lizzie

Thermal bridge calcs for as built epc sap Help please!

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Posted (edited)

Can anyone give me some pointers on thermal bridging at junctions calcs for an as built epc/sap. I started a few days ago from a position of complete ignorance so please bear with me, its a steep learning curve.

 

The house is ready and I have commissioned the epc/sap document.....that is where the problems start. Many many hours on the internet have brought me to a very worried state. I understand that if default values are used for thermal bridge junctions it can be ‘onerous’ and have the result that the value of our highly insulated walls and roof are halved by the calculation for thermal bridging at junctions being innacurate.

 

We had a building regs submission done by architects that included a specification and sap. I only saw the submission a few days ago for the first time.  Our specification was not actually for our house as it was early on and we can get over that by putting in actual items that we have installed. So far so good. The SAP calculation I have no detail. The correct drawings were sent to BC.

 

I know now we have to start again moving from as designed to as built and that is where our problems lie. I had no idea on sap or thermal bridging.

 

I have an MBC house. I have established that MBC do not produce ACD for their frames that is for us to do, they have given us the u values of materials they used so we are OK on u values for slab, frame and roof and  I have window list and thanks to some help from @JSHarris I am sorted on that side of things.

 

It is with the thermal bridge detail that the problems lie. We are able to get a junction value for the easy ones...wall/floor and wall/roof from the information from MBC. and do a drawing to back this up. What I am then left with is all the other junctions, corners, windows etc.  I have a lot of windows and I have an awful lot of corners, the building is not square it has lots of ins an outs. If as I understand it those junctions are added up at the default value and applied against my u value I will have a very expensive thermally efficient house with a lower epc rating than a mass market builder cardboard house. If that is the case I am not at all happy that our ignorance in accurately calculating  technical details of our build will leave us in this position. Hopefully we will get some idea back from assessor later today, if its half decent then I will go with that but if it is marked down then that will need to be addressed.

 

Assuming that my interpretation of the way it works is correct (and please tell me if I am wrong!) if I need to get accurate sap next step is to get paperwork done for junctions to be signed off ‘as built’ to go to the assessor to do the report.  I want to get this epc finalised for submission to BC so they can come and sign us off but I am not going to put in a totally incorrect sap and compromise my rating for the sake of speed. Its taken 2 years to get here another few days will not kill us. We will end up moving in next week without sign off as it is but I do want it done asap. It has a knock on effect on all sorts of things being in the hiatus of house built and occupied but not signed off by BC.

 

How do I go about getting someone to do all these junction calculations for me if I have to have them.  Does anyone know if there is anywhere on line I can go to and get it turned round quickly if needed?

 

Any help at all very much appreciated.

Edited by lizzie

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What did your design SAP specify?  Normally they will say that you build to standard ACDs, then, before doing the final SAP and EPC the assessor will ask you to indicate which details you used.

 

http://webarchive.nationalarchives.gov.uk/20141202102454/http://www.planningportal.gov.uk/uploads/br/wood_frame_illustrations.pdf

 

Using the ACDs allows you energy assessor to apply a value in a table for thermal bridging.  I would not over-complicate this as it is box ticking.

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I have no detail of the design SAP,  I did not see it, I will try to find out thank you.

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OK.  You should have had a SAP assessment carried out at the design stage and it would include details of the u values for all the elements of the house, as well as the heating system and airtightness target.  The cost is about £100 - £150.  They will also produce a Predicted Energy Assessment meaning that if you build as per the SAP your building will comply with part L.

 

Don't sweat on the thermal bridges, just fill in the checklist from the link I posted above that most closely resembles what you have.

 

 

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Thank you I have found out what the problem was. My as designed sap was not my house, I guess mistakes happen and I didnt see the report to BC until a few days ago when I saw spec was wrong. Never seen sap but they have got to the bottom of it now.

Think I am finally there after a tortuous few days.  Looking like a B85 because I dont  have any renewables.   I will settle for that I am completely exhausted by the whole thing.

 

Thank you for your help, that link was really useful.

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Extremely difficult to get an A without PV.  What did your air test score?

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Air test was very good .6

 

Yes the whole thing is skewed in favour of renewables, a nonsense really.

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22 minutes ago, lizzie said:

Air test was very good .6

 

Yes the whole thing is skewed in favour of renewables, a nonsense really.

 

Agree ..!! We have a predicted B for standard construction with higher spec insulation, windows and air tightness. To move that up to A, we need 2 PV panels ..!! Doesn’t matter that they give naff all cost benefit, it gets the extra points on SAP..! 

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1 hour ago, Mr Punter said:

Extremely difficult to get an A without PV.  What did your air test score?

We managed an A95 without renewables and it would be A102 with 3.2kWp PV.

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20 minutes ago, PeterStarck said:

We managed an A95 without renewables and it would be A102 with 3.2kWp PV.

I think if I had all my junctions done properly for thermal bridging I would have been a bit higher, not sure how much, they said cost and time not worth it just take what is being offered so I did.  I  have lost the will to live with it, it has been a tortuous week trying to sort it all out. I just wanted an end, the stress of it all has made me ill.

 

If doing again I would make sure I saw the sap info at design stage and that it was right and related to my build then at least when you get to 'as built' it is just a simple exercise not the marathon of confusion I have waded through to get an end and a certificate.

 

There are lessons for people in my tale.  

 

You did brilliantly well @PeterStarck you should be very proud of your achievement.  I am disappointed with the B, I had hopes of better after all the effort with the build but I am relieved to get an end that is passable after everything the last few days.

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If it makes you feel any better, Lizzie, I had an as-designed SAP and first time around it came back as B84 and failed! This is in spite of having a whole bunch of PV and a sunamp to store the excess energy. The guy didn't seem to know or care too much so I asked him to do the figures again, this time with a thermal store. They also included a bunch of cold bridges as standard.

The second lot of figures came back as a B86 but, more importantly, a pass. I could argue the toss and try and get them improved but i don't care at this stage. I'm not sure that I will be too bothered at the as-built stage, either, TBH, as I don't think it will make any difference to anything for me.

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Not sure if I should be worried or not.  

I didn't have an 'as designed' SAP.  

Building control accepted the plans and no one has ever mentioned SAP

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@Hecateh you will need SAP for your ‘as built’ EPC you cannot get building control sign off without it.  Its easy to fail even if you are not bothered about your rating. You can get your BC stuff in without a sap at the start but you would be wise to be thinking about it and making sure your builder is able to provide any required drawing of what has been done for SAP. ACD they are called. They are not your construction drawings but are relevant to them. You might be as well to get some advice from someone who can run you a Predicted Energy Assessment so at least you have some idea what you will need at the end especially if you might part company with your builder. Someone needs to sign things off for the ACD it could be you as the ‘builder’ but it would be wise if you had some understanding. I was completely ignorant left it to the professionals and somehow errors were made and no one picked it up. I did not even see my BC and Sap submission until a few days  ago and it went in more than a year ago.  I never thought to ask to look at it.  Everyone makes mistakes and thats life it wasnt intentional  what was important is that thankfully this was retreivable without too many consequences beyond a lot of stress.

 

My problem was the ‘ as designed’ SAP that went in to building control was for a completely different house...I am single storey with an angled membrane roof what went in was for two storey house with a metal roof..... when we started to do the ‘as built’ for the completed house it was coming back as nonsense and also failing.....I couldnt understand how the assessor was getting the detail of the house so wrong, it wasnt just different boiler, the glazing didnt match etc. It took a few days to establish what had happened then we had to start from scratch. I was so worried in case the drawings that had gone in with the initial BC submission were for a different house too. Thankfully they were at least our drawings but its taken some untangling and I am massively relieved, the thought of having built a house without any BC approved drawings was very stressful.

 

 

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I've just spent the last hour working on the thermal bridge data in my SAP report. Using the default y value (0.15) gives me a B(87) SAP result, with all bridge lengths calculated and using the psi-values for Accredited Details brings things up to a B(90). Worth having, but I'll still need a kW or so of PV to reach an "A" rating.

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13 hours ago, Hecateh said:

Not sure if I should be worried or not.  

I didn't have an 'as designed' SAP.  

Building control accepted the plans and no one has ever mentioned SAP

I didn't have an 'as designed SAP' either. I had run my design through PHPP and the BCO accepted that instead of the SAP calculations.

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Re linear thermal bridges.

 

The difference between the default y-value (0.15) and the calculated value from a more detailed analysis using ACDs may only about 3 SAP points. However it will make a massive difference in CO2 emissions (DER) and the fabric energy efficiency (DFEE). Probably enough to result in an overall Building Reg failure on these 2 criteria.

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11 hours ago, ADLIan said:

Re linear thermal bridges.

 

The difference between the default y-value (0.15) and the calculated value from a more detailed analysis using ACDs may only about 3 SAP points. However it will make a massive difference in CO2 emissions (DER) and the fabric energy efficiency (DFEE). Probably enough to result in an overall Building Reg failure on these 2 criteria.

 

Good point - TER for the build is 28.4, the default values give a DER pass but the ACDs give a good improvement in this and the DFEE. As this is SAP2009 there is no TFEE

 

y=0.15 (default)

- SAP = 87

- DER = 22.97

- DFEE = 48.84

 

y=0.057 (Thermal bridges ACD)

- SAP = 90

- DER = 17.1

- DFEE = 38.23

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