Green Building Forum - Ventilation gap under sarking board Mon, 25 May 2026 17:56:02 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200410#Comment_200410 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200410#Comment_200410 Thu, 17 Apr 2014 21:31:43 +0100 Ed Davies
The question is, is it OK to full-fill between the rafters up to the sarking with glassfibre or is a ventilation gap above the glassfibre, below the sarking, needed? My intuition is that the glassfibre is sufficiently vapour open that the ventilation gap is redundant but it'd be nice to be a) sure and b) able to prove it to a Scottish BCO.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200417#Comment_200417 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200417#Comment_200417 Fri, 18 Apr 2014 05:47:18 +0100 tony ]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200429#Comment_200429 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200429#Comment_200429 Fri, 18 Apr 2014 08:15:09 +0100 Ed Davies
http://edavies.me.uk/2013/09/roof-rethink/

Going back to the designer to discuss whether using OSB for the sarking layer would eliminate the need for this ventilation layer.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200430#Comment_200430 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200430#Comment_200430 Fri, 18 Apr 2014 08:28:24 +0100 bot de paille Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200486#Comment_200486 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200486#Comment_200486 Fri, 18 Apr 2014 17:29:56 +0100 davidfreeborough
As bot says, with this arrangement & a non-vapour open roof covering, you need to ventilate at eaves & ridge.

David]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200491#Comment_200491 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200491#Comment_200491 Fri, 18 Apr 2014 18:29:09 +0100 Ed Davies
T&G OSB was my original thought, too. House designer originally suggested 9 mm OSB (which I'm pretty sure you can't get as T&G, right?) so I was a bit surprised when the spec came back with the sawn timber sarking boards. Didn't argue until I realized the implications of the air gap.

Haven't heard back from him but it's a lovely day and he's probably busy working on his own house's extension today.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200561#Comment_200561 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200561#Comment_200561 Sat, 19 Apr 2014 12:38:05 +0100 fostertom Posted By: Ed Daviesfully ventilated at the eaves and ridgesthat implies through ventilation above the insulation, below the impermeable covering. Otherwise, why ventilate?

Have we done this one before? Despite what metal roof cos say, WUFI says it's sure death without that through ventilation. Facing a clear sky, even daytime sometimes, the metal roof is a super-cooled condensation plate, and all that permeability just feeds more and more vapour up to be condensed. The metal roofing will be literally dripping - the cause of every caravan's rapid rot fate.

Whereas with copious through ventilation allowing the fabric to dry diurnally and seasonally in both directions, it's very robust and tolerant of that condensation water, even of moderate leakage. No need for vapour barriers/checks anywhere, even 'intelligent' ones - in fact performs better without.

As usual, don't take my word for it (unless paying for consultancy!)- check it out to your satisfaction.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200582#Comment_200582 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200582#Comment_200582 Sat, 19 Apr 2014 17:46:06 +0100 Ed Davies
The question is, would I need another gap below the membrane and timber, but above the insulation? Presumably not open to the world but allowing vapour from under the middle of a sarking board to go 75 mm to the nearest gap between boards. What I was hoping for was an example of timber sarking board of this type (presumably in Scotland as it's a bit of a Scottish habit) with vapour open insulation like mineral wool right up against it. Absent that I'll want to change to OSB but we're getting a bit late in the design process for that.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200587#Comment_200587 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200587#Comment_200587 Sat, 19 Apr 2014 18:32:20 +0100 tony Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200598#Comment_200598 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200598#Comment_200598 Sat, 19 Apr 2014 22:08:11 +0100 willie.macleod
There should be no problem with fully filling between the I beams and up to the inside of the sarking as others have said. Your roof as currently proposed is not a true warm roof as the rafters aren't fully warm, it is more a hybrid, the first drawing you have is a proper warm roof with the rafters inboard.

I don't think anyone will have a problem with changing to OSB instead of 6" sarking boards at this stage and it should be a bit quicker to install and get properly air tight, especially if you are looking to minimise external labour in getting wind and water tight.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200602#Comment_200602 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200602#Comment_200602 Sat, 19 Apr 2014 22:49:50 +0100 Ed Davies
With sarking board I'd do a tony and fold the membrane over at the join but that'd be a big faff on 8 metres of roof on an site where it's breezy more often than not. With t+g I'd just make sure there's a counterbatten over the overlap.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200603#Comment_200603 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200603#Comment_200603 Sat, 19 Apr 2014 23:20:59 +0100 willie.macleod Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200614#Comment_200614 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200614#Comment_200614 Sun, 20 Apr 2014 10:02:43 +0100 Ed Davies Posted By: fostertomDespite what metal roof cos say, WUFI says it's sure death without that through ventilation. Facing a clear sky, even daytime sometimes, the metal roof is a super-cooled condensation plate, and all that permeability just feeds more and more vapour up to be condensed. The metal roofing will be literally dripping - the cause of every caravan's rapid rot fate.Going off-topic on my own thread, why can't you think of it as the metal skin acting as a dehumidifier for the roof structure below? Water condenses on the metal then runs down to the bottom of it or drips onto the membrane and runs down that. So long as it gets out at the bottom, without being dammed by battens or anything, then all is well. All extra ventilation serves to do is to bring in more water vapour.

(Sort of like how the single glazed windows in the static caravan I'm staying in act as a dehumidifier so long as I wipe them dry every morning.)]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200695#Comment_200695 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200695#Comment_200695 Mon, 21 Apr 2014 09:22:48 +0100 willie.macleod Posted By: Ed Davies
(Sort of like how the single glazed windows in the static caravan I'm staying in act as a dehumidifier so long as I wipe them dry every morning.)

Karcher window vac - great for this, gets rid of the water without having to worry about drying towels. Been there done that! Depending on the time of year we are talking about 1.5 tank fulls on a static - 150ml water held on the windows.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200704#Comment_200704 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200704#Comment_200704 Mon, 21 Apr 2014 10:41:56 +0100 Ed Davies Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200726#Comment_200726 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200726#Comment_200726 Mon, 21 Apr 2014 21:56:14 +0100 Timber
If using planks with gaps, it is considered a low resistance membrane and ventillation can be from above.

I don't fully agree with this (if it works for a sheathed timber frame wall, why not a roof), but that is what 5250 says.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200727#Comment_200727 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200727#Comment_200727 Mon, 21 Apr 2014 22:44:56 +0100 bot de paille OSB = ventilation gap needed!

I prefer the idea of the sarking boards over the OSB.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200730#Comment_200730 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200730#Comment_200730 Tue, 22 Apr 2014 09:37:52 +0100 Ed Davies Posted By: TimberIf using planks with gaps, it is considered a low resistance membrane and ventillation can be from above.Thanks - yes, would have gaps (few mm) between the planks and would be ventilated above. Do you know if they say whether the insulation can be against the board or if a gap under the board, above the insulation, is needed?

Posted By: bot de pailleOSB = ventilation gap needed!
That's surprising - I thought various forms of vapour open insulation (sheep's wool, mineral wool, perhaps cellulose fibre) packed in OSB boxes was common.

Just to be clear - you mean a ventilation gap between the OSB and the insulation? Open to the outside world?

With the sarking boards with few mm gap between them I can see the point of a ventilation gap between the board and the insulation which is not open to the outside world so that the insulation under the middle of the board is not as cold as the board but 75 mm (half a 150 mm board width) away from a ventilated gap. With OSB I can't see any equivalent reason for a not-open-to-the-outside-world gap.

Does the thickness of the OSB make a difference?


Edit: spurious and confusing “not” crossed out.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200732#Comment_200732 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200732#Comment_200732 Tue, 22 Apr 2014 10:22:57 +0100 dickster Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200756#Comment_200756 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200756#Comment_200756 Tue, 22 Apr 2014 18:09:39 +0100 davidfreeborough
Tell Building Control that its a hybrid warm roof & if they question it then tell them its no different to a timber framed wall with a ventilated gap on the outside. If you need a reference then choose a breather membrane where the BBA certificate specifically states it can be used "fully supported" over sarking boards.

Looking at the Tyvek Supro BBA there's some interesting wording in Section 15:

http://construction.tyvek.co.uk/Tyvek_Construction/en_GB/assets/downloads/certificates/roof_bba_tyvek_supro_in_warm_non_ventilated_cold_ventilated_roofs.pdf

According to Section 15.1, it is OK to fully support it with softwood sarking boards or insulation. However, it defines the former as a "cold ventilated roof" & the latter as a "warm non-ventilated roof". Does this imply that the softwood sarking boards require ventilation from below? At what thermal resistance does softwood become insulation? Would using 22mm T&G woodfibre sarking boards get around the problem as these are recognised as having useful thermal resistance?

David]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200764#Comment_200764 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200764#Comment_200764 Tue, 22 Apr 2014 21:05:12 +0100 Ed Davies Posted By: davidfreeborough…tell them its no different to a timber framed wall with a ventilated gap on the outside.Exactly - it's what I'll have on my gables. That'll only be 9 mm though … hmm, may need some little noggin things to seal behind the joins.

Woodfibre is an interesting idea which I hadn't previously considered because of cost but if it sorts out any residual condensation risk while providing structure and some insulation it might make sense.

And, yes, vermin resistance is needed. The steel roof people have a scheme for filling in the ends of the profile while allowing ventilation but that's mostly to stop rain getting blown in - will consider if it's likely to stop animals, too.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200766#Comment_200766 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200766#Comment_200766 Tue, 22 Apr 2014 21:29:33 +0100 Timber
HOWEVER BS 5250 does state that if you can model and prove anther construction method, then that is also fine.

So for me, it makes sense to go for T&G OSB, rafters fully filled, vented above breather membrane with a high resistance VCL on the inside. Model it and use that to satisfy BCO. T&G OSB is still sufficiently breathable with a high resistance membrane (well fitted) on the inside, and being T&G will act as a good air tightness membrane on the outside to prevent against wind washing.

For the record - I was stating what the standard says. I actually dislike some of BS 5250 as it confuses things like warm and cold roofs as well as requirements for VCLs on SIPs (to name a few issues with it).]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200779#Comment_200779 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200779#Comment_200779 Tue, 22 Apr 2014 23:08:03 +0100 Ed Davies Posted By: TimberIf gaps between boards, then BS 5250 says that insulation can be fully packed.That's a useful fallback to know about, thanks.

Still prefer T&G but a bit concerned about condensation. A lurker on this forum has just sent me an analysis with software which was a couple of years ago approved for this sort of thing which shows a risk but he's a bit unsure about the resistance of the OSB and also didn't include a vapour barrier, just the higher resistance of my PUR.]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200801#Comment_200801 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200801#Comment_200801 Wed, 23 Apr 2014 11:36:19 +0100 Ed Davies
http://edavies.me.uk/2014/04/osb_vapour/]]> Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200818#Comment_200818 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=200818#Comment_200818 Wed, 23 Apr 2014 15:19:45 +0100 Timber Ventilation gap under sarking board https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=288123#Comment_288123 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993&Focus=288123#Comment_288123 Mon, 15 Mar 2021 19:02:14 +0000 Shevek