Traditional stone walls in Scotland - validation of RdSAP U-value calculation methodology: research

Appendix A: Assessment of stone walls in RdSAP 2012 and RdSAP 10

Existing RdSAP 2012 approach

In RdSAP 2012, version 9.94 (September 2019), U-values of external walls are obtained from Table S7 for Scotland. For stone walls of granite or whinstone, and for sandstone or limestone respectively, Table S7 states that the equations in section S5.1.1 should be used to determine the U-value to be applied in the RdSAP calculation for properties of age band A to D. For Scotland, this covers property ages before 1919 (A) through to 1964 (D).

Section S5.1.1 gives the following equations to calculate wall U-values:

Granite or whinstone: U = 3.3 – 0.002 x thickness of wall in mm

Sandstone or limestone: U = 3.0 – 0.002 x thickness of wall in mm

The adjustment in S5.1.2 should subsequently be applied if the wall is lined. This applies to any type of internal lining on an uninsulated stone, solid brick or cavity wall that creates an airspace behind it, e.g. plasterboard on dabs, or lath and plaster. This requires that the U-value from the above equations, termed U₀, undergoes the following subsequent calculation (where Rdl is the additional thermal resistance introduced by the internal finish, given as 0.17 m²K/W):

Proposed RdSAP 10 approach

The RdSAP10 methodology (February 2024) is similar in principle to that in RdSAP 2012. U-values of external walls are again obtained from Table S7 for Scotland. For stone walls of granite or whinstone, and for sandstone or limestone respectively, Table S7 states that the equations in section 6.6 should be used to determine the U-value to be applied in the RdSAP calculation for properties of age band A to D.

Section 6.6 gives the following equations to calculate the default values of stone walls:

Granite or whinstone: U = 45.315 x W ^-0.513

Sandstone or limestone: U = 54.876 x W ^-0.561

…where W is the wall thickness in mm

The adjustment in section 6.8 should subsequently be applied if the wall is dry-lined or lined with lath and plaster. This requires that the U-value from the above equations, termed U₀, undergoes the following subsequent calculation (where Rinsulation is taken from Table 14, and in the case of drylining including laths and plaster is again given as 0.17 m²K/W):