How to insulate walls?

DeeM82

I'm renovating my kitchen of which all the internal fibro walls need ripping down, replastering and reinsulated. 

As the room gets freezing cold in winter and boiling hot in summer I want to insulate using the highest rating batts (fibro external walls).

Is there any reason why ceiling R7-8 insulation couldn't or shouldn't be used in the walls?

Any advice is gratefully appreciated

EricL

Hello @DeeM82 

Welcome to the Bunnings workshop community. It's sensational to have you join us, and thanks for sharing your question about insulation. 

Just a friendly reminder that before you begin tearing down your walls, if your house was built around the 70's and 80's, I strongly suggest having your walls tested for asbestos. If the test comes back positive, you'll need the services of an asbestos removal specialist. If the test comes back negative, you can continue your renovation plans. 

Ceiling insulation rated at R7–8 is designed specifically for horizontal applications where there is more space available in the roof cavity. Generally speaking, there is nothing that prevents you from putting ceiling batts in the wall cavities, but there are some practical and performance-related reasons why it is not generally recommended.
 

The first issue is thickness. R7–8 batts are typically very thick, much thicker than a standard wall cavity (often only 90–140 mm deep in residential framing). Trying to compress them into that space will reduce their effectiveness significantly because insulation performance depends on trapping still air. When compressed, the insulation loses its loft and R-value, meaning you are not actually achieving R7–8 but something much lower, sometimes close to what a regular wall batt would have given you. You would therefore loose any insulation advantage the R7-8 would have provided.
 

The second issue is fit. Ceiling batts are usually sized wider and longer to span roof trusses or joists. In a wall cavity, you will likely have to cut them down substantially. This creates more seams, insulation waste and potential gaps, which can lead to thermal bridging and reduced performance. Properly fitted wall batts are manufactured to match standard stud spacing so they fill the cavity snugly and maintain performance.
 

The third concern is moisture and vapor management. Walls are more vulnerable to condensation than ceilings, particularly in colder climates or where heating and cooling is frequent. Wall insulation often needs to be paired with specific vapor barriers or breathable wraps designed to manage moisture safely. Using oversized ceiling batts could make it harder to install these systems properly, increasing the risk of dampness, mould, or timber damage inside the wall.
 

In short, while you could physically put ceiling batts into walls, you would lose much of the claimed R-value due to compression, risk poor fit and gaps, and potentially create moisture issues. The better approach is to use insulation products designed for walls at the correct cavity thickness and spacing.

If you need further assistance, please let us know.

Eric