What is Cavity Wall Insulation?
Cavity Wall insulation is a type of insulation applied within the cavity walls (usually the exterior) of residential and commercial buildings. Many people are familiar with the form of cavity wall insulation that involves blowing small foam pellets into the cavity of existing houses, but modern houses are instead often built using Cavity Slab insulation batts - such as Knauf Dritherm 32 and 37. Instead of being applied retroactively, this is added during the process of constructing the wall - literally from the ground up. It is designed to “friction fit” between the cavity walls, in between the steel wall ties, and is an effective thermal insulation.
What Size is Knauf Dritherm 37 (75mm)?
Knauf 75mm Dritherm 37 comes in a pack of eight “slabs” each of which measure 1200mm in length x 455mm width - designed to fit standard vertical wall tie spacings allowing a closed joint with adjacent slabs. Our Dritherm 37 Insulation is a full-fill insulation measuring 75mm so is designed for 75mm cavities.
Knauf Dritherm 37 Cavity Insulation
Knauf Dritherm is a full-fill insulation designed to fill the entire cavity in external cavity walls - to be applied during the process of constructing cavity walls. It is designed to friction fit - meaning it sits snug within the cavity between wall-ties, with no additional fixings required.
Dritherm 37 cavity insulation is a glass mineral wool designed to be a lightweight, water-resistant and highly non-combustible, fire-resistant thermal insulation. This can often be more forgiving than rigid insulation as it’s fibrous and flexible nature allows it to knit together to prevent air gaps which can compromise effectiveness. It is bound with their unique ECOSE® technology to help it form a solid matt, without the added formaldehyde or phenol. This is visible in the brown colour, as opposed to the more yellowish hue common in other glass mineral wools.
The number (Dritherm 32, 37, etc) refers to the thermal conductivity, with the lower number meaning the insulation is less conductive and therefore a more effective insulation.
How to Use Knauf Dritherm Cavity Wall Insulation
Knauf Dritherm is a cavity wall insulation, meaning it is applied within the cavity of masonry cavity walls during their construction. It is built up along with either sides of a cavity wall and added as the courses of masonry and corresponding wall-ties are added. It is cut into 1200mm x 455mm sheets designed specifically to fit between these ties. Ensure you select a thickness that corresponds to the space of the cavity. Designed as a full-fill insulation, this insulation is designed to have full contact with the wall, so special care must be taken to avoid thermal bridging.
What Thickness of Mineral Wool Insulation?
When choosing a thickness of cavity slab mineral wool insulation, it is dictated by whether you are intending to add a full-fill insulation or a partial-fill insulation. Knauf Dritherm 32 is a full-fill insulation, meaning it is designed to fill the cavity wall with no air gap, so typically you should select a thickness of cavity insulation the same thickness as your cavity.
How to Calculate U Value
The U-value (thermal transmittance) of a building component such as a wall, roof, or window, measures its thermal conductivity or how easily heat can pass through it. The lower the U-value, the better the insulation. To calculate the U-value, you typically need to consider several factors, including the materials used, their thicknesses, and any air gaps. A general formula for calculating the U-value:
U = 1 over R
Where:
U = U Value
R = The total thermal resistance of the building component, including all layers of materials and air gaps.
Use the Knauf U Value Calculator.
