Following concerns over the recommendations for insulating the interior stone walls of our house we had a thermal study carried out which suggested we should not be using racking and plasterboard with fibre glass but should be using an 8cm hemp and lime (chaux chanvre) insulating plaster. Followed by a breathable paint.
I've read so much about this and gone backwards and forwards so many times - some say using plasterboard is just asking for trouble with condensation and mould building up over time causing long term problems and others say this isn't always the case. I've attached the note from the thermal study.
Anyone got any experience of using hemp and lime plaster? I know it's almost double the price of putting in the racking and plasterboard with 10cm fibreglass and I've been told to expect a rustic finish.
I'm not expecting to get that totally smooth plasterboard finish but can you achieve an almost fine finish with a lime plaster? I'm pretty sure I've seen this in the UK.
Good site, loads of useful info on there. one of the really good points with the thermal study has been confirmation of the data supporting or clarifying our decisions on the house. The modelling programme has shown that our plans do make our ancient house a bbc house, and discussion with the expert has also shown where it is not worth putting extra money into it. Really knowledgeable and who listened to us rather than giving the usual "thats not how we do it round here" line. We are still going round the vmc options with him about simple or double flux but he backs everything up with facts andfigures which being a numbers geek is speaking my language. Its not all about payback I know some is about comfort but at least we know what energy we should be consuming and how to keep it as low as possible going forwards which is our long term aim. Invest now whilst earning for lower running costs and conmfort in thefuture.
I was probably incorrectly expecting this sort of input from the archi...hence some of my frustrations each time theyve told me something is not necessary, too expensive, too technical for me (yes they did say that) and promptly dismissed my eco concerns. Well now I have a French expert to validate the concerns with the architect instead of just little ole me and I finally feel we are making progress. I will still share my thoughts on here as your inputs and links are also really useful
Suzanne, you have had an expert and a report done so best go with their advice. I am sure you can get a fine finish with lime plaster or polished plaster to suit.
It's a complicated subject so for a good bit of information on the problems of interstitial condensation, have a read up.
Many old French houses have neither foundations nor damproofing courses. There will be, especially in winter, a lot of moisture coming up and through the walls which needs to go somewhere. When insulated the problem gets even worse, as condensation on the interior of old outside walls in winter can be considerable.
If you're going for the insulated plasterboard solution, make sure the airgap is well ventilated from the outside with airbricks and make a good airtight seal with the inside of the house.
If you choose the lime and hemp option, the humidity should look after itself. I know of a local restaurant built recently with a wood frame infilled with lime and hemp, and it's very well insulated and very comfortable and apparently cheap to heat.
We insulated our external north wall with hemp wool panels on wood frames covered with a breathable membrane, air gap and wood panelling. This gives the advantage of using the thermal mass of the metre thick stone wall as a heatstore, stabilising temperature changes quite effectively.
We went ahead with the Hemp and Lime and the comfort is brilliant, the acoustic is also really good given we have a 4m high room thats almost 90m2 and open plan. Really pleased with it. Main negative point so far has been some cracking so we’re leaving it to dry out completely over another Summer and Winter season before we decide on the final plaster finish. We’ve had some haircracks filled already before the Summer but a few more have appeared during the hot weather so not bothering to decorate for a while.
@suz - I think your post got me onto this whole Hemp/Lime thing and in discussion with @JoCo , and now that working outside doing limework is a bit iffy, I decided a few days ago to experiment inside with a small panel of hemp/limework.
It was fun and I did it with continuous formwork and mixed the chanvre 3:1:1 with chaux aérienne cl 90 (3 hemp, 1 lime and 1`water by volume). The dust from the hemp is evil I guess best done outside.
Some photos below and the process involved hacking off all the cement render to expose the stonework, setting up vertical studs and then fixing a shutter. The lime/hemp was mixed by hand in a bucket - I do have a concrete mixer but these small amounts dont deserve the machine.
Its easy work and I love casting the stuff in the formwork - it would be easy to make blocks in casts too.
Nest stage after this cures is to re-plaster with 3:1 sand/lime with a bit of horsehair mixed into the scratch coat.
The shuttering is interesting, I know a house where friends tinted the mix then just threw it at the wall and left that as the finish.
I don’t think more insulation would make much difference in most parts of our place, but what you’ve done would be interesting to try in one niche that’s below a window on the middle floor. I don’t know why the niche is there, unless the original opening was full length; but if it was, why? The house is around six hundred years old and contains many mysteries.
Perhaps the hemp niche can be one for the future, currently replacing an attic floor - no job for old men, but my wife refuses to do it.
It’s not a physically demanding job and really satisfying to do. The scent of the chaux is very nice too and the results are immediate.
Also, if it’s only a small job, you will have a big bag of lime left over with which you can make gallons of limewash paint and then spend a bit of time experimenting with colour. Mud from the garden works as do brambles and tea and whatever. All fun
You can get a good smooth finish to the top coat of chanvre, we used a largish wooden board, (I can’t remember the correct word for it), you can get plastic ones too but I preferred the wood one. If you take your time you can get a beautiful finish, plus if it starts to get difficult just give it a light spray of water.
We curved the edges into all the recessed windows and doors and around any wooden lintels and granite coins. It’s fun stuff to work with, you just need to be careful using hydraulic lime, keep some milk handy , it’s the best thing for lime burns!
The we have similar niches and including chopping the old plaster out, the whole project took about a day and a half to get so far. The final coats of proper render will be quick. Famous last words.
Thanks Debby. I use CL90 chaux Aérienne for intérieur work (the stuff that goes off in air) and the NHL (Hydraulique) on the extérieur because it needs to get wet to cure. (you knew this anyway). Can’t find quick lime or soaked lime in our region…that’s the best stuff and goes all explosive when it meets water but did a while back build a limekiln and made the stuff using limestone rocks and loads of coal or coke.
Building your own lime kiln sounds fun, any photo’s?, ( I’m being nosey).
Did you just use one layer on your walls?, we used 2 . The first layer was rough chopped hemp mixed with hydraulic lime straight onto the exposed granite walls. When it’d fully dried out we then put the finish layer on, much finer chopped hemp mixed with Aerienne lime, the more we ‘polished’ the top layer the smoother and tighter it became. Hard work but enjoyable and satisfying, wouldn’t like to tackle a job that size now though
Hiya…no photos I regret but I remember well the week long course in Fife Scotland at a place called Charleston.
The kiln was similar but a fraction of the size of this one on YouTube. I recall though making it out of bricks and then filling the gaps with mud and then more mud to insulate. Oyster and sea shell would be perfect because of their purity.
With this sample panel, I’m just going to apply a 15 to 20mm scratch coat of lime plaster without any hemp…just with horse hair or fibre glass. Then the finish coat will go on easily afterwards - the in situ hemp is dead straight so the plastering is simple and don’t need to do a first coat to rule out any inconsistencies.
