Quality has become a key watchword in the UK construction industry, particularly within housebuilding, in recent years.
In 2016, the BRE launched the Home Quality Mark, a voluntary standard designed to provide consumers with information about the design, construction quality and running costs of a home. The Each Home Counts report also called for a “quality mark” both for energy efficiency products and the companies that install them.
This focus owes much to ongoing discussions around the performance gap – where the actual energy performance of buildings falls short of the design performance. One of the key sources of this discrepancy is the issue of workmanship and lack of attention to detail. In insulation installations, for example, thermal bridging (caused by breaks in the insulation layer) can significantly undermine the energy performance of a home and potentially lead to problems with condensation.
By following best practices, these problems can be easily avoided. Take, for example, a new timber frame building with a brick masonry outer leaf.
There are two approaches to insulating these properties with rigid insulation, such as Kooltherm K112 Framing Board:
- insulating between and outside the studs
- insulating between and inside the studs.
Regardless of which approach is chosen, best practice should be that the two layers of insulation are fixed so that there are no air spaces between them in construction.
Insulating between and outside the studs – timber frame with masonry outer leaf
Before cutting Kooltherm K112 to slot between the studs, the gap between each stud should be carefully measured to ensure a snug fit. The insulation boards can then be cut to size with a fine-toothed saw.
The insulation should be installed flush with the outer face of the studs and treated softwood battens fixed in place to prevent the boards from moving within the cavity. To limit air leakage, any gaps or penetrations through the insulation layer – such as plug sockets – should be filled with a flexible sealant or a combination of flexible polyurethane foam and flexible sealant or equivalent. A vapour control layer should also be installed on the inner face of the stud. This can be provided by vapour check plasterboard, polythene sheeting or with a couple of coats of drywall sealer.
Timber frame systems come with a layer of oriented strand board or plywood fixed to the external face of the studs, along with a breather membrane. A second, continuous layer of insulation can then be fitted externally to the breather membrane. The insulation boards should be restrained in accordance with the timber frame manufacturer’s guidance. If this guidance is unavailable, insulation boards should be lightly butted to maintain continuity and fixings carefully lined up with underlying timber studs, headrails and sole plates.
Large-headed galvanised clout nails can be used to hold the insulation boards in place before they are tied into the masonry leaf with an appropriate timber frame wall tie.
Insulating between and inside the studs – timber frame with masonry outer leaf
Installers should carefully measure and cut the Kooltherm K112 to ensure a snug fit within each cavity. The stud cavity insulation layer should be flush with the internal face of the studs. Treated softwood battens can again be used to provide stops. Any gaps should be filled with a flexible sealant or flexible polyurethane foam and flexible sealant. The external face of the studs should also be fitted with Oriented Strand Board or plywood sheathing.
Kooltherm K118 Insulated Plasterboard can then be fitted internally. The insulated plasterboards should be cut approximately 5 mm short of the floor to ceiling height. Sheet joints should be lightly butted with fixings no less than 10 mm from the bound edges of the sheet and any joints should lap the timber framing studs by a minimum of 19 mm. Where joints between sheets of insulated plasterboard are unsupported by the timber framing studs, timber noggins should be installed.
The insulated plasterboard sheets should be located centrally over the timber studs and fixed using either drywall screws at 300 mm centres (or 200 mm at external corners), or large-headed galvanised clout nails at 150 mm centres. Fixings should be driven straight with their heads embedded just below the surface of the plasterboard. They should penetrate at least 25 mm into the studs without penetrating right through the timber, care should also be taken not to overdrive nails or screws. The perimeter of Kooltherm K118 and the 5 mm clearance gap at the base of the wall should then be sealed with a flexible sealant or equivalent.
Retrofit
In addition to benefiting new-build projects, insulated framing boards such as Kooltherm K112 are also a common choice for internal wall insulation applications on refurbished buildings.
