Wood Fibre IWI - plaster removal or reinstate?

[offthepiste]

On 21/10/2021 at 12:05, SimonD said:

 

What is the full buildup of your walls?

 

Recent research on gypsum shows that it is indeed 'breathable.' It provides some vapour permeability and it has very good moisture buffering values. The problem tends to be if it get too wet. But as you've mentioned the paint poses some questions and as @larry says you may have a dollop of pva in there too. Lime is actually not very good at all from a moisture buffering capacity, but functions very well in vapour permeability. The difference is down to pore structure with lime having macro-pore structure and gypsum having both micro and macro-pore structure, with larger macro-pores. Gypsum also has higher pore volume than lime. SPAB now explicitly differentiates between the functions of vapour permeability and moisture buffering/capillary action of materials and where they might work best in old buildings.

 

The difference in how the materials work is that vapour permeability works through vapour pressure gradient and with hygroscopicity and capillary materials moisture transport is driven by relative humidity and capillary pressure.

 

It's a thorny topic, mind you.

 

My perspective is that if your whole wall buildup provides for the passing of moisture either as vapour and/or capillary action, then great. If not, look at how the external wall may perform outside of your gypsum layer with potential barriers of paint/pva to ensure moisture doesn't get trapped. Then you can utilise the moisture buffering capacity of your woodfibre and wall finish for indoor air quality - this can actually reduce ventilation demand in the house, but ventilation is nevertheless an essential component part in upgrading thermal performance in your home and should always be done in sync with the upgrades. Just make sure you do a condensation analysis on the proposed buildup before going ahead.

 

Hi Simon,

Thank you for this information .. I have had my head in work .. will probably know more about build up next Feb when we should start in earnest. Cheers

[offthepiste]

On 21/10/2021 at 10:56, larry said:

 

I've used a scraper with a Tungsten Carbide blade recently, screwfix had one for about £9 and it has been absolutely incredible, though more for gloss than walls. Buy a few spare blades if you're doing a large area though 

 

Your logic all seems sound to me but I am definitely not commenting from a position of expertise.

What sort of plaster is it? If gypsum skim then I guess quite likely to have a PVA coating underneath which will not be breathable even if you strip the paint off. 

 

 

Hi Larry, thank you .. interesting comment about PVA .. starting next Feb but may try to get a "sample". Suspect that there its lots of lining paper covering up cracks etc. as well as old Victorian wallpaper. Think major works were done (on the cheap in the 70s's ) but suspect that surface may not have changed others than repainting over the decades with matt emulsion. Cheers

Edited December 8, 2021 by offthepiste
extend

[larry]

 

On 08/12/2021 at 16:18, offthepiste said:

 

Hi Larry, thank you .. interesting comment about PVA .. starting next Feb but may try to get a "sample". Suspect that there its lots of lining paper covering up cracks etc. as well as old Victorian wallpaper. Think major works were done (on the cheap in the 70s's ) but suspect that surface may not have changed others than repainting over the decades with matt emulsion. Cheers

 

Good luck with it all!!