SPECIFYING AND INSTALLING XPS INSULATION IN CAVITY WALLS BELOW DPC
Polyfoam XPS has seen increased interest in the use of extruded polystyrene (XPS) in masonry cavity walls below damp proof course (DPC) level. The use of XPS in this way is a relatively new development, aiming to maintain an existing insulation specification above DPC level, but take advantage of XPS’s moisture resistance in a location that can be subject to water from the ground.
The detailing of ground floor/external wall junctions needs to take into account thermal performance requirements of national building regulations, as well as moisture resistance requirements. While XPS insulation products do not typically carry third-party certification for cavity wall applications, we believe the requirement for certification only applies above DPC.
This blog post looks at how XPS can be detailed in ground floor/external wall junctions to help meet specifiers’ needs.
XPS insulation options in a cavity wall below DPC
Used in conjunction with a partial fill insulation product (such as PIR or phenolic foam) above the DPC, XPS insulation can either partially or fully fill the cavity below DPC.
If XPS partially fills the cavity then the same width of clear cavity needs to be maintained below DPC as is present above DPC. This ensures the required width and depth of cavity is maintained.
If XPS fully fills the cavity below DPC then it needs to be provided with a cavity tray over, to meet the requirements of Approved Document C (or equivalent in Wales or Scotland).
Polyfoam XPS recommendations for below DPC insulation
Polyfoam XPS has a number of key recommendations that we believe are important to using extruded polystyrene below DPC level in a masonry cavity wall. They are as follows.
- Fully filling the cavity below DPC. This ensures the insulation remains in position and delivers the maximum thermal benefit to the junction detail.
- Cavity tray installed over the insulation, due to it fully filling the cavity.
- Continue the XPS insulation to a minimum of 225mm below the top of the perimeter upstand insulation.
- Depending on the relative levels of the ground floor insulation and the base of the cavity, start installing the XPS insulation on the first row of wall ties rather than the base of the cavity/lean mix cavity fill.
These recommendations are based on our best understanding of the Building Regulations, and having consulted details and guidance produced by others in the construction industry. The guidance in this blog post should not replace project-specific specifications, detailing and calculations.
The advice of a building control department, approved inspector or other warranty-providing body should also be sought when looking to implement any of the recommendations.
Nevertheless, we believe adopting these recommendations can help to deliver high levels of thermal performance in the junction detail, avoid the possibility of condensation occurring within the junction detail, and contribute to meeting moisture resistance requirements.
Polyfoam XPS guidance on cavity wall specification below DPC
This is the third and final 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 second post asks: Does cavity wall insulation below DPC require a BBA certificate?
The complete series is adapted from a Polyfoam XPS white paper examining below DPC insulation solutions in detail, including detailed diagrams that illustrate the concepts discussed in this post. 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.