Jeremy Harris Posted July 12, 2018 Share Posted July 12, 2018 @Russdl, for info, here's a scale sketch of an actual MBC nailplate and frame spacing noggin: As you can see, the nailplate has a lot of voids in it and also doesn't extend the full depth of the 89mm x 38mm frame members. Somewhere I have another AutoCad file of just the nailplate, I'll see if I can find it - it's archived from when I did all this stuff 5 or 6 years ago. Link to comment Share on other sites More sharing options...
A_L Posted July 12, 2018 Share Posted July 12, 2018 @Russdl , a couple of comments about your spreadsheet. First 38mm studs at 600mm centres are 6.3% timber by themselves, you have to allow for noggins (if present), wall plate, sole plate, top and bottom of frame, cripple studs at windows etc if these are within the heatloss areas of the walls. BRE recommend a 15% timber fraction for conventional timber frame walls, reducing to perhaps 12.5% if many of these are not present/moved outwith the wall area, into the floor/wall, wall/ceiling etc thermal bridges. See page 8 attached pdf. In balance I believe you have a 'twin wall' timber frame construction, with little/no timber in the 'Cellulose insulation 2' layer. BR_443_(2006_Edition).pdf Link to comment Share on other sites More sharing options...
Russdl Posted July 12, 2018 Share Posted July 12, 2018 @JSHarris I've modelled the nail plate over the central 122mm noggin (it would of course extend either side of that, as per your CAD drawing) but I found that an easy way of modelling it. I didn't consider the fact that it's full of holes but I guess I could just reduce my % of metal in the frame to account for that. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 12, 2018 Share Posted July 12, 2018 3 minutes ago, Russdl said: @JSHarris I've modelled the nail plate over the central 122mm noggin (it would of course extend either side of that, as per your CAD drawing) but I found that an easy way of modelling it. I didn't consider the fact that it's full of holes but I guess I could just reduce my % of metal in the frame to account for that. If you can hang on until later this evening, I can try and find the archived detail from when I did the analysis on this. I concluded that the impact of the nailplates wasn't massive and seemed to have been included in the original analysis for the wall U value I'd been given, as my result was damned close to that. Link to comment Share on other sites More sharing options...
Russdl Posted July 12, 2018 Share Posted July 12, 2018 @A_L Some very interesting observations there and thanks for the attachment, and yes 'twin stud' construction. Link to comment Share on other sites More sharing options...
Russdl Posted July 12, 2018 Share Posted July 12, 2018 @JSHarris Certainly can, off to look at windows now ? The nail plates in my model have very little impact but they are not as accurately modelled as yours. Link to comment Share on other sites More sharing options...
ADLIan Posted July 12, 2018 Share Posted July 12, 2018 The nail plate is shown with a conductivity of 17 W/mK - this is the value for stainless steel. For typical nail plate, in galv steel, this should should be 50-60 W/mK. But, as above, probably has little impact on U-value, probably only in 3rd decimal place and probably less than 0.01 W/m2K. Not sure its worth worrying to much about these slight variances in U-values when effect of linear thermal bridges may be your biggest enemy! 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 12, 2018 Share Posted July 12, 2018 11 minutes ago, ADLIan said: The nail plate is shown with a conductivity of 17 W/mK - this is the value for stainless steel. For typical nail plate, in galv steel, this should should be 50-60 W/mK. But, as above, probably has little impact on U-value, probably only in 3rd decimal place and probably less than 0.01 W/m2K. Not sure its worth worrying to much about these slight variances in U-values when effect of linear thermal bridges may be your biggest enemy! I agree wholeheartedly! I went berserk with modelling every thermal bridge, geometric bridge etc initially. I ended up with U values that were modelled to five decimal places for every element. Pointless really, as when I later came to the trade-off stage, and started changing values of components to see what the impact was on the overall heat loss, I found that seemingly large changes in the values of some components made very little overall difference. I've a personality type that can get very easily interested in digging down to the finest detail, though, whereas I know that most of the time these small details just don't have a massive impact and are swamped by factors that are outside any modelled parameters, anyway. 1 Link to comment Share on other sites More sharing options...
SteamyTea Posted July 12, 2018 Share Posted July 12, 2018 (edited) 13 hours ago, joe90 said: We have a wood burner for winter days Make sure you weigh all the timber you use. Edited July 12, 2018 by SteamyTea 1 Link to comment Share on other sites More sharing options...
MikeSharp01 Posted July 12, 2018 Share Posted July 12, 2018 9 hours ago, ADLIan said: 17 W/mK - this is the value for stainless steel. For typical nail plate, in galv steel, this should should be 50-60 Yes stainless has about 37% the thermal conductivity of steel so using it can make a big difference good / bad depending on the application. Link to comment Share on other sites More sharing options...
MikeSharp01 Posted July 12, 2018 Share Posted July 12, 2018 22 hours ago, Dreadnaught said: What is the form factor? I am guessing close to 2.5. To tidy things up our form factor (Compactness factor) is 3.32. 1 Link to comment Share on other sites More sharing options...
Russdl Posted July 12, 2018 Share Posted July 12, 2018 11 hours ago, ADLIan said: The nail plate is shown with a conductivity of 17 W/mK - this is the value for stainless steel Yeah, I guess I was trying to cheat the system! I've changed it to 55 W/mK and reduced the % of nail plate due to all the holes in them. As you predicted, the impact is minor. The last PHPP expert I spoke to suggested replacing the nail plates with OSB webs (which the manufacturer has agreed to do at no extra cost) but I have to say it seems like complete overkill and I suspect I won't bother doing that. I suppose the bottom line is that I have more faith in @JSHarris's U value calculations than the PHPP expert I spoke to, and I'm starting to feel inclined to ignore the PHPP expert (which means that was an expensive conversation!) Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 13, 2018 Share Posted July 13, 2018 I think the bottom line here is that of all the systems used to build passive houses in the UK, the MBC twin frame is most probably the most common one in recent years. Some of those are certified by the PHI, most probably aren't, but it does show that it's easy enough to meet PHI requirements using that system. FWIW, I looked at using I beam frames, but the thermal bridging through the OSB webs was higher than through the noggin spacers and nailplates of the twin stud. Not by much, but enough to make me consider having large holes drilled in the OSB webs to reduce the thermal bridging. You might also want to note that the geometric thermal bridges in the corners has a great impact than the nailplates. I mitigated this on our build by adding rigid foam insulation into the eaves and fascia ladder frames. The overhanging additional insulation removes most of the geometric thermal bridge in those areas and is relatively easy to do after the frame is up. Link to comment Share on other sites More sharing options...
Russdl Posted July 13, 2018 Share Posted July 13, 2018 2 hours ago, JSHarris said: FWIW, I looked at using I beam frames, but the thermal bridging through the OSB webs was higher than through the noggin spacers and nailplates of the twin stud. Not by much, but enough to make me consider having large holes drilled in the OSB webs to reduce the thermal bridging. That is really interesting because the main player using the I beam construction would appear to have many more certified passive house's out there than MBC using (what you would consider) an inferior construction method to achieve it, that's reassuring as MBC is our company of choice. 2 hours ago, JSHarris said: You might also want to note that the geometric thermal bridges in the corners has a great impact than the nailplates. I mitigated this on our build by adding rigid foam insulation into the eaves and fascia ladder frames. The overhanging additional insulation removes most of the geometric thermal bridge in those areas and is relatively easy to do after the frame is up. I didn't realise you'd done that. As my design has concealed gutters I think I'll be stuffed on the eaves but should be able to get a bit in the ladder frames. Link to comment Share on other sites More sharing options...
Dan F Posted December 30, 2019 Share Posted December 30, 2019 @Russdl I know this was 18mths ago, but I was just looking at exactly his in our PHPP model, and I think I can see your issue. It seems you modelled a single stud wall with 300mm of timber at 600mm centers, rather than a twin-stud wall. The 6.4% applies to the two 89mm sections, but not the 122mm section. The 122mm section does have some timber (the noggins with nail plates), but its a lot less than 6.4% I beleive. I'm not sure if this brings it down to 1.2, but certainly not as high as 1.37. I was a bit concerned when seeing your 0.137 intially, as I know MBC use a "foil/membrane" product in their u-value calculation for their closed-panel/PIR system which has (I'm told) a questionable impact their u-values, giving an advertised 0.14 with the foil vs. closer to 0.18 without the foil! Based on @Jeremy Harris's numbers, and assuming his external cladding isn't particulalry insulating, it would seem this isn't the case with the 300mm sytem though, I hope so anyway! Dan On 12/07/2018 at 09:55, Russdl said: @JSHarris I've no doubt that is accurate, and much different to the figure I've achieved in my PHPP. Can you see any glaring errors in the data I've put in to my spread sheet that has resulted in my U value of 0.137? 1 Link to comment Share on other sites More sharing options...
Dan F Posted February 25 Share Posted February 25 @Russdl Did you complete PHPP in the end? Also, what was your final verdict/number on the MBC twin-wall u-value? I have this, but I think the timber-fractions are too high! The teoretical values at 6.3% and 0.7% (noggins), but @A_L is right that the actual timber-fraction (in walls that have doors/windows more than roofs) is higher. Some of this extra timber does get accounted for in window thermal bridge calculations, but not all of it. Link to comment Share on other sites More sharing options...
Russdl Posted February 25 Share Posted February 25 I did complete the PHPP and here is my final number for the twin stud, similar to yours but different U values for the plasterboard, service cavity and airtight board giving me a different U value to yours. I don't recall how I got to where I got to but I do recall thinking 'that's enough, that will do'. The house was built with no modifications to the twin wall design and performs well, it actually performs better than the PHPP predicted. Link to comment Share on other sites More sharing options...
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