Ive been using an online SAP calculation tool provided by Knauf insulation.

http://tools.knaufinsulation.co.uk/

Its pretty good bit of kit and will populate unknown elements with reasonable assumptions based on averages.

Based on my own research, calculations and systems specifications Ive populated the model with all the design details for a barn conversion Im undertaking. I havent been over-optimistic on U-values (c. 2.0 ish for windows and 0.15-0.2 for walls (340mm brick, 100m Rockwoll, 100mm Durox, 13mm Plasterboard), floors (Screed, 100mm Insulation, concrete slab) and roof (300mm+ Rockwool + Celotex)) but the SAP rating remains stubbornly low at 66 (D).

The environmental impact rating is high (95 A) due to heating system specifications which is nice though.

If anyone here knows how SAPs are actually calculated, can they point me in the direction of which part of the SAP analysis has the greatest contribution to the score? Is it U-values? Window areas? Air circulation? etc

and is the SAP rating proportionate to area and volume or is it lower for bigger buildigs simply by virtue of size?

Also can anyone direct me to an online source of R-Values for the building materials I mention above so that I can double check my U-value calcs?

Cheers

SAP

this might help.

Cheers mrmo but that's not opening for me (might be because Im behind a corporate firewall?) Ill check it out at home.

Its worth noting that my notional DER is around 6 compared with a TER of 20 which seem great. Still cant work out why my SAP is so low though…

Some R-values for insulation and plaster boards here:

http://www.just-insulation.com/index.html
http://www.bbs-ltd.com/drylining_fire.htm

Have also emailed you the relevant page out of this:

geoff – you're a legend, thanks.

Right, if it's a barn I'm assuming it's got 4 exposed walls compared to a flat which in some cases might only have 1 exposed wall. This means the volume can be losing heat from everywhere compared to the flat. I guess it also has 4 unsheltered sides (again something that affects the calc).

Also, your window u-values aren't spectacular – we're looking at 1.2/0.8 U-values for some of our current projects (0.8 is triple glazing). Bringing them down to 1.8 might help for you.

Air permeability also makes a difference – you can put in 15 as a default if you don't want to pay for a pressure test. This will give you a bad result though. If you pay for a pressure test, you'll probably be able to get the building to around 7-8 which will help the result. We are instructing clients to build to 3 for CfSH level 3.

Your thermal bridging Y-value I'm guessing will be at 0.15. Reduce this to 0.08 or 0.04 if possible. The BRE produce a guide on how to achieve this.

Amy – that's very useful info thanks.

Ive been using the default inputs for things I dont know (Such as thermal bridging and Air permeability) so there may be some scope for improvement given the lack of real thermal bridging and opportunity to manage air permeability in the design.

Cheers

You might find that "generic" manufacturer calculations might not really suit your specific case and in order to get a more useable result a consultant might make your life easier, as well as helping you to choose appropriate materials/ solutions. We don't do SAP calcs inhouse but use consultants instead.

I would only bother with a consultant if a) I was planning to sell and was getting bother for not being higher than grade D on SAP (It's A on environmental impact) or b) I really wasnt able to do the rest of the calcs myself to get a more accurate result. Over the next year I will probably study to improve my own calc abilities for things like thermal bridging. Pressure testing will prob require a consultant though.

i can never get my head round pressure testing. I can't remember who, might have been Saint Gobain, but the new factory was built with huge vents to let excess heat out. come pressure test they had to block up the holes?

These are holes that are meant to be there? but they might let air out, yes, that is the F***ing point!

Stoner I used Elmhirst Energy proformas and software for doing SAP audits, and from what I gather, it is the primary heating source and external wall construction that have the greatest impact on the outcome. It is however not an exact science, but that is my observation based on carrying out audits on multiple properties, inc for example a pair of semi detached houses, one with an old back boiler, one with condensing combi, with SAP ratings at opposite ends of the scale!

Stoner, I am in work tomorrow and will try to get some help for you from there.

bh – cheers – I'll try to post latest photos, had a good day on site today 🙂

M-M_T_M: that's what's so daft, the heating source is perfect, but the SAP is still low, mainly I think because we are converting an existing building so are bound to some traditional construction techniques even though we're hitting (just) the U-value targets for 2008+