Internal Insulation Edwardian House

[NWhit]

I am planning to renovate some rooms in my house. I’d like the house to feel warmer in winter and thinking about adding some internal insulation on the external facing walls. The house has cavity walls of about 40mm. I’ve read about wood fibre insulation but it seems to refer to solid walls not cavity walls. Wood fibre also is more expensive. Is it necessary to use wood fibre and lime or will insulated plasterboard be ok? I don’t have any issues with damp or condensation. Thank you.

 

[aockenden]

Let's review your line of reasoning:

1. These rooms get cold in winter
2. In order to make these rooms warmer, I should insulate the external walls
3. The external walls have a cavity so maybe I can use modern insulated plasterboard

Points 1 and 2 strongly imply that you believe your external walls get cold, otherwise why would you bother to insulate them as you attempt to make the room warmer? IF you believe your external walls get cold, then whether the walls have a cavity or are solid is irrelevant. Condensation forms on cold surfaces. So whatever IWI option you go with needs to be able to deal with that condensation.

If you slap a modern, non-breathable insulated plasterboard product directly up against a cold wall’s internal surface (cavity wall or not), and there’s no ventilation between that insulated plasterboard and the internal face of the cold wall, then you should expect to start growing mildew / mould behind that insulation.

So to summarize, if your wall is cold and you’re going to add IWI to it, your options are either to use a non-breathable modern insulation system and retain a gap between that insulation and the cold wall and somehow thoroughly ventilate that gap, or to use a breathable moisture-wicking insulation system (involving lime and / or clay) in direct contact with the cold wall.