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Green Building Bible, Fourth Edition
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General: Would you lime plaster interior walls too?

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  1.  
    • CommentAuthorbgasparotto
    • CommentTimeJun 13th 2024 edited
     
    Hey folks!

    Carrying on with our 1910's Victorian style end terrace renovation we are installing wood-fibre for IWI and plastering the internal side of exterior walls with lime for the sake of breathability.

    However, would it be advised against using a cheaper gypsum plaster on interior walls? We have a long-ish hallway that sits between the party wall and the other rooms of the house which is about 44m2 of wall surface. No damp issues so far and I just insulated the floor and guaranteed good air flow underneath on the about 80cm floor void. We don't expect moisture to be created on the space but it does lead to our kitchen and bathroom (both fitted with extractor fans).

    These walls currently have the original lime after wallpapers removed. Should I do it with gypsum to save ourselves money and time or would we be shooting ourselves in the foot on the long term?

    Here is the link of the image on the right orientation: https://ibb.co/x8xbB3J
    Thanks,
    Bruno
      IMG_20240529_195018629.jpg
    • CommentAuthortony
    • CommentTimeJun 13th 2024
     
    I reckon it will be OK with gypsum plaster
    •  
      CommentAuthordjh
    • CommentTimeJun 13th 2024
     
    We have lime on our exterior (straw bale) walls. We have gypsum on plasterboard on our internal walls. Seems to have worked well.
  4.  
    If the wall has a DPC then gypsum plaster should be ok. If there is no DPC then lime plaster wold be better.
  5.  
    Thanks for the responses everyone! The walls do have DPC (original slate as it seems) and as I said, they are pretty far from the ground. Whilst I was redoing the floors I cleaned the floor gap (dozens of bags of rouble) hoping to encourage more airflow. The floor joists are also separated from the sleeper walls with a layer of slate.
    • CommentAuthorsgt_woulds
    • CommentTimeJun 14th 2024
     
    From discussions I've had with a WUFI assessor; as a skim coat over lime plaster, (if painted with breathable paint) - Gypsum plaster should still allow the wall to breathe.

    However, it is also much stiffer than lime and the difference in movement through moisture absorption might risk cracks long-term.

    Better to use something like Adaptavate Breathaplasta. I had both types in my last house and didn't experience any issues with either on internal walls but the, (uninsulated) external walls with Gypsum plaster did show surface cracking after a year.

    Regarding the IWI: Internal wall insulation will move the dew point within the construction and this may cause issues with the existing structure. The external stone or brickwork and pointing should be assessed and repaired if in poor order to reduce the passage of moisture to what will become a colder wall. Any concrete pointing should also be replaced at this time to enable the wall the breathe adequately.

    Standard U-value calculations will not correctly account for the sorption properties of wood fibres nor their ability to pass on liquid water through capillary action. This means they will always show condensation risk.

    For IWI a WUFI calculation is a useful check - this purely considers moisture issues and how the various elements of the building fabric will deal with the moisture volumes based on site-specific conditions. I always recommend a WUFI assessment for any internal insulation works, although anything below 80mm is usually fairly low-risk.
    •  
      CommentAuthordjh
    • CommentTimeJun 14th 2024
     
    Posted By: sgt_wouldsHowever, it is also much stiffer than lime and the difference in movement through moisture absorption might risk cracks long-term.
    I had missed that there was lime already on the walls. You're completly right to be concerned about the possibility of cracks I think.

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