Re Mar 2013 Tim Pullens report on how to insulate old homes. He says NO VENTILATION/AIR SPACE is required between granite/lime mortar walls and internal plaster and lathe. From the eaves 1st floor i can see right down to ground level between the 2/3″ air gap and could easily fill it with hygroscopic insulation. However this conflicts with everything i’ve read and learned over the last 6 years about leaving a ventilation air gap. Is there any evidence I could obtain to support this new information please? It’s a 1929 detached granite building with lots of walls to insulate at ground level. Insulating internally and externally is not an option. I would use eco friendly breathable insulation. What would be the long term consequence of no air gap to the internal plasterwork?

  • Tim Pullen

    The answer will depend on whether this is a breathing wall or not, and that will depend on whether there is lime mortar in the wall and if the plaster is gypsum or lime. From an thermal aspect if it is a breathing wall then installing a natural insulation will work as any moisture penetration will be removed by the air movement through the wall. If it is gypsum plaster then it is not breathing and moisture will accumulated.

    But, structurally, I don’t know if lathe & plaster will support the insulation. If it is being installed from eaves level then there may be a tendency for the insulation to slump, pushing the bottom of the lathe & plaster away from the wall. I have never tried it and and I have to say that the idea makes me nervous. I have also seldom seen lathe & plaster that was in sufficiently good condition to warrant retaining. I would suggest the best solution would be to remove the lathe & plaster install hemp insulation on a 50mm or 75mm stud and cover with Fermacell boarding.

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