsupported ends anyway are there?

Thanks DJH I see that now. Do you still think that plasterboard is the suitable option?

Plasterboard does need noggins, AFAIK.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278030#Comment_278030 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278030#Comment_278030 Wed, 15 Jan 2020 13:45:05 +0000 fostertom Posted By: bardoWhy are deep ijoists a problem in this case?Not saying they're a problem - well tested technique. Not about heat loss. Just that it puts timber in the outermost insulation zone, when some interstitial condensation is bound to occur, even if only intermittent. Come to think, it may be the slight heat loss via the web, that keeps the outer 'flange' warmer, warm enough to not get condensation even if it's happening in the insulation an inch away sideways.]]> Woodwool board or plasterboard over rafters? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278292#Comment_278292 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16397&Focus=278292#Comment_278292 Sat, 25 Jan 2020 22:41:14 +0000 bardo Posted By: fostertomYou described the inner membrane as 'an intello or Thermofloc VSD airtight membrane', not as a vapour barrier - check?

Pity, that. My tinkering with WUFI some time ago showed that a breathing construction able to re-dry both inward and outward, robustly trumped reliance on any inboard expensive fragile 'designed to fail' vapour barrier.

As far as wetting of OSB sheathing, if min 40% of the insulation value is placed outboard of the OSB, it never wets, in a breathing construction.

Hi fostertom, revisiting the point you made about not requiring a vapour barrier. The surveyor who did the calcs was concerned about condensation within the inner side of the 11mm OSB 3 in winter, and the fact this may be held in the sheepswool. The solution offered was an airtight vapour barrier. What do you think would happen to the condensation that forms on the inside of the OSB which the sheepswool will be resting against? West Wales gets lots of rain in winter and the air can be very damp for several days on end. I'm not sure how this would affect the performance. If I understand correctly, are you against adding the vapour check/airtight membrane? If so would you use any alternative membrane?]]>