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Powell Dobson PassivHaus Papers

Week 3 Insulation

Feb 2021

A highly insulated external fabric is a sure fast way of keeping energy in a building. Building regulation guidance in the shape of Approved Document L sets out minimum performances for thermal transmittance through different elements of the buildings, and in recent months (especially in Wales) they have started to ask for better and better performance. PassivHaus however has been promoting this type of approach since 1992.

As a rule of thumb, a U Value of 0.15 W/m²Kshould be targeted as a minimum for all external fabric elements, but in reality, the performance is usually much better than this, regularly achieving in the region of 0.11 W/m²K. To achieve this level of thermal transmittance, generally the thickness of the fabric is increased, when compared to a building regulations compliant home.

External Wall Comparison 

Building Regulations Home 


U Value

Min. 0.21 W/m²K 

(Typically 0.18 W/m²K)

Min. 0.15 W/m²K

(Typically 0.11 W/m²K)

Thickness (PIR example) 60mm / 70mm 90mm / 130mm

The above exercise shows the difference in performance requirements, and what that means for insulation thickness, using a PIR board in a partial fill cavity wall construction as an example build up for comparison purposes. It is important to note that different insulation types pose different thermal conductivity values, mineral wool for example would need to be specified 140mm thick to achieve a U Value of 0.21 W/m²K typically.

What insulation is typically used? This depends on what type of construction is being pursued. It is really important that as with airtightness (discussed in Paper 2) the insulation is kept continuous, and is able to be wrapped seamlessly around the building. For this reason, it’s usually best practice to stick with ‘fluffy’ or flexible insulation materials, but as mentioned above, these types of insulation usually have higher thermal conductivities, meaning greater thicknesses are needed to hit the target U Values. 

For twin stud or I beam timber frame constructions, pumped cellulose insulation seems to be the go to product at the moment in PassivHaus circles. The stud structure typically provides a 300mm void, which can be fully filled with cellulose. This allows for a nice homogenous layer, which is flexible enough to get into all the nooks within the wall structure, leaving no cold spots. You will see in the image on the PDF attached, that it is a nice junction between the roof and walls, when using a simple system and product.

To install this type of system, high levels of training is required, with checks undertaken to ensure that the density and spread of the insulation with the wall is as required. 

Twin skin traditional masonry walls with a large cavity, fully filled with mineral wool is another route that has been used with good success rates. The image in the PDF attached has been taken from the Denby Dales project. (Worth a look at their blog, this project is responsible for my passion for PassivHaus! – They were real pioneers in the UK).

With large cavities such as this, other design criteria need to be considered. Metal wall ties for example, would undo all your hard work, providing lots of very small but very ‘conductive’ avenues for heat to cross the cavity. Basalt wall ties are regularly specified to overcome this issue, and can be seen on the image to the right. 

That is the end of our introduction to insulation for PassivHaus. As always, this paper is meant to be quite short and sweet, but if you have any queries, or have burning questions, I would love to hear them.

Look out for the next instalment, coming soon.

Next week: Series 1_Paper 4_Triple Glazing

Oliver Henshall, Technical Associate and Certified PassivHaus Designer
E: T: 033 33 201 001



The best way to predict the future is to design it.