Underfloor insulation between joists of varying width?

[old_queenslandr]

I have an old Queenslander house. The floor is hardwood on joists. The underside is open to the air, currently with no insulation. The spacing between the joists seems to vary a bit. About half the house seems to be about 530mm wide (a couple being close to 580), with the rest around 400-420mm.

The only insulation batts product I can find on the Bunnings website for an underfloor that isn't enclosed is https://www.bunnings.com.au/earthwool-r2-5-floorshield-underfloor-batt-90-x-420-x-1160mm-5-4m-11-pac... which only comes in 420mm wide batts.

Can I use this? If so, how do I deal with the 100mm shortfall in width?

Another related question, is how do I deal with possible rodents? I imagine this would be desirable for rodents to nest in during the colder months.

Thanks in advance.

Edit: Doing some more research, I'm thinking the solution (for both the wide bay issue and rodent concern) is to use wire mesh to hold the batts in place. In this way I could add an extra strip to the batts snugly to fill in the wider bays, and the mesh should also hopefully keep out the rodents. Keen to get opinion on this, or if there are better options.

[MitchellMc]

Welcome to the Bunnings Workshop community @old_queenslandr. It's terrific to have you join us, and many thanks for your question about insulation.

You’re right that the Earthwool FloorShield batts only come in the 420mm width, so for your wider joist spacings you’ll need to cut and squeeze in extra strips to fill the gap. Using a string line on nails or small staples to hold the batts in place is a good approach. Functionally, the FloorShield just has a coating to reduce wind flow through it, so in areas where the spacing varies it’s perfectly fine to use standard underfloor batts and they can be easier to trim and fit snugly between uneven joists.

Another option to consider is enclosing the underfloor area after installing the insulation. Adding plasterboard beneath the batts will not only make the space look neater and allow you to use standard batts more easily, but it will also improve your insulation. It will also help with rodents, as they won’t have access to nesting spots. If enclosing the floor isn’t practical, your idea of using wire mesh to hold the batts in place is a solid solution, as it will keep the insulation secure and make it harder for rodents to get in.

Please let me know if you have any questions.

Mitchell