Loft Insulation? Beware condensation!

11/10/2017

The pursuit of ever decreasing U-values

In the drive to achieve the low U values required by current Building Regs it is all too easy to overlook the fact that once you introduce insulation then the risk of condensation increases. Even though the nation’s houses may now be losing less energy through the roof, we could be storing up problems of long-term structural damage if condensation risk is not considered along with insulation.

Condensation occurs when the air cannot hold any more moisture, and droplets of water begin to form on surfaces. Warm air can hold more moisture than cold air: it follows from this that if the temperature of moisture-laden air suddenly drops, or if too much moisture is entering a cold space, then condensation will occur.

A cold ventilated loft has always been tradition...

The traditional way of building a house was to have a cold ventilated loft – cold because it is not heated and ventilated because either there is ventilation at the eaves and possibly the ridge, or sometimes just via draughts coming through the tiles if there is no underlay. Nowadays we are recommended to insulate the floor of the loft with 270 mm of loft roll to prevent heat loss from the rooms below, but this means that the loft is much colder than when uninsulated, and so the condensation risk increases. The cross-ventilation may need to be increased by introducing soffit vents or at least ensuring that the loft roll is not blocking up the ventilation at the eaves. If condensation continues to be a problem, then possibly a ridge vent or even tile vents may need to be introduced.

So if a breather membrane is being used?

If a breather membrane certified for use in an unventilated loft is used as the roofing underlay then ventilation may not be necessary, though this might well depend on whether a vapour barrier has been installed underneath the joists, plus factors such as the local climate, the depth of loft roll present and how much moisture is entering the loft. Any gaps around the penetrations, e.g. pipework, can let through moisture-laden warm air, and seemingly obvious precautions such as having a badly-fitting lid on the cold-water tank or not having extractor fans ducted into the loft are sometimes found to be the culprits where condensation has been a problem, but if there is still condensation with a breather membrane, then introducing ventilation would be the only answer.

Insulation at rafter-level as well?

If there is insulation on the loft floor, then it's sensible to also have insulation at rafter level, as recommended by some DIY websites? If even a breather membrane used as roof covering can result in condensation in an unventilated loft, then anything that breathes less - even the fully breathable TLX Gold – would be even more likely to cause it. Certainly if materials such as polystyrene, or the silver vapour barrier multifoils were used they would be guaranteed to cause it, as any moisture has no way to escape.

But not if the space is ventilated...

Can rafter-level insulation be used if the loft is ventilated? By ventilating the loft enough to prevent condensation, cold outside air is being introduced, so having insulation above this is somewhat pointless – a bit like standing under an insulated gazebo on a windy day. Even if a fully breathable reflective insulation such as TLX Gold is used any benefit is uncertain, and this is not a recommended way of using it.

You see from the above that even for an unused cold loft there are factors to be considered, and the answers to “But I want to board the roof for storage” and “I want to use the loft as a room but don’t want to do a loft conversion” will be covered in another blog.

12-15 litres of moisture enter the loft space

TLX Silver loft insulation multifoil blog

It is estimated that 12-15 litres of moisture per day enter a loft space from below. The loft either requires ventilating to sweep this moisture away, or else relies on the combination of an effective vapour barrier underneath the ceiling (to prevent moisture entering) with a breather membrane to facilitate water vapour leaving.

Insufficient ventilation will cause condensation

Using TLX Silver to insulate a loft conversion

With insulation fitted on the floor of the loft the temperature is much colder, and if there is insufficient ventilation condensation will result.

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