The Mad Surveyor

It seems that we get more and more enquiries these days on how to tackle the problem of cold bridging of rafters in ceilings, now that "cold bridging" is being paid much more attention and defined and covered by the Building Regulations as implemented from April 2002 (particularly relevant here is part L). Cold bridging is the buildings ability to transmit heat from the inside to the outside through bridges of material (e.g a steel wall tie) that offer relatively high thermal conductivity compared to the rest of the fabric of the building and therefore represent a potential source of heat loss from the building. So, stopping or eliminating any bridges during new construction is the general idea but dealing with them to minimise the impact for thermal loss from a building is the practice and art of design. Cold bridging has to be allowed for in the overall ‘U’ value calculation, clearly avoiding bridges wherever possible is to be desired.

For those not in the know, cold bridging takes place when a thermal conduit is formed. Simply, heat can be transmitted through the make up and construction materials used in the building structure to the outside air and it is the rate of heat transmission that is crucial since nothing is a perfect insulator, including polyurethane foam good though it is as an insulator. Obviously the slower the heat is conducted through the walls, floors, ceilings, roofs, windows and doors the better as less energy will be used to keep the building warm. What can ruin a well insulated building’s ability to keep heat in is cold bridges, points in the building that effectively become a conduit for heat transmission from inside to outside. So, the Building Regulations now require a ceiling at rafter level not to be allowed to form a so called "cold bridge" but to be so constructed to form a warm roof or warm deck. The idea is that the rafter itself forms a potential cold bridge and must be prevented in the method of construction from transmitting heat easily to the tile and then on to the outside world.

In roofs, when plasterboard is put up against the roof ratfter to form the ceiling a cold bridge will be formed. Heat can be transmitted through the plasterboard to the rafter, then through the rafter and then on through the roof tile and then on to escape to the outside air even if you insulate between rafters. Now, even though (most) roof rafters are made of wood, which has a relatively low heat conductivity, there is still sufficient concern that the rafter can form a "cold bridge", i.e. it will act to conduct the heat out of the building. Obviously, the wood is still too good at transmiiting the heat so must be stopped. And the Building Regulations now specifically deal with this problem when ceilings are at roof rafter level. A warm deck has to be constructed with the absence of cold bridges to avoid heat being sucked out of the building through bridging.

So, that is the problem, but what is the solution? Well, using polyurethane spray foam, the foam can be sprayed in the gap on top of the rafter where the laths are nailed, typically the gap there is between is 20 to 30 mm, and then the foam is built up to a depth of 75 mm between rafters. Now it could be argued that the laths that are nailed directly to the rafter can still tramsmit heat to the roof tiles but in practice the surface area of contact is so small that the cold bridge thus formed is insignificant and this method of insulation will meet the Building Regulations requirement for cold bridging and exceed the required ‘U’ value of 0.25 watts per square metre or less (before 2002 was 0.35). But wait, there is now another proposal on the table, which is to get roofs down to 0.16 by 2005. That will be a challenge!

Testing initiated by Isothane indicates that a polyurethane foam depth of 75 mm will meet the 0.25 requirement and meet the requirement to avoid cold bridging by the method of installation which is to spray in between rafter and lath gaps and then to build the foam up to a depth of 75 mm from the tile between rafters.