DOES CAVITY WALL INSULATION BELOW DPC REQUIRE A BBA CERTIFICATE?
Thermal insulation materials used in cavity walls “should be the subject of current certification from an appropriate body”, as per Approved Document C in England.
Cavity wall solutions typically feature a single product type throughout the whole wall construction. Because that product continues below DPC, it may seem as though the requirement for third-party certification applies both above and below DPC.
At Polyfoam XPS, we are seeing increasing numbers of enquiries for the use of our moisture-tolerant insulation boards in cavities below DPC level. Specifiers are looking to meet both the thermal and moisture resistance requirements of national building regulations. They then specify a different cavity wall insulation above DPC level.
In this blog post, we’ll look at whether anything in regulations precludes such a solution.
What does ‘typical’ cavity wall detailing look like below DPC?
The following generic details illustrate common cavity wall insulation solutions where the insulation continues down the cavity, past DPC level.
These details meet the technical guidance in Approved Document C because:
- the DPM is continuous with the DPC; and
- the DPC is a minimum of 150mm above the adjacent ground level.
For the partial fill solution(s), the cavity also continues down at least 225mm below the level of the lowest DPC. The clear cavity width must be in accordance with the exposure zone of the building’s location.
For both insulation types, the manufacturer must be able to provide third-party certification confirming the product’s suitability for the application.
How does this typical cavity wall detailing correspond with the increase in specifications for extruded polystyrene (XPS) below DPC?
Are there any XPS insulation solutions certified for use in cavity walls?
At the time of writing, there is no XPS insulation product with third-party certification for use as a partial fill cavity wall insulation. Our view, however, is that this only precludes the use of XPS above DPC level.
To recap our first blog post in this series, the structure of Approved Document C breaks the wall down into two distinct areas.
- Below DPC, where it is necessary to resist moisture from the ground.
- Above DPC, where it is necessary to resist external moisture, and where the requirement for third-party certification is specifically mentioned.
It is the below DPC ‘zone’ where specifiers and contractors are looking to use the moisture resistant properties of XPS insulation to provide added reassurance that their external walls will resist moisture from the ground. Now we have outlined why there shouldn’t be an issue with that, the next post in the series looks at how to detail XPS to best effect.
Polyfoam XPS guidance on cavity wall specification below DPC
This is the second blog post in a series looking at the detailing of cavity walls below DPC level, and suitable insulation solutions to help meet both energy efficiency and moisture resistance requirements.
The first post asks: How should external walls below DPC resist moisture?
The third post looks at: Specifying and installing XPS insulation in cavity walls below DPC
These blog posts are adapted from a Polyfoam XPS white paper examining below DPC insulation solutions in detail. The white paper addresses a wider need for information about this area of detailing, as specifiers look to ensure compliance with all areas of national building regulations.