ADLIan

Offence taken. Bear in mind that SAP is not a detailed design tool. It is simply the Govt's preferred method of showing compliance with the Building Regs by assessing energy use under standardised conditions.

As another energy assessor (among other things) +1 to Clive Chitty. SAPs are now considerably more complex/time consuming if a correct assessment of the linear thermal bridges is done.

See also individual manufacturers BBA certs - these will give more details of type of construction and max exposure limits. @JSHarris - I am aware of the procedures for injected CWI having developed blown in cavity products, taken them through the approval process and approved contractor status.

Problem with blown insualtion is that any defects (voids, density variations, clumping) are hidden - the method for 'guestimating' the adequacy of fill is woefully innacurate and would not identify any of the above. With retrofit in old walls I take the view that anything is better than nothing so the odd void may not be an issue. In new build this is not the case - at least with built in cavity batts you can see any issues/problems and correct them.

Ask for the manufacturers declaration of performance (DoP) - this should state the product meets all necessary requirments/standards, identify the products even down to serial number and give the manufacturer details, name and address. I suppose even these can be faked but if you have a problems getting the DoP then alarm bells should ring. I believe Trading Standards are the policemen for CE Marking - best of luck taking action against any naughty importers/manufacturers via this route.

Re Andi & Fergie house. I worked for the manufacturer of the cavity wall insulation (pink colour) at the time this was built. Nothing special as I remember just basic Buildng Regs for the time. Ian

Perhaps the solid timber here is fire fire reasons - internal fire breaks thru window and could then travel within hidden cavity?? ian

I've not seen JSH detail but noting his attention to detail I'm sure it'll work! My concern is with the diag you show at the top which fails to show the vent required to this airspace - I find this a little misleading from the manuf. ian

Both knauf render panel requires drained/vent airspace behind. See manuf instructions and BBA certificate. They omit to say how to deal with this detail above window however and diag is misleading. As JSH says TRADA will have more details (I don't have these to hand however). ian

+1 on mech fixings & fire issue when using D&D/foam. See plasterboard manufacturers instructions.

While since I worked in pur foam industry but I'm sure the gases generated by HW cutting are pretty nasty. Perhaps ok if well vented room and on a small scale.

Pur/pir foams are NOT classed as non combustible.

Good point! Phone call or email to your insulation manufacturer of choice tomorrow outlining your proposed wall and you'll get the number crunching done in accordance with the BS and BRE 443 which is all you need to keep your BCO (and SAP assessor) happy. All of the mainstream insulation manufacturers will provide accurate calculations along with detailed breakdown of the numbers used. Ian

I've been out for a couple of beers and come back to a bit of spat!!! Back to the top of the page. BS EN (DIN) 6946 gives method for calculating U-values for walls, roofs etc and thermal resistance of air spaces/cavities. For U-values it includes the effect of repeating thermal bridges (timber studs in TF construction, mortar beds in aircrete blocks etc), mech fixings penetrating the insulation layer (wall ties in masonry, fixings through insulation in rendered insulated systems, flat roofs etc) and air gaps in the insulation layer. BR 443 from the BRE gives guidance and how the Standard applies to our Building Regs/Standards. The thermal (not cold) bridge is accounted for but can be mitigated if there is an insulation layer internal or external to the bridged layer. As for ProDave's calculation linked to above - it's wrong. Mainstream UK insulation manufacturers would give accurate numbers (multi foils excepted!) Ian

I was being critical of the company doing the calculation. They mention EN ISO (DIN) 6946 which includes accounting for repeating thermal bridges but then totally ignore the effect. A layer of insulation internal or external to the studs mitigates the effect but the UValue will be higher than that shown - not at my PC at the moment so can't check but would still guess nearer 0.15 than 0.13W/m2K.

@ProDave Is this wall 'standard' timber frame? - the calculation does not appear to account for the thermal bridging of the timber studs. The default is 15% bridging and this would increase U-Val nearer to 0.15 W/m2K. Also there is a layer of '25mm spruce' - seems strange to have an inch of solid timber at this point! Should it be 25mm air space & batten.

Polyurethane & polystyrene foams have no acoustic properties. Must be mineral wool, glas or rock.

Have a chat with the insulation manufacturer on Monday morning. Their technical services will do the number crunching for you and send a nice pdf print out that BCO will accept. Ian

You could try CIGA the cavity wall insualtion guarantee people, www.ciga.co.uk.. If regisitered they will have details of the job and any guarantee. Heard mixed reports of how they handle compliants!! Ian

Thanks Crofter Ian

Interesting discussion guys Can someone (ProDave?) point me to the reference exempting portable buildings upto 100m2 from building regs/building warrant? Thanks in advance.

@Barney12 Lots of information missing from the linked spreadsheet, it barely scratches the surface of the required input data!! An accredited asssessor should not accept the input data from any 3rd parrty - not even another accreedited assessor. Any errors are then yours and continued accreditation may be at risk. Personaly I value my professional reputation, status and accreditation too much to risk this course of action. A good asssessor will not just do the number crunching but will also offer independant advice and guide you through the myriad of options available under SAP. I use NHER Plan Assessor and find it a great bit of software. Elmhurst is good too just takes ttime getting used to sometihng different. Ian

Difficult to say if the numbers are making sense without knowing the raw input data in detail. Unless you are very good with Excel and understand the complexities of SAP in great depth I wouldn't bother compiling my own spreadsheet!! Also remember that SAP software should be BRE approved - this ensures all software from all suppliers is churning out the same results. Without some form of checking against recognised sofware your spreadsheet may be giving meaningless numbers. Software, even free trial versions, from the major approval bodies should give a warning if wrong info is used - if you are getting erroneous figures either the software is not up to scratch of you need to study the SAP document in more detail - insomnia cured!! Ian

If its saturated I would suggest replacing it. Perhaps best if you contact Rockwool for their comments. Ian

Hi Info on SAP (new build houses) and rdSAP (existing houses) is available from the BRE website - full SAP/rdSAP guide and conventions for both schemes can be downloaded. Re SAP ratings - this is based on fuel cost so choice of fuel will have a big impact as will space/water heating system efficiency. Fuel costs are given in Table 12 of SAP 2012. Ian