Insulating Plaster - is it worth it?

[wardling]

Hi all.
Currently in planning stages for IWI in 1900 solid brick semidetached home in Belfast and speaking to various suppliers about options.
Two have come back suggesting solely using insulating plaster is the best shout for our project and have sent on modelling for depths:u-values etc.
Just wondering what people’s real world experience has been with having it installed in their homes? What depth did you use?
Every external wall has either original window boxes which we were hoping to keep or chimneys with original marble fireplaces that we are loathed to try and remove to later reinstate so think the thinking was that insulating plaster allowed us to vary depths depending on the wall. Modelling suggests a 70mm layer which we could prob use in most places (except around the fireplace) would get us a uvalue of 0.43 which I thought was reasonable and most certainly better than it is currently. It’s obviously a fair investment though so paying thousands to be underwhelmed worries me.
Grateful for any insight people would have! Thanks in advance!

 

[fernicarry]

If you have 70mm to play with then you'd be better off with wood fibre or similar. Roughly speaking insulating plaster is about half the insulating value of the various natural insulations like wood fibre, hemp, sheeps wool etc. so you'll either get twice the performance at the same thickness or same performance at half the thickness. I do use insulating plaster here but only as a levelling coat on the rubble masonry or where depth is very limited such as around window reveals.

As for whether its worth it? Its never going to be new build levels of insulation, so don't expect that, but every little bit helps. What you really need to pay attention to is air tightness. No point in insulating the wall if air is leaking out around the windows or under the floor. The analogy would be buying a really expensive down jacket but not zipping it up...

 

[Maria D]

We used wood fibre board in attics rooms that had no features that we wanted to preserve, but used insulating plaster on the external walls of all our other rooms as we wanted to keep coving. Also, we found the wood fibre to be a very costly and time consuming to install. We also had less room to spare in our bedrooms than you seem to have, so that was another factor for us. In terms of mm, our walls had scud coat, then insulating plaster and then a finish coat of lime. I asked the plaster to apply the insulating plaster thick but really not sure of that happened. So far, I think it has worked well but if the option to use wood fibre throughout had been available to us, I think I would’ve gone for that.