30mm Vs 70mm Insulated Plasterboard on External Wall

Internal insulation is a sticking plaster when a full wrap is what the doctor ordered. 👍️

We dry lined all external walls on our semi with 120mm foil backed insulated plasterboard. It's worked remarkably well with very stable temps without any heating since early April.

Do it right and pay attention to board sealing and opening sealing with airtightness tape and it can be highly effective.

Can also be an entirely DIY job that will cost a quarter of what external insulation will cost. It's not as good as external - but can you afford the extra cost for the relatively small overall gain.

I heard people say that there's not much gain after 60mm since the various leakage paths would start to dominate, but I cannot say if that is so.

Airtightness is half the job and if you externally insulated and don't go and seal all air pathways you are wasting a lot of the effort - at least you get to do it in one pass with dry lining.

I used foam glue adhesive to glue mine to the walls. Get a glue gun for this and also a tin of glue gun cleaner to use between glue cans.

In order to be fire safe you also need to add at least 4 mushrooms to each board. These are 150mm long (different lengths for different boards) and hollow, they are pressed out of galvanised metal and the spike has a slot down its side which should be hammered in with the slot downwards. This allows any condensation to drain out of the shaft. In the case of a fire these will hold the board even if the foam glue melts.

Procedure is;

• spray a permimeter bead of foam on the back of the board and at least two more from top to bottom. Make this continuous to create a closed aircell on the back of the board.
• Lightly wet the wall with a hand spray bottle (aids adhesion).
• get a second person to lift the board into place and hold it there, you will want a board lifter for this.

• drill your mushroom holes and hammer in your mushroom plugs until they are just below the surface of the board. Try to judge your hole depths so they don't punch through your block and create a air pathway to the cavity, failing this will significantly compromise the end result.
• spray a dab of foam down the shaft of the mushroom. Spray foam into any large uneven gaps between the boards.
• air-tightness tap over the heads of each of the mushrooms and down the seams between boards and at least along the top where it meets the ceiling.

This is the cheapest airtightness tape available in the country and very good. Its more of a cloth and very sticky. You will want special window tape for sealing round windows

https://www.ecologicalbuildingsystems.com/product/tescon-vana

Hope that helps.

Thanks @10-10-20 and @Shoog - appreciate the steer!

Just to be clear, I never suggested sending the mushrooms in at an angle - the slot in the mushroom should be on the bottom when they are hammered in horizontally.

I have a semi d that was wrapped. Cant beat it.

I got an extension on the side with 92mm insulated plaster board which is grand but nothing compared to the wrap. Im getting the extension wrapped asap.

You are assuming g that the cavity is open.

In any event, with 70 mm IWI, it wont be moist air, it will be water, from outside and inside as unless you go with a proper A/T layer and a service cavity, there are always holes in the foil layers.