Alan Ambrose Posted July 30 Share Posted July 30 Hi, anyone have any experience of either (a) calculating thermal bridge psi values themselves or (b) having an inexpensive service calculate those for you? Link to comment Share on other sites More sharing options...
SteamyTea Posted July 30 Share Posted July 30 Do you want an exact value (certification) or an approximate value, I think the building regs have some charts to calculate it. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted July 30 Author Share Posted July 30 I wouldn't mind an 'exact' value i.e. some calculations rather than someone's guess. Link to comment Share on other sites More sharing options...
MikeSharp01 Posted July 30 Share Posted July 30 You can do it with Therm. Here is a natty video on how it could be done. Link to comment Share on other sites More sharing options...
ADLIan Posted July 30 Share Posted July 30 Therm is not very intuitive and great care is needed in building the model. The results need further manipulation to get the psi-value and the added bit of maths varies with junction type if I remember correctly. Also refer to BR497 from BRE on the calculation conventions. I use Psi-Therm 2D and 3D - expensive but easy to use. Link to comment Share on other sites More sharing options...
SteamyTea Posted July 30 Share Posted July 30 3 hours ago, SteamyTea said: Do you want an exact value (certification) or an approximate value, I think the building regs have some charts to calculate it. I did write up a method on here a while back, can't remember what the title was though. It basically took a topological approach to turn a 3D shape into a 2D shape, but keeping the dimensions and thermal properties the same. Trouble is I have no way to verify it. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted July 31 Author Share Posted July 31 Sounds neat - I can see that working as heat doesn't know or care whether it's going up or down or side-to-side . A couple of related threads here: https://aecb.net/forums/topic/psi-therm-software/ https://aecb.net/forums/topic/psi-therm-software/ The Therm video is fun - reminds me that some in the world still use BTU / ft^2 / degree F ! Link to comment Share on other sites More sharing options...
Alan Ambrose Posted July 31 Author Share Posted July 31 Ah the second thread should be: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11107&page=1#Item_0 FYI I'm trying a simple case with Autodesk Fusion to see how/if that works. Link to comment Share on other sites More sharing options...
Duncan62 Posted July 31 Share Posted July 31 ThermInstructions.pdf 1 Link to comment Share on other sites More sharing options...
Dan F Posted July 31 Share Posted July 31 There a few commerical products that make them easier to calculate, but none are exactly cheap. If what you need to do is farily limited though, you could potential do them within a trial period. The only low cost (free) approach I know is Therm though, but it takes some time to learn and you need to ensure you use the correct boundary materials and temperatures. Then on top of that you need to caculate thermal bridge psi value seperatly. There is some guideance here: https://peterwarm.co.uk/resources#therm The altnerative approach is to get hold of a book of standard details and use the psi values for these if you are not doing anything really complex/different. Using values taken from standard details is AFAIU acceptable for PH certification if you go down that route, and it's not a requirement to calulate every single bridge. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted July 31 Author Share Posted July 31 Thanks @Duncan62 I feel I'm making some progress but it seems a hard nut to crack. It appears that most paid-for FEA software is actually a nice user interface sitting on a public domain back end e.g. Nastran. All quite expensive considering it's 'required' for SAP, PHPP etc. As @Dan F says, standard details are available if that's what you have. Link to comment Share on other sites More sharing options...
SteamyTea Posted July 31 Share Posted July 31 OpenFoam is the big free CFD package. Not used it but a mate of mine was a heavy user of it and thought it was brilliant. www.openfoam.com Link to comment Share on other sites More sharing options...
Duncan62 Posted August 1 Share Posted August 1 (edited) Junctions picture showing all the TBs that need consideration: Appendix B had a list of all the TBs required: S10TP-05 - Treatment of thermal bridges - V1_2.pdf Corners are quite straight forward: Windows are more involved. The frame conductivity needs to be calculated and input as a custom material for the frame in the materials library. If the head/jamb/sill have different UValues then you'll need to drop it for each. Ground slab calculations are a two step process. I think they are explained in the previous ThermInstructions.pdf I uploaded above. Edited August 1 by Duncan62 Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 1 Author Share Posted August 1 Thanks for that, and yes, those instructions are well written. Fusion makes some pretty temperature plots (in 3D) (see below) but doesn't do the last step of calculating total heat flow. Do you have an idea of how well the standard details match real cases? Presumably if they work for standard modern estate builds they're a 'bit high' for better insulated designs. I see from the doc that the BRE have doubled some of the numbers between SAP 2012 & 10. They say part of the rational is to encourage people to do their own calcs. Well great if that was easy. Re: windows I found that e.g. Rationel publishes U-Values and psi-values. I assume other suppliers do the same although I have not looked. How reliable are they - anyone's guess. Link to comment Share on other sites More sharing options...
Dan F Posted August 1 Share Posted August 1 (edited) @Alan Ambrose Remeber that the SAP and PHPP calculations are different. One uses internal measurements and one uses external, also the boundary conditions and/or external temps might be different (can't remember). I've seen defaults of 0.040 used as slightly conservative a number for(presumably good spec and well installed) windows and 0.1 used for thresholds. Great installation and this could be less though, and if you have a blind system or poor insulation this could be more, but I think those are good starting points. If rational publish psi values then that will be based on assumptions about the installation situation and fabric make-up, so may not be accurate or valid. Also need to check if rational numbers use SAP or PHPP methodology. Window/doors installation of course aren't the only thermal bridges, you have all the building juntions too. 🙂 Edited August 1 by Dan F Link to comment Share on other sites More sharing options...
Duncan62 Posted August 2 Share Posted August 2 18 hours ago, Dan F said: One uses internal measurements and one uses external, also the boundary conditions and/or external temps might be different (can't remember). Yes this is correct. For example the corner detail: PHPP uses the external wall length. SAP uses the shorter internal dimension. Boundary conditions and Rse values differ. From memory the external conditions are 0degrees and 0.04Rse. i.e. there is no allowance given for ventilated facades. This is from memory so if anyone has a reference, please share. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 2 Author Share Posted August 2 Yeah, I was sort of aware of that (I don't know much about SAP). Seems to me that PHPP is a bit more sensible in that, if you don't put in any bridges, it probably overestimates the straight U-Value heat loss and partially compensates for the missing bridges. Have installed Therm, so will give it a go. Link to comment Share on other sites More sharing options...
Duncan62 Posted August 2 Share Posted August 2 (edited) You can create a DXF for your structure that you'd like to model in Therm, then trace it in Therm with the auto snap feature in Therm 7.8. That makes it easy. Look for "underlay" or similar in the menu Edited August 2 by Duncan62 Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 2 Author Share Posted August 2 Ah, nice. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted August 6 Author Share Posted August 6 OK some progress. Now I need to check all my inputs and back out the psi thermal bridge value... Also had to trace my dxf file for now (rather than getting a direct import). Couple of hours work so far. Link to comment Share on other sites More sharing options...
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