Pitched and Flat Roof Condensation Solutions


What causes condensation in a roof?

Condensation is caused by changes in the temperature profile of a roof. When hot air meets cold air, the air particles can no longer hold onto suspended water molecules, therefore droplets of water are deposited on surfaces. It is estimated that the average household generates approximately 15 liters of water vapour per day. Throughout the day, the warm air containing water vapour will rise towards the roof and if there is no clear service channel for this water vapour to escape, when met with the cold air within the roof space, condensation will occur, causing moisture in the roof space.

Managing condensation risk using multifoil insulationHow to stop condensation

Condensation can be very difficult to reverse, especially if it left unnoticed for a prolonged period. Not only is it unsightly, but it can also cause rotting of timbers which can be very costly to repair and can have hazardous structural implications. There are various steps one can take to limit the chances of condensation, such as the use of extractor fans and avoiding drying clothes indoors. However, the most important step is to ensure insulation and ventilation is correctly positioned within the roof to manage the condensation risk before any issues begin to arise.

How can TLX help to manage condensation risk?

TLX Silver and TLX Gold multifoil insulation improve the thermal performance of a building whilst preventing condensation risk. This is managed in different ways for each of our products.  

Preventing condensation with multifoil insulation

Installing a good vapour barrier underneath the rafters in a roof will prevent warm air from entering the roof void from below. Multifoil insulation has shiny, reflective layers to keep heat inside the building, ensuring no risk of condensation.  It is also the optimum thickness at 30mm, which allows it to provide the best thermal performance without compromising on space. TLX Silver is completely airtight, meaning there is no risk of water vapour rising into the rafter space and therefore no risk of condensation if installed in accordance with our approved detail.

How can TLX Gold help?

TLX Gold the 2 in 1 insulating breather membrane is installed above the rafters within a roof, either draped or pulled taut and counter battened. The fully breathable, shiny undersurface faces inwards with an optimum 20mm unventilated air gap underneath to ensure the reflection of heat. Whilst occupying the coldest part of the temperature profile, water vapour entering the rafter space can escape through the breathable layers of TLX Gold and transported out of the building before it is able to cool and condense.

The air gap debate?  

You may be wondering, if a 50mm ventilated air space is required below non-breathable felt, why then does TLX Gold require a 20mm unventilated air gap underneath it? Well, the breathable undersurface of TLX Gold enhances the core R value of the air layer next to it when heat is reflected across it. Therefore, the U value of the overall structure is greatly improved when these two layers interact with each other.

Since TLX Gold is breathable, water vapour does not require a channel to escape the roof as it can pass through the layers of the multifoil into the tile batten space above and out through the tiles. Ventilation is only required above TLX Gold where the tiles are non-breathable, to prevent condensation occurring between the tiles and the upper surface of TLX Gold.

Preventing Condensation in a flat roof with TLX Silver multifoil insulation

Insulating a garage or a shed roof? Or perhaps you are planning to construct a flat roof dormer in your upcoming loft conversion and need to be assured of the condensation risks before attempting the work? Insulating a flat roof can be tricky, not least because they typically lose more heat than their pitched counterparts, therefore extra care is required when tackling the insulation, ensuring an airtight, thermally efficient structure with the minimal risk of condensation. Condensation within a flat roof can cause significant damage, and if left unchecked, can lead to rotting of timbers and can affect the performance of insulation within the roof structure. Therefore, it is critical to ensure condensation risk is limited, of which installation plays an important role. 

How to approach a flat roof with Multifoil insulation?

Our recommended build up for a traditional flat roof system is to install TLX Silver beneath the rafters, with ventilation below the felt and timber deck. Additional insulation is then fixed between the rafters to enhance the U value of the roof structure and to ensure the ventilated air gap is maintained. TLX Silver is a 2-in-1 vapour barrier and multifoil insulation, preventing moisture from entering the roof space from the inside. If fitted in accordance with our installation guide, it provides a completely airtight layer, meaning there is no risk of surface or interstitial condensation. TLX Silver is comprised of reflective outer layers, reflecting heat back into the building which contributes to the overall energy saving efficiency.

Why is ventilation so important?

Building regulations require a minimum 50mm ventilated air layer beneath the timber deck. This is vital as it protects the roof space from extreme temperature variations which can result in the build-up of moisture.

On the warm side of the roof, TLX Silver keeps the system airtight and therefore contributes to the prevention of condensation. The reflective surfaces enhance the R value of the air layer next to it, therefore unventilated air layers are required on either side to maximise the thermal performance of the structure. This is achieved by securing TLX Silver with 38x38mm battens; setting them crosswise will in turn limit the degree of thermal bridging. Inserting PIR board is in-between the rafter space helps maintain the ventilated air space above and protects the unventilated air layer below it. 

How long does it last?

TLX Silver is BBA certified and has a lifespan equivalent to that of the roof structure in which it is incorporated. 

For more information, our LABC Approved Solutions guide offers a variety of buildup recommendations and U value calculations. If your requirements are bespoke, our helpful team of product scientists are on hand to help on our thermal hotline, 01204 674 730.  

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