SAP Process, Air Tightness Test etc

[benben5555]

Hi clever people,

 

We are finishing the design of our self build house and about to submit planning. We have deposited a building notice with the local authority so we will be working to the existing Part L rather than the 2022 version.

 

Can anyone provide a 'nutshell' summary of the SAP and air tightness process. Do we send the drawings to a SAP assessor and they give us a yes or no answer? What then about air tightness (we are using timber frame), once the build is complete do we just arrange for the test and again it is a yes or no? Are there any other parts to this process I'm missing?

 

Our design utilises 140mm mineral wool batts in the wall frame and 50mm PIR internally, 450mm mineral wool in the loft and 100mm PIR in the floor. ASHP, 26% of the elevations are glazing.

 

Thanks

[JohnMo]

Two stages.

 

Before build, done to your drawings and your airtightness prediction.  If above 15 (I think) no airtight test required.

 

You build the house, have an airtight test complete, if required  Any changes to spec are advised to EPC person, you or the builder sign a form that the details are accurate, send back to EPC person.

 

They issue as built EPC.

[Iceverge]

I'd move to 300mm EPS floor installation. 

 

Also @Thorfun used a similar buildup to you recently in the walls. Maybe their comment is worth considering before you commit. 

 

https://forum.buildhub.org.uk/topic/27181-tf-house-about-to-start-and-last-minute-pondering-insulation/?do=findComment&comment=412180

Edited May 25, 2022 by Iceverge

[nod]

It’s quite simple Sends your drawings to the sap accessor 

and will produce a sap report for planners 

Once you have finished there is an as built sap  report Total nonsense as they rely on what you are telling them to be true 

[benben5555]

2 hours ago, JohnMo said:

your airtightness prediction

I've not heard this before. Who does this and does it need to be submitted to building control?

[Conor]

4 hours ago, benben5555 said:

I've not heard this before. Who does this and does it need to be submitted to building control?

The SAP assessor will assume a value (max allowable by the regs) unless you specify otherwise. We had a design assumption of 3, and achieved 1.4.

[JohnMo]

It will also alter the way you are allowed to ventilate the property.

 

Better than 3 drives you are driven to MVHR, 15 a simple extractor in wet rooms and kitchen or even passive stack ventilation.  Different airtightness and ventilation affects your sap EPC score.

 

So if you predict 5 and get a 2.5 in Scotland, you would have to retrofit MVHR or a similar balanced system.

 

Also if you choose 15 you will have to make sure you meet or exceed the trial house within the sap EPC report, with extra insulation and renewables etc.... otherwise you could fail the EPC criteria.

 

It all fun and one said it was easy, the more you ask the bigger the learning curve.  You architect should be steering and helping on this.

[ProDave]

Our design SAP was done with an assumed air change of 4.  MVHR was always planned.  The actual air test result came out at 1.4, which I was disappointed with, but the tester nearly wet himself with excitement as it was the best he had personally tested so far.  Then when all the figures were sent back to she SAP assessor, he issued the as built SAP with an improved figure over the design SAP.

 

In Scotland at least it appears an air test is a requirement, and I know of one builder nearby who's air test came out better than 3 and he was forced to retro fit mvhr.  So plan for MVHR from the outset, or get the air test done before the plasterboard goes on.