bissoejosh Posted January 4, 2018 Share Posted January 4, 2018 (edited) Currently our design is based on using 15mm Egger DHF for our external sheathing but having received some prices it seems an expensive option. Any huge issues (assuming the engineer OK's it) switching to either 11mm or 15mm OSB? I'm sure others have used OSB and it comes in at roughly half the price so a substantial saving on 300m2. My only thought was weatherproofing during construction as I know the Egger is pretty water resistant. For reference the rest of the make up as follows: PB & Skim Service Cavity 12mm VapourBlock OSB (again quite an expensive option) 245mm i-Stud 15mm Egger 38mm Cavity 20mm Cladding Many thanks Edited January 4, 2018 by bissoejosh Link to comment Share on other sites More sharing options...
IanR Posted January 4, 2018 Share Posted January 4, 2018 (edited) Are you planning on the (T&G?) Egger DHF being your Air tight layer? If so perhaps its cost is not so high, What would be your alternative airtight layer if you didn't use DHF Do you need a Service cavity? You could use foil-backed PB as VCL rather than OSB and the service cavity. Perhaps just put the Service cavity where you need it. Electrics can run through insulated cavity, only plumbing (and MVHR) needs to be kept out of the insulated cavity. 38mm Is a small vent gap to external cladding. Edited January 4, 2018 by IanR Link to comment Share on other sites More sharing options...
bissoejosh Posted January 4, 2018 Author Share Posted January 4, 2018 Hoping to use the VapourBlock OSB as our airtight layer and VCL for durability and ease of construction. Service cavity is currently 25mm directly behind PB. I thought minimum external vent was 25mm - BCO upped it to 38mm. I could up it to 50mm if necessary? Link to comment Share on other sites More sharing options...
A_L Posted January 4, 2018 Share Posted January 4, 2018 15mm OSB instead of the Vapourblock and Panelvent DHF instead of the Egger ? https://www.ecomerchant.co.uk/panelvent-structural-vapour-permeable-sheathing-board.html Link to comment Share on other sites More sharing options...
bissoejosh Posted January 4, 2018 Author Share Posted January 4, 2018 1 minute ago, A_L said: 15mm OSB instead of the Vapourblock and Panelvent DHF instead of the Egger ? https://www.ecomerchant.co.uk/panelvent-structural-vapour-permeable-sheathing-board.html Another option, although the frame is drawn to allow for the 12mm internal board and changing now would be a bit of a pain. Panelvent instead of the Egger looks interesting - same sort of product just a little thinner and hopefully cheaper? Link to comment Share on other sites More sharing options...
Dudda Posted January 4, 2018 Share Posted January 4, 2018 Internally the 15mm OSB wouldn't be as airtight as the 12mm VapourBlock OSB. I wouldn't go changing that considering this is your airtight layer. 1 Link to comment Share on other sites More sharing options...
A_L Posted January 4, 2018 Share Posted January 4, 2018 (edited) 6 minutes ago, bissoejosh said: Panelvent instead of the Egger looks interesting - same sort of product just a little thinner and hopefully cheaper? Actually made by Egger, not T&G and not 'branded' panelvent_dhf_technical_data_sheet_1.pdf Edited January 4, 2018 by A_L minor typo Link to comment Share on other sites More sharing options...
bissoejosh Posted January 4, 2018 Author Share Posted January 4, 2018 6 minutes ago, Dudda said: Internally the 15mm OSB wouldn't be as airtight as the 12mm VapourBlock OSB. I wouldn't go changing that considering this is your airtight layer. Agreed, I think this stays as it is. It may be the case that the Vapourblock is also sufficient for racking strength which makes the external sheathing rather redundant except for holding insulation in place and providing protection during construction. Link to comment Share on other sites More sharing options...
IanR Posted January 4, 2018 Share Posted January 4, 2018 10 minutes ago, bissoejosh said: Hoping to use the VapourBlock OSB as our airtight layer and VCL for durability and ease of construction. Service cavity is currently 25mm directly behind PB. I thought minimum external vent was 25mm - BCO upped it to 38mm. I could up it to 50mm if necessary? I hadn't seen OSB used as an Air tight layer, but have just checked and MBC have updated their build-up and appear to use the OSB inner layer for air tightness and VCL, When I considered using them (a few years ago) they used an additional membrane for this. If that is a robust solution then both it and T&G Egger DHF would not be necessary. But, if you go airtight layer on the inside skin you do need a service cavity to avoid penetrations. If you went airtight on outer skin you could avoid the extra cost of service cavity - other than where you can't avoid plumbing routing along/down an external wall. With regards external vent gap I was encouraged to go 50mm minimum. Link to comment Share on other sites More sharing options...
IanR Posted January 4, 2018 Share Posted January 4, 2018 21 minutes ago, bissoejosh said: Hoping to use the VapourBlock OSB as our airtight layer ...another thought. Are you going with blown cellulose? If so that's a lot of holes in you airtight layer to re-seal. Link to comment Share on other sites More sharing options...
ProDave Posted January 4, 2018 Share Posted January 4, 2018 The designer of my house specified OSB3 as both the racking layer and the air tight layer (with taped joints) on the inside of the frame. I later decided to add a proper air tight membrane on the inside over the OSB. Then later the structural engineer upgraded it to 2 layers of OSB to give the required racking strength. Definitely stick with a service void. If changing the external layer, check with your SE re racking strength, particularly if moving from a T&G product to a square edge product. Link to comment Share on other sites More sharing options...
bissoejosh Posted January 4, 2018 Author Share Posted January 4, 2018 1 minute ago, IanR said: ...another thought. Are you going with blown cellulose? If so that's a lot of holes in you airtight layer to re-seal. Sadly not, I couldn't make the budget work on this occasion so using mineral wool. Link to comment Share on other sites More sharing options...
IanR Posted January 4, 2018 Share Posted January 4, 2018 2 minutes ago, ProDave said: Definitely stick with a service void. Why's that Dave? it's a lot of extra cost. Link to comment Share on other sites More sharing options...
A_L Posted January 4, 2018 Share Posted January 4, 2018 Quote "I've put my faith in the words I heard from Peter Warm, one of UKs three (?) PH Assessors - 'I've never yet found a building that failed PH airtightness because of OSB" from :- http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15470 fourth comment from top Link to comment Share on other sites More sharing options...
ProDave Posted January 4, 2018 Share Posted January 4, 2018 1 minute ago, IanR said: Why's that Dave? it's a lot of extra cost. It's the cost of some battens. You will have the OSB anyway (or whatever you change it for) so omitting the service void just mean you put the plasterboard straight onto the OSB. It then makes it a pain to run cables through the frame (drilling the web of the I beams) and then cutting through the OSB and plasterboard for back boxes. Then there is the issue of making it all air tight. I have omitted the service void in a couple of places where I know there are no services (stairwell head wall and bathroom outside wall) just to gain an extra 25mm in the rooms, but all other rooms have the service void. Link to comment Share on other sites More sharing options...
bissoejosh Posted January 4, 2018 Author Share Posted January 4, 2018 9 minutes ago, ProDave said: The designer of my house specified OSB3 as both the racking layer and the air tight layer (with taped joints) on the inside of the frame. I later decided to add a proper air tight membrane on the inside over the OSB. Then later the structural engineer upgraded it to 2 layers of OSB to give the required racking strength. Definitely stick with a service void. If changing the external layer, check with your SE re racking strength, particularly if moving from a T&G product to a square edge product. I've asked the question re racking. Did you have any external sheet materials other than the fiberboard? It seems to me that the Egger DHF is redundant for the most part but excluding it would mean I have to insulate from the outside as I would need the internal board up right away for strength - I can't decide if this is an issue. Link to comment Share on other sites More sharing options...
IanR Posted January 4, 2018 Share Posted January 4, 2018 (edited) 13 minutes ago, ProDave said: It's the cost of some battens. You will have the OSB anyway Nope, it's the cost of battens, OSB and labour. My build up from out to in is: Cladding vent gap breather membrane T&G DHF Egger (airtight layer & racking ) I-Joist Foil-backed PB (VCL) & skim So unless the loads on the structure require a structural layer inside and out the extra OSB is just closing off the cavity to hold the insulation. Edited January 4, 2018 by IanR Added external breather membrane layer Link to comment Share on other sites More sharing options...
bissoejosh Posted January 4, 2018 Author Share Posted January 4, 2018 6 hours ago, IanR said: Nope, it's the cost of battens, OSB and labour. My build up from out to in is: Cladding vent gap breather membrane T&G DHF Egger (airtight layer & racking ) I-Joist Foil-backed PB (VCL) & skim So unless the loads on the structure require a structural layer inside and out the extra OSB is just closing off the cavity to hold the insulation. Lots of thinking after this post. Perhaps I'm missing a trick not detailing in a similar manner - Cullen design asked me a while ago if I wanted to go down the airtight on the outside route but I had already gone 'traditional' in my mind. Any downsides having done it this way or things you would do differently? I think I'd be tempted to still opt for a cheap VCL+ batten to create service void & help support the mineral wool in our frame. Link to comment Share on other sites More sharing options...
IanR Posted January 4, 2018 Share Posted January 4, 2018 1 hour ago, bissoejosh said: Lots of thinking after this post. Perhaps I'm missing a trick not detailing in a similar manner - Cullen design asked me a while ago if I wanted to go down the airtight on the outside route but I had already gone 'traditional' in my mind. Any downsides having done it this way or things you would do differently? I think I'd be tempted to still opt for a cheap VCL+ batten to create service void & help support the mineral wool in our frame. I think the build-up is very cost effective and I would go the same route again. A service void is nice to have, but is it worth the extra cost and wall thickness? That's for you to decide. There are no downsides that are specific to this buildup. If you get your air tightness from a rigid layer then your detailing of corners and junctions needs to be good. I'm sure CTD will get the design correct, so you'll need to ensure the build quality is up to the job. I was told an external breather membrane was needed for the roof but not necessarily for the walls, but chose to cover both to protect from the weather while cladding and roof was going on. Electrical 1st fix was very simple within the cavity (pre-insulating), but any penetration through the DHF (wires for outside lights, door bell, MVHR ducts to outside etc.) need an air tight gasket. I'd question your decision for mineral wool. Blown cellulose will improve air tightness and I assume has a higher decrement delay (I've not checked mineral wool). You should also perhaps get some advice or a Wufi analysis on mineral wall to make sure it's suitable behind the DHF for clearing any moisture out. Maybe the insulation manufacturer will be able to support this or can confirm it's suitable for this application. Link to comment Share on other sites More sharing options...
JamesP Posted January 4, 2018 Share Posted January 4, 2018 On our wall build up I have 11mm OSB on the inside for racking, not sure why you would need the Egger and OSB. My wall build up from outside Larch cladding 25mm batten 25mm vertical batten Breather Membrane 40mm Pavatherm 275mm Timber with Rock wool Flexi Vapour Barrier 11mm OSB 25mm Service void 12mm Plasterboard Link to comment Share on other sites More sharing options...
bissoejosh Posted January 5, 2018 Author Share Posted January 5, 2018 14 hours ago, JamesP said: On our wall build up I have 11mm OSB on the inside for racking, not sure why you would need the Egger and OSB. My wall build up from outside Larch cladding 25mm batten 25mm vertical batten Breather Membrane 40mm Pavatherm 275mm Timber with Rock wool Flexi Vapour Barrier 11mm OSB 25mm Service void 12mm Plasterboard So your Pavatherm acts as a retaining layer for the RockWool. We're not having Pavatherm so the question is does the RW need sandwiching or will it stay put with only one (external) side covered by OSB? Link to comment Share on other sites More sharing options...
JamesP Posted January 5, 2018 Share Posted January 5, 2018 3 minutes ago, bissoejosh said: So your Pavatherm acts as a retaining layer for the RockWool. We're not having Pavatherm so the question is does the RW need sandwiching or will it stay put with only one (external) side covered by OSB? The Rockwool Flexi 140mm slabs are very stable and would stay put. Personally I would use a cheaper option to the Egger board on the outside. Some wall build ups have OSB just on the inside or outside, the breather membrane would seem a bit vulnerable with out it. I used Pavatherm to increase the insulation and have a warm roof. My project below. Link to comment Share on other sites More sharing options...
bissoejosh Posted January 16, 2018 Author Share Posted January 16, 2018 (edited) Bit of an update... We've upgraded to 300mm jji beam walls and blown cellulose throughout. To manage this from a cost point of view we've opted to use OSB instead of the egger dhf and also to use 11mm osb and a membrane internally with just a membrane on the ceiling. My next question for those that have sheathed with OSB...Did you use tongue and grooved or square edged? Our egger dhf was t&g which I guess eliminated some cutting for joints etc but I hope square is fine as I believe most are done this way? Edited January 16, 2018 by bissoejosh Typo Link to comment Share on other sites More sharing options...
jamiehamy Posted January 16, 2018 Share Posted January 16, 2018 We used square edge for the upstairs part. Make sure you order the right size and don't get caught out with 2440x1220 boards (rather than 2400x1200) which will not tie in with 600mm centres. Link to comment Share on other sites More sharing options...
bissoejosh Posted January 16, 2018 Author Share Posted January 16, 2018 25 minutes ago, jamiehamy said: We used square edge for the upstairs part. Make sure you order the right size and don't get caught out with 2440x1220 boards (rather than 2400x1200) which will not tie in with 600mm centres. Good advice, I'll make sure I specify and the supplier confirms! Link to comment Share on other sites More sharing options...
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