What lime mortar /plaster (internal)

[Jblakes]

Good afternoon,

I'm looking for info on what the best method to sort out.

We had damp issues in bedroom, so I've knocked the gypsum off and now left with a wall that need bricks cementing in, then plaster on top.

What's the best lime mortar and plaster to use for this? Also been looking at wood board to use against the stone wall and plaster, anybody done this, is it better insulation than just using lime plaster, and more importantly does it breath. We had damp issues from gypsum, don't want to do something that will cause damp again.

Kind regards
James

 

[Drayton]

Good afternoon,

I'm looking for info on what the best method to sort out.

We had damp issues in bedroom, so I've knocked the gypsum off and now left with a wall that need bricks cementing in, then plaster on top.

What's the best lime mortar and plaster to use for this? Also been looking at wood board to use against the stone wall and plaster, anybody done this, is it better insulation than just using lime plaster, and more importantly does it breath. We had damp issues from gypsum, don't want to do something that will cause damp again.

Kind regards
James

View attachment 9100

Have you worked out what has been causing the damp?

 

[Jblakes]

Yeah, it had a gypsum skin on top with multiple paints over that, our final thick paint was the last straw, it was condensation getting trapped.

Once all the gypsum removed. Not had any signs of damp. So all good. Now just about to decorate and wondering the most cost effective, quickest way to insulate external walls.

Kind regards
James

 

[TudorCottage]

I had a big hole in one of my internal walls. I constructed a lath and used this lime mortar first, built up in stages.
I then used this lime plaster to finish off. I’m no plasterer but found them both easy to use and was chuffed with the results.

 

[Jblakes]

Anybody know how wood fibre board/lime plaster skim compares to just lime plaster for insulation purpose?

Kind regards
James

 

[malcolm]

Woodwool is 3 times better at insulating than insulating lime plaster for the same thickness. I used 60mm woodwool board in my kitchen and it gave me a U value of 0.7 by itself. I think the building regs maximum U value for new build walls is about 0.3.

A properly dry brick wall with ordinary lime plaster should have a U value around 0.73. This is based on an English Heritage measured value rather than the value for soaking wet cement mortared walls which are more than twice as bad.

I calculated that with dry lime mortared brick walls and nice lime plaster I would end up around a 0.34 U value in total. 58% reduction in heat loss through the wall.

Changing the 1/2 inch of lime plaster for insulated plaster would bring that down to 0.67. 9% reduction in energy loss through the wall.

I've put my spreadsheet for calculating this sort of thing on https://www.periodproperty.co.uk/data/u-value-calcs2.xlsx It's a bit broken just now for whole house energy calculation but the U value stuff at the top should still be good.

 

[Jblakes]

That confused the hell out of me, wood fibre boards are 3x better at insulation than lime plaster, but a dry lime mortared brick and lime plaster give better U value.

Unless I'm reading that wrong

Kind regards
James

 

[Westholme]

A properly dry brick wall with ordinary lime plaster should have a U value around 0.73.

When I had 4" of EWI installed on some of my house, the final U value of 0.3 assumed initial U value for 9" solid wall of 2.1 (possibly too high in reality but a lower value makes little difference).

 

[malcolm]

That confused the hell out of me, wood fibre boards are 3x better at insulation than lime plaster, but a dry lime mortared brick and lime plaster give better U value.

U value is a measure of the thermal performance of the whole wall. Lower is better. If you add insulation with a U value of 0.7 to a wall that already has a U value of 0.7 you end up at total U value of 0.35.

The U value depends on thickness of insulation. the insulated plaster is 1/3 as effective as woodwool board for the same thickness. But you'll not find woodwool as thin as that.

If it's only a bit of plaster I would use any old lime for the repairs. No point stripping off everything for a 10% reduction of energy loss through the wall. For smaller repairs the readymix lime plasters are the way to go.

 

[malcolm]

When I had 4" of EWI installed on some of my house, the final U value of 0.3 assumed initial U value for 9" solid wall of 2.1 (possibly over-pessimistic).

Yes they assume the bricks are soaking wet as is modern practice. The 2.1 didn't work for me as I would have more energy loss through the walls than goes in to the entire building. This sort of thing works better for me in the calcs:

https://www.spab.org.uk/sites/default/files/documents/MainSociety/Advice/SPABU-valueReport.Nov2012.v2.pdf

 

[Jblakes]

So looking at the picture I posted, would it be better to just fill in with lime stone,

Or knock back to the bare wall, fill gaps with mortar then 15 or 25mm wood fibre boards then lime plaster over the top, I assume this one will give alot better insulation?

Is it easy to fit wood boards as I've never done it.

Kind regards
James

 

[Jblakes]

If so, what wood fibre boards do I get?

 

[Jblakes]

Bump, want to ideally get cracking this weekend

 

[a twig]

If you search for woodwool boards on this forum you will find plenty of posts from people who have done it (me included).
If it were me in your case I would:
1) Fill the gaps using stone and NHL based mortar then give the mortar at least a week, maybe two to do its thing
2) Fix woodwool boards to wall using screws, plugs and fibre reinforced washers (this sort of thing, other vendors available)
3) Skim with an NHL based render and use fibreglass mesh to avoid movement cracking in the longer term.
All the above has been detailed multiple times in threads from the likes of me, zebra, malcom and others, usually with plenty of links / images

 

[Jblakes]

Thank you for the info, just want to clear a few things up before i go bodging it up. By fill gaps you mean mainly build the wall back up, if so already have stones and mortar.

Do you mean fix fibre boards to the existing lime plaster, or knock back to the wall. (any issues either way)

And my lime render you mean plaster? Isn't render for external use?

Probsbly daft questions, but might aswell get it right first time.

Kind regards
James

 

[a twig]

Thank you for the info, just want to clear a few things up before i go bodging it up. By fill gaps you mean mainly build the wall back up, if so already have stones and mortar.

Do you mean fix fibre boards to the existing lime plaster, or knock back to the wall. (any issues either way)

And my lime render you mean plaster? Isn't render for external use?

Probsbly daft questions, but might aswell get it right first time.

Kind regards
James

Yep

Either / or - up to you, won’t really make a difference other than potentially when you are trying to make it all level

I just used the word render because for a lot of people plaster means “fat lime” and render means NHL and some seem to be against using NHL indoors. No idea why, and mix is the same, except you might want a finer sand internally if you want a smoother finish. Or buy a pre-mix and add water.

This thread (among many others) might help https://www.periodproperty.co.uk/forum/threads/plastering-question.16738/

 

[Lacan07]

Bump, want to ideally get cracking this weekend

Those gashes in the wall will have to be fixed with stone and lime mortar as Twig has indicated, then dub out all bare masonry with suitable base coat lime plaster until its flush with the current old plaster. This will be your base for the wood fibre boards. You have a choice of manufacturers but Steico is popular, go for 40mm or 60mm thickness of board. Its generally considered that anything thicker runs a certain risk to the external brick wall, you have to allow some heat to escape and warm the bricks during winter, and besides 40/60mm wood fibre board has a certain amount of 'give' in it which can be handy if your wall isn't entirely flat which it most likely won't be. Most vendors that sell these boards will have the fitting instructions in PDF form on their website, its pretty standard, and Steico have their own instructions readily available for example.

Most vendors recommend something like Lime Green Solo for the finishing coat, I'm not sure why specifically. It has a strong lime content that dries to a lovely creamy rock hard finish. I thinks its considered desirable because you can put a second coat straight over the first coat and this helps with the mesh application. For example, apply first coat - trowel in the mesh - apply second coat. The problem is it tends to go of like a bitch. Others on here have used alternative plaster finishes though and I'm sure thats just as good.

If you've never used lime plaster before and intend to do it your self then good luck! The dubbing out will be fairly easy once you get the hang, the finishing coat not so. No matter what you use you'll be 'chasing the gear' but don't worry its common with lime, even the professionals have to 'chase' lime and they get it on the wall 10x faster than you and me.

 

[a twig]

Just as a counterpoint - I’ve never found any need for anything thicker than 25mm depends what you are trying to achieve I guess

 

[Lacan07]

Just as a counterpoint - I’ve never found any need for anything thicker than 25mm depends what you are trying to achieve I guess

Well I guess if you're going to the trouble of doing it you might as well go as thick as your space will allow for better insulation. But I would imagine 25mm is a popular choice if your worried about reducing room space which is one of the issues with IWI.

 

[Jblakes]

Just a quick one. Does it have to be limestone to build up the internal wall? I'm not entirely sure I have enough, is anything else as breathable and can be used?

Regards
James