U-Value & Condensation Reports

Modern living under thatch has changed in the last 10 to 15 years, with the introduction of modern central heating, double or secondary glazing, power showers and many more people wanting fully vaulted ceilings.

Additionally the requirements and associated recommendations from Building Control and insurance companies see the requirement for fire barriers and increased levels of insulation to be installed within the roof structure.

All of these areas have an effect on the performance of thatched buildings in terms of thermal and vapour, therefore all components of the roof structure need to be built into any calculations/reports.

U-Values

When looking at sample reports and information, care needs to be taken regarding the type and thickness of thatch used in the examples. If you need further clarification, request that a report is issued showing all of the proposed products and components, also check things such as the type & thickness of thatch and the density of the insulation etc.

U Values are expressed in units of watts per degree of temperature difference W/m²K. Overall ‘U Values’ will vary according to actual thickness and density of the thatch.

  • 300mm thickness of Water Reed – 0.29 W/m²K
  • 300mm thickness of Long Straw – 0.23 W/m²K

(Source: Based on conductivities from CIBSE GuideA3)

For comparison, the standard in Part L of the Building Regulations (2010) for replacement roofs insulation at rafter level is 0.18 W/m²K. It is therefore clear that a typical thatched roof on its own can come close to meeting modern standards for thermal insulation.
(Source: English Heritage – Insulating thatched roofs)

In terms of a suitable insulation product, at TAS we recommend the Thatchbatt® product for the following reasons:

  • Thatchbatts® are non-combustible
  • High Density
  • Offer additional protection in terms of fire resistance
  • Thatchbatts® are manufactured for TAS under special agreement by Rockwool®

On the basis of additional insulation being required, we have listed below some example U-Values from previous reports using a range of materials. See ‘report section’ below for more information relating to reports.

Fire Barrier Insulation & thickness Type of Thatch & Thickness Example U-Value
TAS100 Thatch Fireboard 50mm Thatchbatts® 300mm Reed Thatch 0.20 W/m²K
TAS100 Thatch Fireboard 100mm Thatchbatts® 300mm Reed Thatch 0.16 W/m²K
Thatch Firewall 50mm Thatchbatts® 350mm Reed Thatch 0.18 W/m²K
TAS100 Thatch Fireboard 150mm Thatchbatts® 400mm Reed Thatch 0.12 W/m²K
TAS100 Thatch Fireboard 100mm Thatchbatts® 300mm Straw Thatch 0.14 W/m²K

Notes: As shown in the above listed examples, many things can have an effect on the final U-Value that is achieved. Due to the individual nature of thatched buildings, many other components are listed in these reports such as air gaps, size and type of timbers, internal finishes and provisions of suitable vapour controls.

Vapour Control

It is widely accepted in the construction industry that in addition to sufficient levels of insulation, vapour control layers are required to control vapour and reduce the risk of condensation.

Many thatched buildings that have a fireboard type system installed are either new build or fire damaged, either way the main fabric of the building will have been subjected to large amounts of water and therefore as part of the drying process larger amounts of water vapour will be created. Subject to the design of the roof, consideration should be given to installing the vapour check before the Thatchbatts® and TAS100 Fireboards are installed, this is with a view to reducing the risk of these materials from the effects of water vapour and therefore not prolonging the drying out process.

In addition to the drying out period a correctly fitted Vapour Check will play an important part in the way that the building continues to perform for the longer term. Many products are available ranging from foil backed plasterboards to high performance membranes, but it is important to install the correct system that best suits each individual building.

We are happy to put you in touch with a range of suppliers and manufacturers, they will be able to discuss the exact requirements of the building and recommend the most suitable products.

Reports

Every thatched building is different both in terms of its construction, type of materials used and location within the country.

Based on the data that we hold we are able to issue you U-Values based on most different roof types and designs. If however a bespoke U-Value & Condensation Risk Analysis report is required we are able to arrange for these to be commissioned for you.

For more information on this or to discuss your exact requirements in more detail, please contact us on 08455 20 40 60 or use our enquiry form.