Kevin Dawson Posted August 20 Share Posted August 20 Hi - starting work on a long term ambition here - self-managing a greenfield passivhaus build (SIPS panels over an insulated raft, timber clad and standing seam roof), aiming for certified low energy (PHPP model says we can't meet Classic levels). Have done several house refits and improvements/extensions over the years but this is a in different league so hoping to be able to pick some collective wisdom from the forums here. 1 Link to comment Share on other sites More sharing options...
IanR Posted August 20 Share Posted August 20 High @Kevin Dawson, and welcome What's made you choose SIPs? Link to comment Share on other sites More sharing options...
Kevin Dawson Posted August 20 Author Share Posted August 20 Repeated visits to the NSBRC in Swindon ended up with us deciding MBC’s offering worked for us. Link to comment Share on other sites More sharing options...
IanR Posted August 20 Share Posted August 20 11 minutes ago, Kevin Dawson said: ended up with us deciding MBC’s offering worked for us. I wasn't aware MBC offered a SIPs option, perhaps I'm out of date. Would you mind linking to the option you are going with? Link to comment Share on other sites More sharing options...
Kevin Dawson Posted August 20 Author Share Posted August 20 My bad - playing a bit fast and loose with the terminology. It's the MBC Passive Wall structure sitting on their Passivhaus certified slab - we've just gotten a bit lazy describing it to friends. Link to comment Share on other sites More sharing options...
IanR Posted August 20 Share Posted August 20 (edited) 47 minutes ago, Kevin Dawson said: It's the MBC Passive Wall structure sitting on their Passivhaus certified slab That's cool, saves a long discussion about whether SIPs are the best choice. Now my next question, why: 3 hours ago, Kevin Dawson said: aiming for certified low energy (PHPP model says we can't meet Classic levels). Have you got a really complex plan profile? I can't imagine how MBC's twin-stud wall + an insulated raft couldn't mitigate most positional, orientation and profile issues. Edited August 20 by IanR Link to comment Share on other sites More sharing options...
Kevin Dawson Posted August 21 Author Share Posted August 21 The floorplan is a simple 20m by 6m rectangular "barn" and whilst the final PHPP is still being worked on and reviewed by the certifier the initial modelling showed a significant likelihood of overheating so we reoriented the long axis more northwards and debated various shading options but the design issues were a projected heat demand of 24kWh/(m2a) versus a limit of 15 for Classic certification and a projected heat load of 14kW/m2 (versus a limit of 10). So it looks like it'll be certified PHI Low Energy rather than Classic. Link to comment Share on other sites More sharing options...
IanR Posted August 21 Share Posted August 21 Overheating can be resolved. Ideally with Brise Soleil, but if planning restrictions don't allow (as was the case with me) then external venetians should resolve the issue. Obviously, reducing South-West to South-East window areas and roof lights are the cheaper options, but if you want to keep these then spending some budget on shading is needed. What windows are you specifying, are there solar control coating options? Surprised at your "heat" demands/loads. Especially if you've got lots of solar gain. Just to be sure, you are going with the Twin-Stud MBC PH Wall, with 300mm of blown cellulose insulation? Does your PHPP modeller have experience with this build method and with insulated raft foundations? Have they correctly calculated thermal bridges or are they using some defaults? Link to comment Share on other sites More sharing options...
garrymartin Posted August 21 Share Posted August 21 I'm with @IanR on this one. My gut says something doesn't seem right or we don't have some important piece of information yet that will enable us to go "ah, that makes sense now!" 😉 Link to comment Share on other sites More sharing options...
garrymartin Posted August 21 Share Posted August 21 (By the way, it will be 14W/m2 and not 14kW/m2. I'm guessing you know that anyway, but just for anyone else who may be reading). Quick thought, is this perhaps a design with large vaulted, double-height spaces? What's the usable m2? Link to comment Share on other sites More sharing options...
Russdl Posted August 21 Share Posted August 21 Agree with the above. Something does not sound right. We’re in Wiltshire, not that far from East Somerset. We had one meeting with a PHPP consultant who was based in West Wiltshire (even closer to East Somerset…) He did not know his arse from his elbow and made some very very basic mistakes in his first iteration of the PHPP. We quickly moved on and completed the PHPP ourselves. I hope you’re not using that individual. 1 Link to comment Share on other sites More sharing options...
Kevin Dawson Posted August 21 Author Share Posted August 21 Good questions @IanR Windows are Rationel triple-glazed Auraplus (haven't considered coating options but will check). Brise soleil was added (to the mostly glazed face that is now mainly north facing after we rotated the house clockwise 25 degrees to improve the view and reduce the solar impact) but there is still a fair amount of glass on the "western" face (about 17m2 - about 20% of the wall area) and slightly less on the eastern face. To @garrymartin's points - good catch, yes 14W/m2, the house is upside down in as much as the living/dining/kitchen are all upstairs and open plan and yes that is a vaulted space to the ridge along with the stairwell. According to the architect the total GFA is 108m2 upstairs and 98 down. Site layout including the orientation attached for reference. Note also that the left hand end also has a 3m roof overhang covering an upstairs balcony and downstairs patio) which obviously shades that end and its two sets of 4m glazed doors. @Russdl the PHPP consultant is a certified PH designer within the architectural practice and there is an external/independant certifier reviewing the model for certification purposes so hoping that between them it should be reasonably representative. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 21 Share Posted August 21 Looks like a nice design, have some elevations? And a good compass orientation - in fact, similar to ours, which was chosen for PV & solar gain reasons. The 'high' heat demand is probably just a function of the glass area - we also made similar choices. You can get great heat demand results if you don't have any windows at all . You could ask for a copy of the spreadsheet and play around with variations and drill down to see where the bulk of the energy use is. Or you can start with the J Harris spreadsheet, which is 3% as complicated, and will get similar-ish results to PHPP for heat calcs. Link to comment Share on other sites More sharing options...
Kevin Dawson Posted August 21 Author Share Posted August 21 @Alan Ambrose strange isn't it, buy a plot with great views and then not have any windows - what are we thinking? What is the J Harris spreadsheet? Interested to try it out and yes, will see if I can get a copy of the PHPP one. Link to comment Share on other sites More sharing options...
Adrian Walker Posted August 21 Share Posted August 21 I agree with all the comments about overheating. Even without PHPP you can look at a design and its orientation and have a good guess about overheating. If the architect has designed a house that overheats I would question their skills, I presume that the brief to them was to design a PH that met the classic levels. Overheating is greater than 25ºC for a max of 10% of the year (36.5 days!) Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 21 Share Posted August 21 Jeremy was (he no longer posts here) a Buildhub 'living treasure'. See here: https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=18333 Like the design a lot If the architects are sniffy anout giving you the whole PHPP analysis ask for a printout of the section down to cell Q25 at the top of the 'Annual heating' tab. e.g. (this is ours which has an inadvisable amount of glass) 1 Link to comment Share on other sites More sharing options...
IanR Posted August 21 Share Posted August 21 (edited) Looks a nice project. If you are interested in drilling down into the building performance, it may be worth starting a new thread, perhaps in https://forum.buildhub.org.uk/forum/115-energy-efficient-sustainable-design-concepts/ and get some wider forum input. It really feels to me that something is not correct with your calcs. I'm all on a single storey (so much less efficient) with a higher percentage of glazing and roof lights and achieved the the classic targets. I've not got an MBC structure, but my I-Joist structure on insulated raft performs only marginally better than MBC's twin-stud, but it's a negligible difference. The I-Joists did allow me to go to 350mm deep on the roof to mitigate the inefficiencies of being on a single storey, but even without this I was almost at the targets, much closer than your PHPP calcs are showing. Is the plan view orientation shown above after it was changed? The below comment suggests it's round the wrong way. 1 hour ago, Kevin Dawson said: Note also that the left hand end also has a 3m roof overhang covering an upstairs balcony and downstairs patio) which obviously shades that end and its two sets of 4m glazed doors. Wouldn't the balcony with overhang and patio better on the South-east elevation, rather than North-west? Or is it the views that are setting their position? Edited August 21 by IanR Link to comment Share on other sites More sharing options...
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