Most cavity wall openings have four sides!
The previous 2006 version of ADL1A contained a system of Accredited Construction Details (2006 ACDs) that addressed many areas where significant heat loss was occurring. However, even with the 2006 ACDs deployed, the remaining heat loss was still too high – commonly roughly equivalent to that from the floor and roof combined.
The option to improve detailing beyond the 2006 ACDs was always possible, and if one did, it was reflected in the DER. What ADL1A has done is effectively make this calculation mandatory. Thus, one must focus attention on all the key junctions applying their individual psi values and basing the calculations on the length of such junctions. Every opening through a cavity wall suffers heat loss at the head (lintel level), the reveals and the sill. In all areas (known as envelope opening junctions), it is possible to improve (reduce) potential heat loss.
At head level, the use of separate lintels can provide an alternative build approach. The common heat loss path that exists with metal combined lintels that link both skins is eliminated. The essential DPC arrangement is provided by a self-supporting Type C Cavitray that requires no building into the inner skin. As the petheleyne material does not promote heat transmittance the arrangement maintains separation integrity, which can be further enhanced with a compressible acoustic/thermal infill barrier between lintels in the head cavity. The use of these separate elements reduces the heat loss at head level as identified within NHBC Technical Details issue 03 table 2.
Vertical thermal integrity of the reveals can be increased using a Continuity Closer instead of an ordinary reveal closer. The Continuity Closer introduces immediate advantage by overlaying and blocking the heat loss path that regularly occurs between ordinary closer and partial fill insulation. Building Regulations Approved Document L1A 5.9 states there must be no reasonably avoidable thermal bridges caused by gaps. When tested after completion, openings built with exposed gaps will not meet the required standard. The Energy Saving Trust when reviewing the “100mm Cavity Debate” advised: “Air gaps (between sections of insulation) are a serious issue with current forms of cavity construction and can affect all types of insulation products………performance discrepancy is present with foam insulation, due to the potential occurrence of large air gaps.”
Sill level provides another opportunity to introduce a compressible acoustic/thermal infill barrier between skins. So doing completes the perimeter thermal zoning of the break junctions affecting every opening through a cavity wall. Moreover, if the sill is cast, tile or masonry, the necessary thermal zoning can still be maintained by using a Type U Undersill Cavitray with an integral bonded insulation break.
Typical preformed Type U Undersill Tray
with integral bonded insulation break.
Under ADL1A 2010 builders, designers and energy assessors will ultimately select from the formal options available:
2006 ACD’s + 0.02W/(mk) or 25% penalty – whichever is greater.
Option to model and calculate own junctions + penalty as above.
Addition of 0.15W/(mk) to overall heat loss of dwelling.
(Penalties do not apply until Secretary of State has approved scheme).
NHBC Technical Extra, Issue 03, September 2011*.
Cavity Trays technical manual volume 22.