What height of insulation should I use between joists in an MDF floor?

Question

Background

I am starting my first serious DIY project - converting our garage into a playroom for the kids. The garage floor is concrete.

My plan is to run timber (Sawn Kiln Dried Timber (L)3000 x (W)75 x (T)47mm - B&Q) joists across the concrete floor. Then put down an MDF floor, put down carpet mat and carpet.

Question

I want the room to be warm all year round (we live in Ireland), so I'm going to put insulation between the joists in the floor. The insulation will be Jablite Flooring Polyboard. It comes in both 50MM and 75MM.

As my joists are going to be 75MM tall, should I opt for the 50MM or 75MM insulation polyboard and why? (will it make the floor more stable using 75MM, or will it decrease the insulations effect not having an air gap?)

Answer 1

Static air is what gives you the insulation. If the air is able to move, then it doesn't insulate. So you want the entire space to be filled with insulating material (which contains small air bubbles within the material) to get the maximum insulation effectiveness.