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.