MikeSharp01 Posted October 18, 2016 Share Posted October 18, 2016 Hi ALL. I am working through our wall design and am concerned to make sure we don't get moisture build up. Our architect seems happy enough but because we need great air tightness so an air tight barrier on the inside I have been working with WUFI to see how the wall performs I only have the free versionof WUFI, limited materials and only Grenoble climate data. I have read loads around WUFI (good and bad) and dew point calcs plus run a few simulations. The wall build up (inside - out) is: 12.5mm Platerboard 35mm service void. Air tight barrier. 300mm I Joists filled with DRITHERM .032 insulation 12mm OSB (4) 40mm Kingspan K12 Tyvek UV breather membrain 25mm Verticle battens (Ventilated) 25mm Horizontal battens (Ventilated) Marley Internit Thrutone cement tiles. PHPP U value is .104 (WUFI says .098) My key concern is the OSB because as I understand it this construction scheme (outer sheath of insulation) requires the insulation to dry out to the inside because the Kingspan is not vapour permiable - makes me wonder why I have a breather membrain and I think the OSB outer will reach dewpoint on occassions when it is very cold outside or if the sheathing is not installed perfectly. Any thoughts anywhere? Link to comment Share on other sites More sharing options...
IanR Posted October 18, 2016 Share Posted October 18, 2016 I have the same concern. Dritherm is vapour permeable, but the K12 is not, so your OSB is in trouble. Even with a vapour check layer on the inside (foil backed plasterboard???) I would have thought you should still have reducing vapour resistance to the outside if you have a vapour permeable main insulation. Link to comment Share on other sites More sharing options...
MikeSharp01 Posted October 20, 2016 Author Share Posted October 20, 2016 Sorry Ian have been out today. Yes I think the approach is basically a standard US scheme but they don't routinely have an airtight layer on the inside. I was not intending to have foil backed plasterboard but rather use something like Intello across the I-joists taped and held in place by the vertical battens forming the service void. Is your scheme similar? I guess I will have to find a WUFI professional to check my work. Mike Link to comment Share on other sites More sharing options...
IanR Posted October 20, 2016 Share Posted October 20, 2016 Is it a standard build up including the Kingspan K12? This seems to go against the advice from Knauf. From Knauf's timber frame document: Quote The importance of condensation control A vapour control layer is essential on the ‘warm’ side of the insulation to reduce the risk of condensation forming within the timber frame. The vapour control layer may be incorporated within the plasterboard or installed separately. Joints should be as few as possible and sealed, with special care taken around penetrations for services. A vapour permeable membrane on the outside of the sheathing board is also necessary to protect the timber frame from water penetration whilst allowing water vapour to escape. As a rule of thumb, the vapour resistance of the materials on the ‘warm’ side of the insulation should have at least five times the vapour resistance of the materials on the ‘cold’ side of the insulation. Alternatively, calculations should be carried out to assess the risk of interstitial condensation by referring to BS 5250 : 2011. If in doubt call the Knauf Insulation Technical Advisory and Support Centre for a free condensation risk calculation. Bold format added by me Since the K12 is vapour impermeable, it does not follow this guidance. Might be worth taking advantage of the offer of condensation risk analysis. I have a Touchwood frame, and their standard build up is (inside-to-out): Foil backed plasterboard (VCL) 300mm/350mm JJ I-Joist with Warmcel Eggers DHF Fibreboard (Airtight layer) Breather membrane Link to comment Share on other sites More sharing options...
MikeSharp01 Posted October 20, 2016 Author Share Posted October 20, 2016 Hmmmm... Interesting, we didn't, in the end go with with Touchwood - did go and see a site ran by Reubens brother, very impressed but we cannot get the U value we need with just Warmcell, hence the outer sheathing. I read their research on outer air tight layer which I also liked but then we need the extra sheathing. Yes I will call the Knauf support center and let them have a look at. I have been worried about this build up from the start but our architect seems to think it will be fine! Link to comment Share on other sites More sharing options...
DavidFrancis Posted October 21, 2016 Share Posted October 21, 2016 Do you really need the Kingspan at all? Have you worked out the payback period? Link to comment Share on other sites More sharing options...
MikeSharp01 Posted October 21, 2016 Author Share Posted October 21, 2016 I think the payback period is less of an issue than achieving passive house certification. We are very tight on roof thickness because we have an absolute ridge height we cannot exceed and between that and the ground level (which we cannot go below because of possible surface water) we cannot get the two clear stories in. In the end IanR put me on to Knauf who have been back to me an said the build up will work, in their words ' The resistance from the vapour control layer will comfortably outweigh any resistance from the cold side.' They have given me their model printout - great guys at Knauf. So I am happy but I still want to get my WUFI model to say the same thing. Link to comment Share on other sites More sharing options...
DavidFrancis Posted October 21, 2016 Share Posted October 21, 2016 Didn't realise/remember you wanted certification. Link to comment Share on other sites More sharing options...
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