DIY Doctor UK DIY Forums

We have a summerhouse/shed, built for us last autumn. We have had various quotes for insulating it and finishing it off, but we are confused because of conflicting advice from different builders. We need to make a decision urgently between two different companies and obviously want to get it right.

The floor has Celotex insulation underneath the chipboard; the walls are just shiplap wood cladding on a wood frame with no breathable/Tyvek membrane. The outside is painted with Sadolin breathable paint. The doors and one window are double glazed, with one trickle vent.

Should we use rockwool or Celotex to insulate the walls? Celotex=not breathable, expensive, very good insulation; rockwool=breathable, cheap, not as insulating - will it also act as a moisture bridge across from the outside wall to the inside cladding? Do we need to have a Tyvek lining put on before rockwool? Is this possible, now the building is all bolted together?

Will rockwool, tightly packed, still drop a little over time, or will it be fine?

Should we use plasterboard or MDF? Will plasterboard swell and crumble if it gets damp? Is there much of a price difference between MDF and plasterboard? Will MDF swell in the damp??

Any comments would be VERY gratefully received!!! Please help to clear up the confusion!!

The important question is. "What are you going to use it for?"
If its a typical cold wet summer, will you be using it?
Do you intend to heat the summerhouse?

If you intend to use it through the summer and into the autumn, then you need heat, insulation and draft proofing.

The heat will best be an electric radiant heater, this is instant and it will help you feel warm.

The celotex under the floor is good, but how thick?

The walls are useless as they are, and using it will not be pleasant, except on the warmest of days.

Presuming you will only use it for short periods, with electric heating, then there will be no question of condensation, so go for polystyrene.

Polystyrene has the same form as Celotex but is cheaper, the insulation you get from polystyrene is a constant from the day you fit it to some point long in the future when you get rid of it.

With Celotex and the other proprietary brands you pay more for a thinner product and the insulation is guesstimated on a 15 year deterioration., (back down to the polystyrene level)

An inch of polystyrene will save about 90% of your heat, will keep out the drafts and will heat up quickly. Two inches will be better. (Make sure it is a tight fit. Gaps equal loss of heat)

Each of use produces about 1.5 liters of water vapour each 24 hours, more if you are active, less just sitting down and watching the grass grow.

From this you will realize that moisture/water vapour will not be a problem. Therefore, forget Tyvek or similar. And block that vent.

However, if you use it as a gym with a lot of activity this would change.

Plasterboard is a very good fire barrier, it holds a lot of water vapour and will hold back a fire for 30 minutes.

If you varnish all sides in and out, of the summerhouse to protect it from water vapour, you will help to extend its life. If you use MDF, woodchip or oriental board these too need to be painted/varnished on both sides and all edges, to keep the water vapour out.