The Invisible Force
Earthwool DriTherm is the leading brand of mineral wool cavity slab for the thermal insulation of masonry cavity external walls in residential and non-residential buildings.
It is the choice of leading Passivhaus designers and builders for insulating masonry cavity walls and we couldn’t wish for a better endorsement of Earthwool DriTherm than this.
In Passivhaus it is essential that the building fabric performs to the design expectation. This is achieved by a combination of dependable product performance, the intended construction arrangements and details minimising the risk of thermal bypasses and the product being simple to fit so the construction details are easily realised. Earthwool DriTherm clearly fits the bill as the list of projects where it has been used testifies.
Of course DriTherm is not a new product that is hard to source, it has been a mainstay in masonry cavity construction for nearly 40 years and it is so widely available it could almost be called ubiquitous. The reasons why it is the perfect choice for Passivhaus construction apply equally to mainstream construction where performance levels may not be as high but meeting the design expectation is still essential. It is available in a range of thermal conductivities to meet the varying demands of customers and building regulations.
Earthwool DriTherm 32 Ultimate has been developed as the highest performing product in this range with a thermal conductivity of 0.032 W/mK. This allows building regulation compliant U-values to be achieved in normal cavity widths with all types of blocks.
Build with DriTherm - Case Studies
The following case studies demonstrate the use of DriTherm both in Passsivhaus and mainstream constructions over several years. This includes projects dating from before the introduction of our Ecose Technology1.
1Our Glass Mineral Wool with ECOSE Technology has a lower environmental impact in a number of indicators in Life Cycle Assessment including Global Warming Potential and energy compared to our traditional Glass Mineral Wool.
Building Regulations have driven improvements in the energy efficiency of buildings, whether they are completely new buildings, or extensions to existing buildings. A major contribution to this has been the improvement of the thermal insulation of external walls.
Building regulation compliance for building fabric insulation focusses on calculated Thermal Transmittance of the fabric; U-values of plane elements and PSI values for junctions and openings. Of course thermal comfort is delivered by the realisation of the calculated performance in the actual construction.
U-values and psi values explained
Thermal transmittance, also known as U-value, is the rate of transfer of heat through a structure (which can be a single material or a composite), divided by the difference in temperature across that structure. The units of measurement are W/m²K. The better-insulated a structure is, the lower the U-value will be.
The U-value is calculated from the reciprocal of the combined thermal resistances of the materials in the element, air spaces and surfaces, also taken into account is the effect of thermal bridges, air gaps and fixings.
In a masonry cavity wall The DriTherm component has the greatest influence on the U-value. However the other components can make small but critical differences.
The block type generally has the next biggest influence and if aircrete blocks are included they would improve performance and dense concrete would worsen it.
Using wet plaster rather than plasterboard on dabs worsens the calculated U-value and using a block and render outer leaf rather than brick improves it.
It is also important to include the wall tie characteristic in the U-value calculation.
Of course the wall construction will have an influence on the overall airtightness of the building and wet plaster is generally regarded as providing the highest and most predictable airtightness performance which are required for Passivhaus construction.
The Knauf Insulation Technical Support Team provides U-value calculations and condensation analysis in accordance with the BBA/TIMSA U-value Competency Scheme
Heat loss through non repeating thermal bridging is commonly referred to as `linear thermal transmittance value` or the psi value (Ψ-value).
New Build Homes
In New Build there are different requirements for Domestic and Non Domestic Buildings in all countries in the UK and additionally building regulations compliance is in the first case based around the building emission rate calculated in SAP or SBEM.
The wall U-value is not fixed so long as it does not exceed a maximum value, commonly called a backstop. To focus on Dwellings, in England and Scotland the building is unlikely to comply with a wall built merely to the backstop requirement where as in Wales it would.
At Knauf Insulation we have considerable experience of completing SAP analysis for customers and understand that the suggested U values will normally result in an overall compliance.
The table below shows the requirements for new build dwellings for each country and the universal solution that achieves the U-value no matter what configuration of outer leaf, inner leaf and lining is chosen alongside it.
Because there are so many variations in the type of block and the other factors that influence the U-value there are a large number of possible to solutions. We have created detailed tables of U-values for different options for England, Wales, Scotland Northern Ireland and Northern Ireland. If you require advice please refer to our detailed U-value tables or call our Technical Support team on 01744 766666 or email firstname.lastname@example.org
In extensions to existing buildings there are different requirements for each nation in the UK. Indeed to make matters more confusing in Scotland there are three depending on the type of building and in dwellings the performance of the existing building.
The table below shows the requirements for each country and the universal solution that achieves the U-value no matter what configuration of outer leaf, inner leaf and lining is chosen alongside it.
Because there are so many variations in the type of block and the other factors that influence the U-value there are a large number of possible to solutions. We have created detailed tables of U-values for different options for England, Wales, Scotland Northern Ireland and Northern Ireland.
If you require advice please refer to our detailed U-value tables or call our Technical Support team on 01744 766666 or email email@example.com
Exposure to Wind Driven Rain
Earthwool® DriTherm Cavity Slabs are approved for use in all exposure zones by the British Board of Agrément. The possibility of wind driven rain penetrating the outer leaf of a cavity wall depends on the exposure level of the site or building. This varies according to where it is in the country, topography and various site factors.
It is widely recognised that as cavity widths increase the likelihood of rain penetration reduces.
Under the rules in Approved Document C it is permissible to build with full-fill insulation behind tooled flush jointed fairfaced masonry providing the cavity is a minimum of 150mm wide. However, this is not the case with Warranty provider technical standards where the restriction on full-fill remains.
For clarity, the requirements discussed above are summarised in Table 1. It should be noted that recessed jointing on masonry flush cills and copings are not permitted in very severe exposure zones. Similarly, all other aspects of good practice concerning rain resistance and masonry cavity walls should be respected.
NHBC2 and LABC Warranty Requirments
Approved Document C
Fairfaced Masonry - Full fill Insulation
|Minimum 150mm Full Fill Cavity|
Fairfaced Masonry - Partial Fill Insulation
|Minimum 75mm Residual Cavity||Minimum 75mm Residual Cavity|
Rendered - Full Heigh of Wall
|Mimimum 75mm Full Fill Cavity||Minimum 75mm Full Fill Cavity|
Rendered - ABove Ground Storey
|Minimum 150mm Full Fill Cavity||Minimum 150mm Full Fill Cavity|
Impervious Cladding - Full Height of Wall
|Minimum 50mm Full Fill Cavity||Minimum 50mm Full Fill Cavity|
 NHBC and LABC
 Note NHBC requirements apply to England Wales and Northern Irelas, in Scotland NHBC Warranty does not permit Full Fill insulation in masonry cavity walls with any type of outer leaf.