Thermal bridging

[Paddocks532]

Hi all

Just when we thought we could crack on😥

Our sap calculation man has told us we've failed. He thinks🤔 because of possible cold bridging. He says he can't do the design for us we need to pay a professional designer😭 possibly costing £2000. Is this right. How can we overcome this quickly and do we really have to pay someone more money in the design stage. We've already been through the whole nutrient neutrality and buying credits etc which held us up for 3 years on top of all the normal stuff 😭

[Russell griffiths]

I don’t know what build method you are proposing, but if it’s a new house and you have failed the sap then you need to go back to the beginning and start again because something is very wrong. 
 

you either have an architect who thinks it’s 1960 or you have chosen a poor build method, 

 

How are you thinking of building this thing. 
 

please don’t say brick n block with 100mm cavity and 75mm mineral wool insulation. 
 

we all started somewhere, c’ mon show us what you got. 

[Paddocks532]

Hiya

Thanks Russell

No we're timber frame. Just about to put 220mm insulation + 75 ml edge insulation into our base over the next few days then dpm and ufh pipes then 100ml concrete slab in 4 weeks time. We will have a brick skin up to window height then black stained cladding on the top half, we are a single storey dwelling. We have lots of insulation everywhere😁💰+ mvhr, ashp, solar and a battery. One thing we haven't decided on is windows but looking at our budget already it will have to be a middle of the road to lower end price point. We have a lot of glazing but will be having shading on the south and west side over windows. I have to go to work but will put drawings on this evening.

[IanR]

From your January post you were up to ground level, so can we assume you now have your timber frame up?
 

Do you really have an issue or have you just not supplied sufficient info for thermal bridges to be calculated so worst case assumptions have been used.

Can you post a labelled section through the offending thermal bridges. Did an Architect do the overall design?

Added: re-reading your OP, is it possible he doesn't know how to calculate the psi values (hence you'll need a professional) and has just realised that the new SAP rules for cold bridging make using the default assumptions an almost definite fail.

Edited yesterday at 07:17 by IanR

[Paddocks532]

Hi Ian

Oh how I wished we were that far. 🫣 It took a long time to nail down a company to do our ashp, ufh, mvhr and solar package. So we're nearly ready to put in our insulation (which we've just had delivered this morning) prior to our concrete pour in about 4 weeks time. Probably the fact we haven't given sufficient info🫨. Yes an architect done our drawings. But can't remember seeing detailed drawings for these junctions. 

[IanR]

8 minutes ago, Paddocks532 said:

Probably the fact we haven't given sufficient info🫨. Yes an architect done our drawings. But can't remember seeing detailed drawings for these junctions. 

To offer help here, we're going to need the junction sections, but if you didn't supply them to who was doing your SAP calcs, you probably don't need our help, you need your Architect to produce the construction drawings including junction sections.

With regards your original question, do you need a professional "designer" costing around £2K. If that's just for the thermal bridging calcs, that's probably a bit high unless you have lot's of different, unique details in different areas of the build (which you drawings suggest you don't). But if the £2K is covering creating the Junction sections, and possible construction drawings, as well as doing the psi calcs, that's very cheap.

Edited yesterday at 07:41 by IanR

[ADLIan]

I don't understand how this is suddenly an issue now the work is underway as this should have been sorted as part of the design stage SAP calculation and your Building Reg application. Has the SAP assessor taken the lazy option and used the default (worst case) thermal bridge numbers? Most insulation, block & lintel manufacturers have their own sets of psi-values that can be used and these will be a lot better than the default values. The TF manufacturer/designer should also have bespoke psi-values. Producing bespoke psi-values is very time consuming - if there are a lot of junctions to assess costs could easily be £2k plus!

[Russell griffiths]

If you have lots of glazing I would not be looking at the lower end price wise. 

[Redbeard]

3 hours ago, Paddocks532 said:

One thing we haven't decided on is windows but looking at our budget already it will have to be a middle of the road to lower end price point. We have a lot of glazing but will be having shading on the south and west side over windows. I have to go to work but will put drawings on this evening.

 

I am sorry you are having this faff. As others have said, the dwgs may enlighten us a bit. Lots of glazing = lots of potential thermal bridges at the structural opening, which could be mitigated well or badly. There may of course be plenty of other reasons for the technical 'failure', but the openings would probably reward good scrutiny. 

 

I have not 'done SAP' for some years but surely your 'calc man' can offer some more insight. 'Because of possible cold bridging' is a pretty wide field. He's done the input. What are his suggestions for mitigation?

[JohnMo]

4 hours ago, Paddocks532 said:

One thing we haven't decided on is windows but looking at our budget already it will have to be a middle of the road to lower end price point. We have a lot of glazing

What is the assumed U value for your windows and doors? Lots of glazing is different things to different people and can make a huge difference to the heat loss.

 

I would go through the SAP report line for line to see what assumptions have been made, sounds shite in shite out to me somewhere. 

 

Post your report 

[Paddocks532]

We aren't using a timber frame company. My husband is a carpenter and will be making the timber frame himself, mostly onsite. He was a contracts manager for big builds. But never got involved until after the design stage, when he got handed the drawings and instructed to build the building in question. Which was mostly commercial properties. He stopped working for them 3 years ago and went self employed doing small carpentry jobs in the building game so is a little unfamiliar with new building regs.

We've only just given the sap assessor the spec of heating and hot water etc. so have only just found out that we have this issue. 

[saveasteading]

He says you've failed but doesnt know why?

He put the numbers in.

It doesn't sound like an expert, just someone with a licence for a sap spreadsheet.

I've come across one such. He wasnt interested in the reality and just wanted to enter conservative defaults.

 

Your construction should pass but tell more.

Insulation spec in floor and roof?

[Redbeard]

27 minutes ago, JohnMo said:

I would go through the SAP report line for line to see what assumptions have been made, sounds shite in shite out to me somewhere. 

 

Certainly sounds like full mitigation has not been implemented. If you (OP) know nothing of SAP you may need to try to find a good independent who can do the appraisal of the original SAP report. ( @saveasteading has posted as I write. Have not read his post yet). *Have now, and agree with it!

Edited 23 hours ago by Redbeard
later post

[Redbeard]

15 minutes ago, Paddocks532 said:

My husband is a carpenter and will be making the timber frame himself, mostly onsite.

 

Solid timber studs, or I-beam (which latter builds in a degree of thermal bridge mitigation)? BTW, if you have not 'done' this level of building before, treat with some caution claims that designs or execution have 'got rid of the thermal bridges'. Generally, while you can make them 'less worse' ('mitigation') you can't necessarily get rid of them without removing the element altogether.

[saveasteading]

2 hours ago, Redbeard said:

you can't necessarily get rid of them

Whatever we build, it is a thermal bridge to greater or lesser degree.

BH  can sort this

And carpentry skills will be of huge benefit to the final product even though the SAP doesn't acknowledge build quality.

[Redbeard]

29 minutes ago, saveasteading said:

BH  can sort this

 

Oh yes, sorry, never meant to suggest otherwise! 'We *got rid* of the thermal bridges' in project presentations is simply a pet hate of mine, as of course they didn't!

[saveasteading]

1 hour ago, Redbeard said:

'We *got rid* of the thermal bridges'

I  think some presenters have something to sell, but they can't fill 10 minutes with the facts so exaggerate. The less they understand it the more they exagerate.

 

It's as if those prescious but flighty heat particles were rushing through a piece of timber as if it was a big hole in the wall.

 

 

[SteamyTea]

If there really is a problem, can it be mitigated by extra insulation in other places?

[IanR]

19 hours ago, Redbeard said:

BTW, if you have not 'done' this level of building before, treat with some caution claims that designs or execution have 'got rid of the thermal bridges'. Generally, while you can make them 'less worse' ('mitigation') you can't necessarily get rid of them without removing the element altogether.

 

"Thermal bridge free" is a common term used in low energy home design, but its use relies on the person reading/listening understand what the term means. It just means that the losses at junctions have been reduced to a level that they have no effect on the overall structure's heat loss and can therefore be considered as a continuation of the floor/wall/roof U Value.

The PassivHaus threshold for "Thermal bridge free" is a PSi value of 0.01W/(mK). The PHPP thermal modelling package considers any thermal bridges with that Psi of 0.01W/(mK) or lower as not needing to be individually calculated into the building's overall heat loss. They obviously do still need calculating to show they are under 0.01W/(mK), but their heat loss does not then need to go forward to the total building heat loss calc.

[saveasteading]

To put it into real life context, in the Steading we left a significant proportion of 600mm original wall exposed internally. This was for aesthetics and a historical nod.

That would never pass Passivhaus which is an extreme target. I have no intention of sacrificing lots of money and aesthetics for the certificate that comes with a few % energy benefit.

But many do and it's their choice and money (never regained in £ payback).

 

We did get respect from the bco who agreed it was a good design feature, and we balanced the insulation by extra elsewhere.

 

You are building all new, so can more easily tweak the insulation. Should be easy enough esp with a proper analysis with expertise.

 

You say lots of insulation. What products?