DIY Doctor UK DIY Forums

Hi there,

I am building a conservatory with a 75mm cavity and am really stuck on the type of insulation to use.

The cavity wall to be insulated is approx 13 M2 and so is quite a large part of my small conservatory. Its important to me that the conservatory is well insulated so I can use it all year round. Which type of insulation would you recommend?

Im I correct in thinking if I use insulation boards such as Celotex (for 75mm cavity) I will need 50mm thickness as to leave a 25mm clear gap? And if I use something like Rockwool cavity batts I can fill the whole 75mm?

If there is not much of a difference in U values then it will then come down to the difference in costs.

The reason we have cavity walls is, wind blown rain, on hitting the outside of a wall at 60mph and above will spread over the wall and will often find its way inside. Then you have water running down the inside of the outer wall.
A cavity filled with anything that is not totally water proof will cause the inner wall to become damp.
As heat always moves to cold, a damp cold wall will strip a lot of the heat from inside the home.
On the other hand, a cavity that is more than 16mm wide, will start up an air circulation that will move heat across the cavity.
Therefore, a multi cell product like polystyrene that is almost waterproof, will if fitted properly (no gaps - gaps filled with foam) provide a helpful insulation. Five inch thick polystyrene or similar will save the transmission of all useful heat.
Consider putting the insulation on the inside of the room, where it will be the most effective.

I would recommend using 50mm kingspan with a 25mm clear cavity.