Specifying extruded polystyrene insulation in masonry cavity walls below DPC level

1 Introduction to below damp-proof course cavities
2 What are the moisture resistance requirements in the Building Regulations?
- 2.1 Moisture resistance of floors
- 2.2 Moisture resistance of external walls
3 Does cavity insulation below DPC level require third-party certification?
4 What are the options for using XPS insulation below DPC level?
5 Using the advice in this article
6 Related articles on Designing Buildings

[edit] Introduction to below damp-proof course cavities

Detailing masonry cavity walls below damp-proof course (DPC) level has seen greater focus across the construction industry, including with major housebuilders. What should design professionals look for when seeking to maintain existing wall insulation specifications above DPC, while improving the robustness of their detailing below DPC? And how can extruded polystyrene (XPS) insulation help?

Detailing junctions where a ground floor meets the external walls must consider thermal performance. The psi-value of the junction, along with avoiding risk of condensation and mould growth, is a well-established part of the Building Regulations.

Equally well-established is the moisture resistance of floors and external walls, including the provision of DPCs, covered by Approved Document C in England and Wales. Moisture resistance of floors is relevant because the position of thermal insulation and damp-proof membrane (DPM) within the floor influences the detailing of the wall construction below DPC level.

Like its predecessor, Approved Document L1A 2013, Approved Document L1 2021 says the building fabric should be constructed “to a reasonable standard so that the insulation is reasonably continuous over the whole building envelope.” Unlike its predecessor, the 2021 document includes extra guidance that says, “moisture-resistant insulation should be fitted below damp-proof course level and extend to the foundation block/ structure.”

It is for this reason that more and more designers and specifiers are paying closer attention to their below DPC detailing.

[edit] What are the moisture resistance requirements in the Building Regulations?

Part C of Schedule 1 of the Building Regulations in England sets out requirements relating to site preparation and resistance to contaminants and moisture.

Requirement C2, ‘Resistance to moisture’, says that a building’s floors, walls and roofs “shall adequately protect the building and the people who use the building from harmful effects caused by: (a) ground moisture; (b) precipitation including wind-driven spray; (c) interstitial and surface condensation; and (d) spillage of water from or associated with sanitary fittings or fixed appliances.”

At the time of writing, the Building Regulations in Wales adhere to the same Requirement C2. Section 3 of the Scottish Building Standards, ‘Environment’, has its own moisture resistance requirements, with technical guidance published in the domestic and non-domestic technical handbooks.

[edit] Moisture resistance of floors

Section 4 of Approved Document C gives guidance on how ground-supported floors featuring a concrete slab (‘solid floors’) and suspended concrete and timber floors can meet requirement C2. Different guidance applies if the floor is subject to groundwater pressure. A DPM is an essential part of a solid floor construction and might need to be part of a suspended floor design. The guidance for solid floors requires the DPM, which can be placed above or below the concrete slab, to be “continuous with the damp-proof courses in walls, piers and the like.”

[edit] Moisture resistance of external walls

Section 5 gives guidance on how external walls can be constructed to resist moisture from the ground, and from outside the building (i.e. precipitation). As with floors, different guidance applies if the walls are subject to groundwater pressure.

A wall meets the requirement if it has a DPC that is continuous with any DPM in the floor, and the DPC is a minimum of 150 millimetres above the adjacent ground level.

If the external wall is of cavity construction, the cavity should continue down at least 225 millimetres below the level of the lowest DPC; or a “damp-proof tray” (cavity tray) should be provided to prevent precipitation passing to the inner leaf (with weep holes to aid the passage of moisture out of the wall through the external leaf).

Section 5 also describes suitable outer leaf materials for cavity walls, what materials and components can bridge a cavity, and how exposure zones affect the width of clear cavity that should be maintained.

As well as this, it details the conditions that a thermal insulation material must meet to be considered suitable for use in a cavity wall, including that rigid insulation boards “should be the subject of current certification from an appropriate body”.

Generally, that means an Agrément certificate issued by a third-party. This is usually the British Board of Agrément (BBA), although other Agrément certificate providers are available.

[edit] Does cavity insulation below DPC level require third-party certification?

Cavity wall insulation tends to feature a single solution/ product type, whether full fill or partial fill, which continues down the cavity past DPC level. As a result, it can seem as though the requirement for third-party certification applies to the complete wall construction.

To recap, the structure of Approved Document C breaks the wall down into two distinct areas. The first is below DPC, where it is necessary to resist moisture from the ground. The Second is above DPC, where it is necessary to resist external moisture, and where the requirement for third-party certification is specifically mentioned.

The below DPC ‘zone’ is where specifiers and contractors look to use the moisture-resistant properties of XPS insulation to provide added reassurance that their external walls will resist moisture from the ground.

At the time of writing, there is no XPS insulation product with third-party certification for use as a partial fill cavity wall insulation. But our view is that this only precludes the use of XPS above DPC level.

[edit] What are the options for using XPS insulation below DPC level?

Used with full fill insulation, XPS can fully fill the cavity below DPC. Used in conjunction with a partial fill insulation product, 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. This ensures the required width and depth of cavity is maintained.

Polyfoam XPS recommends fully filling the cavity below DPC level to ensure the insulation remains in position and delivers the maximum thermal benefit to the junction detail. You should also install a cavity tray over the insulation.

Depending on the relative levels of the ground floor insulation and the base of the cavity, you should consider starting the XPS insulation on the first row of wall ties rather than the base of the cavity/lean mix cavity fill.

Following these recommendations will, we believe, help you deliver high levels of thermal performance in the junction detail, and avoid the possibility of condensation in the junction detail.

[edit] Using the advice in this article

The use of XPS insulation products in masonry cavity walls below DPC level is a relatively new development. This article represents a summary of our best understanding of the Building Regulations and offers advice based on details and guidance produced by others in the built environment sector.

The details included here should not be used as a detailed specification; project-specific specifications, detailing and calculations should always be produced.

The advice of a building control department, approved inspector or other warranty-providing body should also be sought when looking to implement any of these recommendations.

This article appears in the AT Journal summer issue, 2024, number 150 as 'Specifying extruded polystyrene (XPS) insulation in masonry cavity walls' and was written by Rob Firman, Technical and Specification Manager, Polyfoam XPS Ltd.

--CIAT