Skeilings part 2: Avoiding the risk of condensation

25/07/2017

When re roofing, there is a requirement to insulate to the current Part L standard of U = 0.18 W/m2K where possible, if the existing insulation is not better than 0.35 W/m2K. Where there is a skeiling section present with a breather membrane above, it needs to achieve the best U value possible given the limitations of the rafter depth at that point. However, careful consideration should be given as to how to best insulate the other sections, as the wrong choice can create problems.

Common scenarios are those of chalet bungalows where there has been an attic conversion carried out some time ago, or mansard roofs in older properties, where there are existing rooms below and sometimes generous loft spaces above. There can also be issues with new builds, where the lower U values often required mean that the condensation risk is greater if the risks are not fully understood.

Complying with BS5250

The ideal insulation strategy according to BS5250 (Code of Practice for Control of Condensation in Buildings) is to run the insulation at rafter level from eaves to ridge, keeping the voids warm also. Even if the insulation is limited in the skeiling sections by what can be accommodated, it may be possible to improve the U value of the rafter insulation over the voids by having a greater depth of PIR there.

TLX Gold used in a skeiling application

The skeiling insulation between the rafters should be designed to be the best possible. For a new build this would mean having insulation between the rafters and a vapour barrier under the plasterboard to prevent warm moist air migrating to the cold side of the insulation, where it could condense. Where there is an existing room below, however, it is not possible to install a vapour barrier. In this case using TLX Gold is the best way of avoiding condensation risk, along with checking that all the penetrations (e.g. pipework) through the ceiling are sealed. The main source of heat loss is in fact through air movement.

Solution with 100mm rafters + 50mm PIR; 400mm centres

The above solution shows a typical TLX Gold buildup, using it draped between 100mm rafters with 50mm PIR board under, and the accompanying condensation risk calculation – without a vapour barrier in place – indicates that condensation does not occur.

What about the loft voids?

Having decided upon the skeiling insulation, the builder then often thinks all there is to do is to put mineral wool in the loft void and triangular eaves voids. On the face of it this would appear to be a cheaper solution than continuing the rafter-level insulation, but when the other aspects are considered this may not be the case. Reserve this “hybrid pitched roof” for where there are significant loft voids.

Having a warm roof buildup in the skeiling and cold roof sections in the voids means that these should be regarded as entirely separate, so that at the junction between these sections the air gaps -  under the breather membrane and under the TLX Gold  - should be blocked off by an upstand of PIR for example – so that there can be no air movement between the roof sections.

That still leaves you with the problem of how to insulate the voids, which require U = 0.16 W/m2K on the floor and U = 0.28 W/m2K on the dwarf walls. The recommendation when using mineral wool on the floor of a loft is to use 270mm mineral wool: 100mm between the joists and 170mm laid crosswise over the top. New builds requiring lower U values may have even more, making the loft very cold as heat is retained in the rooms below. However, to avoid condensation there has to be some means of either preventing moisture migrating up from below into the loft space or some means of getting rid of it as quickly as possible.

TLX Gold can be used in skeiling application

The traditional way of doing this is to ventilate the loft so that cross-currents of air can sweep away the moisture, but this is not possible where there is a skeiling. So you either need separate ventilation of the voids – eaves vents for the eaves voids, and tile and ridge vents for the loft void – or rely on a breather membrane certified for use in cold non-ventilated roofs, ideally with a vapour barrier below.

Managing the Condensation risk

The occurrence of condensation is always a balance between how much moisture is entering the voids and how much is leaving, plus the temperature of the loft and the local climate. For a new build, where a lot of drying out takes place initially, NHBC recommend the use of a vapour barrier plus an air-open breather membrane in a cold loft. For a retro-fit where there is no vapour barrier, this advice is also an option where vents cannot be fitted. There is no problem switching from TLX Gold to a different breather membrane where the skeiling joins the upper part of the rafters.

So consider how much moisture is being produced within the house - for example is there a shower room below – and is there a vapour barrier preventing it going into the loft void? The amount of mineral wool loft insulation will determine the loft temperature. Is the house in the freezing North-East or the balmy South-West? Is an air-open or a conventional breather membrane being used? Is it possible to fit ridge and tile vents?

There is not one definitive answer to the best buildup, but the above factors should be taken into account.

View the full (LABC approved) technical solution HERE

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