Plastering - hemp or lime.

[Abecedarian]

Hi,
As my money pit restoration begins to stir into life I find myself planning wall coverings. Currently one bedroom is back to bare brick on the two outside walls, a gypsum plaster on one wall and lime (I think!) on the other.
Having decided against insulating our outside walls in order to maintain the thermal mass of the solid brick I come to thinking about plastering....

What are the pros and cons of hemp or lime? As I understand it hemp has some insulating properties, which I realise goes a little against my "maintaining thermal mass" plan.
Would hemp offer some insulation without causing issues from thermally isolating the brick walls?
Are there any other factors to consider?

Also, can anybody recommend a "teach yourself plastering" tutorial please? I think I'm gonna have a go at this myself.
I've found the Natural Building Centre videos on YouTube, are there any other/better ones out there?
Many thanks,
Paul.

 

[TheForge]

The binder will be lime in either option, the question is really is it worth the extra expense of adding hemp. In terms of insulative value you’d need about 50mm (and more likely 100mm) minimum total thickness to even make a dent on the performance of the wall so I would suggest irrelevant.

Hemp plaster is a bit like porridge, if you like working with that consistency then it is easy to use but not necessarily anymore so than pure lime plaster.

All will need tending and the preparation and after care (especially with shrinking and cracking) are as important as application to a successful job.

The type of lime and aggregates are also important and lime putty or hot lime mixes are more workable and ultimately more permeable (in most circumstances) to hydraulic limes and preferable for plastering. Aggregates should be washed and well graded to avoid salt leaching and premature failure. Cheap, ‘normal’ builders sand is not normally suitable, particular if moisture is coming from the substrate.

 

[a twig]

Assuming you are buying pre-mixed, a lime plaster with hemp attracts lower VAT as it counts as an insulation product so price difference is minimal.

Personally I liked the hemp as it was good solid stuff, hard work but I was able to do dubbing out and base cost all in one go, and then put a finishing coat on, so 2 layers rather than the traditional three. Your mileage may vary

Personally I've since settled on woodwool boards with an NHL based skim over, I've posted somewhere else about it, but the rooms I've used this method for have ended up much drier and warmer, and have been quicker to do than the traditional fat lime 3 coat method.

 

[a twig]

Here we go:

https://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=16738&p=227203#p227203

 

[Abecedarian]

Thanks to both for the input.

If I could question further.....
In another room - also an upstairs bedroom - the plaster is in reasonable condition but has several hairline cracks, can I key the surface with a devil float and then skim with a finish coat?
The walls were papered over paint. I think with a stiff brush the vast majority of the old paint is going to come off without too much of a fight.
I'm hoping a skim over will be good enough?

Cheers,
Paul

 

[MatthewC]

Also, can anybody recommend a "teach yourself plastering" tutorial please? I think I'm gonna have a go at this myself.
I've found the Natural Building Centre videos on YouTube, are there any other/better ones out there?

I would recommend a one day course at Mike Wye - I'm sure they still do the one day "Introduction to Lime" course. I did it over 10 years ago (times flies...) and it also got me 10% off all of their own products (i.e. lime mortar/plaster) which, in my case, paid for the course many times over! You also get free lime advice on the phone.

NB Lime plastering does require (or will develop) stamina and strength in the arms!

 

[MatthewC]

the plaster is in reasonable condition but has several hairline cracks, can I key the surface with a devil float and then skim with a finish coat?

Just saw your later question! One answer to this is to use stuff called DG27 whch is breathable and gives a key for a skim of plaster.

I used it when I had to remove huge areas of old plaster due to water damage etc but wanted to keep as much old plaster as possible. We applied new plaster to the right thickness and put DG27 on the old, then a skim of fine plaster and it worked a treat. DG27 is a bit expensive and I was pretty annoyed when the plaster's mate used it on the whole wall, including the NEW plaster!

 

[Abecedarian]

DG27 looks like the answer.
Cheers,
Paul.