When turning to the internet to find answers about insulation, one of the most asked questions is as follows:
My house is framed with 70mm studs, leaving a 70mm gap for insulation. Can I still install a 75mm insulation batt, such as a high density Earthwool 14kg/m3 75mm insulation? Or will I risk damaging the plasterboard?
Before we answer that question, let’s take a look at the two most common options available to the home owner who wants an R2.0 insulation in their wall cavity.
Standard R2.0 Wall Batts
A typical R2.0 wall insulation batt is 90mm thick. These thermal wall batts are most commonly installed in the exterior wall cavities, providing an effective thermal barrier around the home. A standard R2.0 insulation batt has no particular acoustic (sound deadening) benefits, though the mere presence of glasswool or other bulk insulation in a wall cavity will always to some extent reduce the amount of noise which finds its way through the wall.
Acoustic Wall Batts
Being only 75mm thick, the Earthwool 14kg/m3 75mm acoustic wall segment is an excellent option for home owners who want to address both the thermal and acoustic issues, i.e. they want to protect their home from both excessive temperatures and also from the transmission of unwanted sound. As such, acoustic wall batts are commonly installed in the interior walls of the home, between bedrooms, bathrooms, laundries and of course TV rooms and music studios.
… back to the 70mm Cavity Issue
So you’ve decided you want R2.0 acoustic in the interior walls, and a quick check with the measuring tape confirms what you already suspected – the gap is 70mm, while the insulation you plan to install is 75mm…
While in the bag, insulation batts are typically compressed – either slightly or greatly – to a thickness which is below their officially stated out-of-the-bag thickness. Insulation brands such as Knauf Earthwool have developed ‘compression packaging’ which allows a significant number of insulation batts to be squashed into a single bag, reducing storage and transport costs, and of course reducing the number of bags that insulation installers need to lug around the building site. Now the theory is that insulation batts, whatever their squashed dimensions, will expand out of the bag to their full, recommended thickness. But the reality may be somewhat different. In fact, it would be fair to a assume that a significant % of R2.0 acoustic batts will never expand beyond 70mm thick, making them perfectly safe to install in a wall cavity the same size. And even if they did expand to the full size – it’s hard to imagine the extra 5mm causing sufficient pressure within the wall cavity to have any impact on the plasterboard or other wall lining.
While the official advice will always be ‘never install bulk insulation which is thicker than the wall cavity’ it’s safe to assume that the likelihood of any issues occurring from installing a 75mm insulation batt inside a 70mm cavity is very low.