Logic behind U Values.. New Build: New Element.

[champagnecharly]

Can anybody offer any logic behind why New build U values requirements appear less stringent than those in for new elements in existing?
Is it the assumption that doing so will increase the U value of overall property if other thermal elements are upgraded? If so is there/ can there be a trade off? It seems daft that a completely renovated property with new roof would have a better u value that that of a new build sticking to min regs.

simple table for comparison here..

http://www.energy-saving-experts.com/wp-content/uploads/2011/01/U-Value-fact-sheet.pdf

Also curious as to required U value for intermediate floors. Regs state floor. Does it really mean it require the same insulation between floor and suspended as between first and floor also? Or would some 100mm of R45 or similar suffice?

 

[jeds]

The thermal performance of new housing is designed on a whole house basis. Because you are starting from scratch you have a lot of scope in how that performance is met and in practice you will find the U-values of walls will often be significantly more than the minimum requirement. 0.24 isn't unusual. When adding to existing buildings you don't usually have that much scope so the minimum is set slightly higher to push a better performance.

PS. Intermediate floors aren't normally insulated for thermal. But they are insulated for sound which does provide some degree of thermal as well. I say normally because there is an option to calculate a SAP based on individual units rather than an overall but it is rarely done this way.

 

[champagnecharly]

Thank you. That makes sense. More sense than what BC stated today who said the values stated in tables need need to be improved on by 20% anyway. and some sort of 'PAS' test has to be done. To be honest the conversation was so hurried that I didn't quite grasp the point and had to rush to the next question.

I'm curious if 'all' the thermal elements updated (property is empty shell). Is it permissible to use the values given for a new property across the board and if so what test and 20% would BC have been referring to?

 

[jeds]

Never heard of a pas test. Did he mean SAP test? The new regulations will change the way we deal with major renovation or where 50% of the thermal elements are upgraded. I haven't read the new documents in detail but I think I'm right in saying that thermal performance will be calculated as if it were new build. There's always the over-ride with existing though that this applies only to the extent that it is technically, functionally and economically feasible.

 

[champagnecharly]

Well at least I got the letters right even if not the order.

It may be that going this route would be more economical and more logical. However BC disagreed that the U values were more relaxed for new build on the basis of a 20% improvement upon given figures? What could he have meant by that?
I do note that there are 2 tables listing U values in L1A
Table 1 Bottom of page 23.
Table 2 Page 15. (limiting fabric parameters)
Which should be used?

 

[jeds]

For new build the overall must be calculated by SAP and table 2 (of L1a)gives the minimums for any element. So table 2 for new build - but remember they are minimums and in practice are likely to be much tougher.

New elements of extensions to existing can be done under elemental values quoted in table 2 of L1b.

Retained (upgraded) elements of existing should de done under table 3 of L1b.

 

[champagnecharly]

Thanks.

What does table in appendix a refer to then? page 23 for? These values again are very different.

Have downloaded a copy of fsap and am slowly going through the required entries.

When you say likely to be much tougher. What do you mean. BC can Impose over the minimum?

 

[jeds]

That table is just an example of how you might present the actual values used in a SAP calc to building control. i.e. so they can easily look up what values have been input to the software.

When I say much tougher; when designing and calculating a sap you usually find the first run fails. In fact it can fail time after time and you have to juggle and adjust values to make it comply. One option it to improve the u-value of an element. So you just keep increasing a u-value until is passes. Or more likely increasing several u-values. So the design you end up with often has much tougher u-values than the minimum requirement.

 

[champagnecharly]

So the table shows the minimum accepted for a singular element as opposed to the minimum being practically applicable to each element. Which would in tern explain why BC said expect a 20% increase on these values. All very much like the 0.45x0.45 for egress. as mins they would never work to achieve the 0.33.
Crystal clear now. Thank you very much for the explanation.

Is it worth my while continuing to punch numbers into FSAP or should I call it a day and be thankful of L1b tables?

 

[champagnecharly]

but remember they are minimums and in practice are likely to be much tougher.
.

So i've continued to play around with FSAP.. and you're not wrong with what you said! Getting the DER level down anywhere near TER is proving rather tricky

 

[tony1851]

When I say much tougher; when designing and calculating a sap you usually find the first run fails. In fact it can fail time after time and you have to juggle and adjust values to make it comply.

Exactly.

This is one of the chief problems with our thermal insulation legislation. It is now so complex for new-build that it can only be done on spreadsheets and - increasingly - only by closed-shop 'experts' in carbon emissions, SBEM, CfSH, BREEAM and all the other environmental codswallop.

If all the money spent on specialists was spent on better insulation of elements instead, it might make energy-efficient buildings cheaper.

 

[jeds]

Have downloaded a copy of fsap

As a matter of interest, where from. And how are you getting on with it?

 

[champagnecharly]

I downloaded from: http://www.stroma.com/certification/software/sap-software-fsap

How am i getting on? well. its hard without precise values for 'everything' and the default SAP don't tend to pass. Other than that and the creation of bespoke surfaces (for which I far prefer build desk on grounds of combination of material i.e 80 Insulation : 20 Timber for insulating between joists). However as a whole its great. The ability to tweak various components makes it simpler to see how conformity is attained and or why it fails. However I get the feeling a spreadsheet with check boxes would help basic basic compliance for thoese who hadnt done it before.
In general I'd rate the app 9/10 although creation of new elements is very slow.
I do find it ironic that a passive house can be created (not my case) but conformity is not reached without alternative energy sources/ solar panels.

What you said with regards to calculating SAP on an individual room basis.. is interesting . I would like to read cases/ guidelines on where the same is possible and what the limitations are.

 

[champagnecharly]

One thing that is not so easy is the air permeability. I cant grasp how to set specifications to achieve this.

 

[jeds]

When I said calculating sap on an individual basis I meant individual flats not individual rooms. You can do a sap of a whole building or you can do each flat individually.