Refurbishment can be a technically challenging area. The Celotex Technical Centre speaks to a wide range of customers every day – many of whom are looking for design advice around a refurbishment project.
Whether it is an architect working on a whole building refurbishment or a homeowner who is looking to upgrade the insulation in their own property, there is often a lot of confusion and even fear around topics such as moisture and condensation.
One area that we are often asked about is pitched roof refurbishment. When it is time to renew the roof covering, homeowners are often keen to take the opportunity to increase the thermal comfort of their home – especially if they have lived for many years in a property with little or no roof insulation.
In this blog we will look at some of the challenges of pitched roof insulation, touch upon some frequently asked questions and introduce an exciting new system – Celotex Rafter Gold.
What’s the difference between a warm and a cold pitched roof?
These terms can be used differently by different people, which can add confusion.
When discussing pitched roofs, the normal usage is that where insulation is positioned at the line of the sloping rafters then this is defined as a warm roof.
Where insulation is positioned at the line of the horizontal ceiling joists then this is referred to as a cold roof.
Today we will be looking at warm pitched roofs – where insulation is provided at the line of the sloping rafters.
Building Regulations for pitched roof – Approved Document L1B
A common question is what the building regulations require in terms of insulation when refurbishing a pitched roof.
Part L states that where more than 50% of the roof is being renovated the insulation value should be upgraded to achieve a U-value of 0.18 W/m2K.
This can often be difficult to achieve where there are constraints due to working with an existing building.
Often this U-value could only be achieved by either raising the roof height (to accommodate additional insulation above the rafters) or by stripping all the ceilings (to accommodate additional insulation below the rafters).
Neither of these measures are desirable and may even be practically impossible.
However Part L goes on to state:
“Where the standard… is not technically functionally or economically feasible, then the thermal element should be upgraded to the best standard that is technically and functionally feasible…”
What this rather wordy paragraph means is that building control is often happy to accept a lesser U-value as long as you make reasonable provision to achieve the best standard possible.
Relaxation of the regulations may also be available for historic or traditional buildings.
One important point to note is that you should always look to achieve a U-value of no worse than 0.35 W/m2K.
This value is outlined in other areas of the building regulations with respect to minimising risks of harmful surface condensation.
Risk of harmful condensation
This is an area that quite rightly concerns both homeowners and building designers. It is also a requirement of the building regulations that pitched roofs are designed and constructed to minimise risks of harmful condensation.
Upgrading the insulation value of the roof can increase the risks of harmful condensation – as the difference in temperature between the heated building and the roof covering is increased.
Normally risks of condensation are controlled by ventilating under any non-breathable roof covering or membrane and by providing a vapour control layer to the warm side of the insulation material.
A vapour control layer (VCL) is a layer within the construction that has a high resistance to the passage of water vapour. A polythene membrane would be a classic example. The job of the VCL is to prevent water vapour migrating into the insulated construction where it could lead to interstitial condensation.
Here we have another challenge, as often the roof has been constructed without a vapour control layer. Introducing one may require replacement of all the ceilings in the property. This would be expensive, disruptive and often beyond the scope of the project.
Enter Celotex Rafter-Gold…
Celotex Rafter-Gold is a unique system designed for re-roofing of existing properties using a combination of high performance Celotex GA4050 or GA4075 insulation and TLX Gold – the unique intelligent 2 in 1 insulating breather membrane.
Celotex Rafter-Gold allows the insulation of a pitched roof without the need to raise the roof height and does not require a vapour control layer.
Celotex Rafter-Gold is designed to be installed when the roof covering is being replaced. This allows the system to be installed from above without replacement of existing ceilings.
U-values as low as 0.26 W/m2K can be achieved with a 125mm rafter – without the requirement to raise the existing roof height. All systems can achieve a U- value within the recommended “backstop” value of 0.35 W/m2K.
Control of condensation without a vapour control layer
TLX Gold is Sd intelligent (the more moisture, the more it breathes) which is why there’s no condensation risk, even without a vapour barrier.
As part of the development of the system, Celotex Rafter-Gold was assessed by the BRE to ensure that it was robust in terms of controlling the risks of harmful condensation.
Standard condensation risk analyses are not sufficient to analyse to risk of condensation within complicated structures where insulation is interacting with structural members such as roof timbers.
The BRE assessment utilises a specialised model which assesses the risks of harmful condensation, thermal bridging and mould growth using dynamic real world data.
Celotex Rafter-Gold is LABC Accredited and both Celotex GA4000 and TLX Gold are BBA Certified giving peace of mind that the system is safe and effective.
Whilst a vapour control layer is not required, it is important that there is a “well sealed ceiling”. This means that any gaps or cracks in the existing ceiling should be made good – to minimise vapour transport into the construction by convection.
More information on Celotex Rafter-Gold can be found by visiting
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