Renewing and insulating lime ceiling

[faulkneralder]

We've recently moved into a Grade II listed house. The bedrooms have part-sloping ceilings that are directly under the roof slates (the roof was originally thatched so is a steep angle), and we believe it is just lath and lime plaster between us and the tiles, so it is VERY cold.

The plaster has detached over the decades and ceiling paper + paint is holder large bulges of it in place, so it needs replacing anyway. If replacement has to be lath and plaster due to Grade II planning rules (I haven't asked them yet), is there any way to insulate it at the same time without adversely impacting the character?

I should mention that it is a black-and-white house, built approx. 1680, so has exposed beams around the ceiling too.

 

[Cubist]

Fact is that almost any effort to insulate those ceilings will increase the final depth from the underside of the tiles to the new exposed face of the ceiling by quite considerable amounts depending in large on how effective you might want the insulation to be. In some cases that can be over 100mm and more. Frankly, with these old timber frame houses that can be almost impossible to achieve if significant portions of the timbers must remain exposed. You would be wise to have a chat if you can with your local Conservation Officer to seek his/her opinion and perhaps suggestions. Frankly put, whoever got rid of the thatch robbed you of that roofs best insulation capability.

 

[faulkneralder]

Thanks Cubist. How about something like Spacetherm? It seems very thin but is permeable.

We will of course take your suggestion and speak to the Conservation Officer.

 

[Cubist]

I don't know that material and so couldn't comment beyond that an air gap between it and the tiles or rather any membrane below them may still be needed.

 

[paulc]

How about something like Spacetherm? It seems very thin but is permeable.

On paper Spacetherm is quite good at just 0.015 W/mK, but make sure you are sitting down when looking at the price.

 

[montyjohn]

Your lathe and plaster is going to be around 30mm thick I guess.

Trying to stary low profile with something that might be acceptable to the CO:

• Do nothing and repair like for like - U ≈ 33 W/m²·K (so terrible).
• 20 mm woodwool with a skim of lime: - U ≈ 4.55 W/m²·K (could be a similar profile to existing at about 35mm thick with a 10mm air-gap, very affordable, big improvement on insulation)
• 20mm Woodfibre, overlaid with 20mm of woodwool to help lime stick, and then a skim U ≈ 1.39 W/m²·K (55mm thick, so would stick out 25mm more than existing, might be acceptable????). Heat loss would be 24 times less than existing so might be able to win the CO over.

To give you an idea of how much these still fall short, current standards are U = 0.18 W/m2. you obviously can't get anywhere near this you could go from being 183X to 8X the current standard.

 

[IHateDamp]

What paint is covering the wooden beams?

General question to the experts; isn't paint not recommended on structural old timber because it traps damp thus encouraging rot??

 

[Cubist]

What paint is covering the wooden beams?

General question to the experts; isn't paint not recommended on structural old timber because it traps damp thus encouraging rot??

Yes it is. I don't think it's a paint but as bad would be varnish. An oil or perhaps wax, but the OP may confirm.

 

[faulkneralder]

We've only moved here 3 months ago, so I can't really comment on what people previously have done. The beams look stained and varnished to me, but I'm no expert.

@montyjohn thanks very much for your suggestions, I'll keep those in my back pocket for discussions with the CO.

 

[IHateDamp]

About painted beams: https://www.periodproperty.co.uk/forum/threads/powdery-beam-what-would-you-do.20551/

 

[mh12]

Your lathe and plaster is going to be around 30mm thick I guess.

Trying to stary low profile with something that might be acceptable to the CO:

• Do nothing and repair like for like - U ≈ 33 W/m²·K (so terrible).
• 20 mm woodwool with a skim of lime: - U ≈ 4.55 W/m²·K (could be a similar profile to existing at about 35mm thick with a 10mm air-gap, very affordable, big improvement on insulation)
• 20mm Woodfibre, overlaid with 20mm of woodwool to help lime stick, and then a skim U ≈ 1.39 W/m²·K (55mm thick, so would stick out 25mm more than existing, might be acceptable????). Heat loss would be 24 times less than existing so might be able to win the CO over.

To give you an idea of how much these still fall short, current standards are U = 0.18 W/m2. you obviously can't get anywhere near this you could go from being 183X to 8X the current standard.

Fantatstic thread
Would be grateful if you could given an idea of how the approximate U values are calculated. Need to get an idea of improvement in U value for my anticiapted roof insulation but I'm oblivious at present to how much u-value will be improved OR how how much insulation will be required. (breathable insulation in mind).
Is there some sort calculator which helps achieve this. I know the PIR manaufacturers offer online calculations but unsure if there is one for breathable insulations.
Thank you

 

[larry]

Fantatstic thread
Would be grateful if you could given an idea of how the approximate U values are calculated. Need to get an idea of improvement in U value for my anticiapted roof insulation but I'm oblivious at present to how much u-value will be improved OR how how much insulation will be required. (breathable insulation in mind).
Is there some sort calculator which helps achieve this. I know the PIR manaufacturers offer online calculations but unsure if there is one for breathable insulations.
Thank you

I've used this one. https://www.vesma.com/tutorial/uvalue01/uvalue01.htm
Can't vouch for accuracy, but I've not spotted problems

It also highlights potential condensation risk. Obviously approximate as not all potential materials are listed (Rockwool is similar to wood fibre insulation in U value but of course wood fibre has thebenefit of hygroscopicity so mitiging some of the condensation risk). I'm sure there are others.

In fact I'm sure if you used Chat GPT or Gemini....

 

[Mulder]

I'm doing this exact thing on my sloping ceiling. I'm using 60mm woodfibre boards with lime finish.

You will need LBC. It helps that you can largely leave the existing plaster intact and just daub out the dips and bumps with some more lime plaster. Doesn't need to be neat as you're covering it with the boards. This may help convince the CO as it is a semi-reversible procedure unlike removing all the existing plaster (and therefore historic character).

 

[faulkneralder]

Thanks for the suggestion @Mulder . Have you finished it yet? If so, does it make a big difference during these chilly nights?