A client came to us after receiving SAP calculations with an unusually heavy insulation specification: 800mm in the loft, 500mm in the floor and 300mm in the walls.
They wanted to know whether it was really necessary. We rebuilt the SAP assessment from scratch and reviewed the details behind the result. The revised specification passed all three SAP targets with substantially less insulation.
We estimate the revised insulation specification saved our client £11,792 in materials alone, excluding VAT.
The specification that prompted a second look
The client was concerned about the cost of the specified insulation and asked us to check whether the design could meet Part L with less.
We did not start by deciding how much insulation to remove. We started with the drawings, construction details and proposed systems, and built a fresh calculation.
What we looked at differently
The review focused on three areas:
- Thermal bridging: we paid close attention to the junction details and the values appropriate to the proposed construction.
- Airtightness: we reviewed the air permeability input alongside the ventilation specification.
- Product data: we used the proposed air source heat pump and MVHR product specifics rather than relying on generic system assumptions.
These inputs affect the assessment alongside the insulation. If you concentrate on adding insulation without reviewing the rest of the model, you can miss other reasons why the design is struggling.
This was a fresh review of the whole assessment, rather than changing one input and attributing the entire saving to it.
How much insulation did the revised design need?
| Element | Original specification | Revised specification |
|---|---|---|
| Timber frame walls | 300mm | 170mm |
| Loft/roof | 800mm | 270mm |
| Floor | 500mm | 150mm |
The revised assessment passed the emissions, primary energy and fabric energy efficiency targets. The reduction achieved a comfortable pass across all three metrics.
Every home is different. What worked here might need a little tweaking on your project. The insulation you choose, how the building fits together and the heating and ventilation systems all play a part in the result.
Why the whole SAP calculation matters
Better insulation is one way to improve performance. It is not a substitute for getting the rest of the assessment right.
A SAP model needs to represent the intended building and systems as accurately as the available information allows. Junctions, airtightness, ventilation and heating equipment deserve the same attention as the wall and roof U-values.
If an assessment leads to a specification that looks impractical or unexpectedly expensive, ask what is driving the result before ordering materials. There may be a good reason for it, but it is worth understanding that reason.
For this client, a second look produced a much more manageable insulation specification while still meeting all three targets.
Does your insulation specification look excessive?
Our new build SAP calculations include advice and reasonable revisions. If you already have an assessment and want a fresh review, send us your drawings, specification and existing report. We can review the scope and quote for the work.
