Adding IWI to house with EWI.

[micar]

Have a 1950s 3 bed semi which I had EWI done about 8 years ago.

I am in the middle to adding 100mm rigid boards insulation under the suspended wooden floor.

While I was at it, I thought I may as well add some IWI to the bedrooms and to the front downstairs.

I've taken the plaster off the walls and reveals.

My plan was to screw battens to the wall and add either fit 25mm rigid boards or rockwool.

Also have airtighness tape around the windows.

Over all that i'd put 12.5mm foil backed plasterboard.

Been reading online that it's either 1) pointless or 2) ok to do.

From what I read, the EWI keeps the heat in the block work which is gradually directed back into the house.

Since I only want to add 25mm, would that prevent the transfer of heat back to the house.

A house in the same area same as mine got EWI done in the last 2 years and I'm almost certain that they got IWI as well.

 

[Micks'r]

Have a 1950s 3 bed semi which I had EWI done about 8 years ago.

I am in the middle to adding 100mm rigid boards insulation under the suspended wooden floor.

While I was at it, I thought I may as well add some IWI to the bedrooms and to the front downstairs.

I've taken the plaster off the walls and reveals.

My plan was to screw battens to the wall and add either fit 25mm rigid boards or rockwool.

Also have airtighness tape around the windows.

Over all that i'd put 12.5mm foil backed plasterboard.

Been reading online that it's either 1) pointless or 2) ok to do.

From what I read, the EWI keeps the heat in the block work which is gradually directed back into the house.

Since I only want to add 25mm, would that prevent the transfer of heat back to the house.

A house in the same area same as mine got EWI done in the last 2 years and I'm almost certain that they got IWI as well.

Don't do it (until you fully understand what is going on). You're risking introducing interstitial condensation into your walls which gives a high risk of mildew / mould behind the iwi.
If you still have significant heat loss after the ewi was done, then either the primary heat loss mechanism was not conduction through your walls to begin with or the ewi was not detailed or done properly. If the former then iwi won't help and if the latter then my first point above is even more relevant.