Using fibre insulation in a garden room

In the plans we have created as part of The Complete Self Build Garden Room Guide, we suggest using a rigid insulation such as Celotex and Kingspan. We recently had an email from one of our Self Builders, Tom who wondered if it was possible to replace this rigid insulation with a ‘woolly’ insulation system such as fibreglass or sheep’s wool.

The answer is, of course, yes you can. Using a fibreglass insulation will probably work out significantly cheaper than using a PIR rigid insulation like Celotex, so could be a good choice if you are budget conscious. If you are looking to create a ‘green’ garden room, then a natural insulation such as sheep’s wool or hemp would be a good option.

The reason we have suggested a rigid insulation in our sample plans is that this is the system adopted by most of the professional garden room designers who use a traditional timber frame system. ‘Woolly’ insulations are still used by some garden room designers but have largely been superseded by this new system which is considered to perform better.

The thing with the rigid PIR insulations like Celotex is that they offer excellent thermal performance from a relatively thin profile. This means depending on your choice of board you could get the same performance from 50mm of PIR insulation as you might achieve from 150mm of a fibreglass insulation.

Don’t forget your vapour membrane

When using a ‘woolly’ insulation such as fibreglass you need to ensure you have your vapour membrane in place and that any joints are sealed with the special membrane tape.

Rigid insulation is still a great option for the roof of a garden room

You could certainly use insulation like fibreglass or sheep’s wool on the roof of your garden room as part of a cold roof build-up. The problem is with this build-up is that you need to incorporate ventilation into the roof. Matt insulations also limit your options when it comes to recessing lights into your ceiling.

You can mix a warm roof build-up, which incorporates rigid insulation sheets and still use fibre matt insulation in the walls and floor of the garden room.

We talk about the insulation systems used in garden room design in the Self Build Garden Room Guide eBook.