To discuss your project give us a call on 01392 861763

Sustainable Building Forum

The place to share best practice advice about all things sustainable building. Use the search icon to find the topic you’re interested in or register as a member to comment on topics and ask your own questions.

Notifications
Clear all

If I have cement render inside or outside of my solid walls, should I strip it all off before insulating?

 
Chris Brookman
(@chris)
Member Admin

Cement renders and plasters are actually not that vapour impermeable when compared to limes and so assuming the external surface of the wall is in good order and not letting in any rain then there should be no need to remove an external cement render. In some cases the render is actually beneficial as it will prevent rain driven moisture from penetrating the walls, keeping the insulation drier.

Internally cement plasters are also not a problem as they can often provide a useful surface into which the insulation fixings can hold. Removing the cement plaster can also do a lot of damage to soft brick and stone and so we usually suggest it is left in place. However, any pink gypsum plaster should be removed as it can attract moisture and cause elevated moisture levels in the insulation, resulting in decay in wood fibre insulation or other natural fibre insulations.

If the existing render or plaster is failing then it would be best to remove it and replace it with a lime based render, such as one from the Baumit range. Contact us for more details.

Quote
Topic starter Posted : 09/03/2021 5:44 pm
cognis1
(@cognis1)
Member
Posted by: @chris

However, any pink gypsum plaster should be removed as it can attract moisture and cause elevated moisture levels in the insulation

Hi Chris,

I have a 1960s bungalow which I want to bring to near Enerphit standards. It has cavity walls with Thermalite block inner leaf and gypsum plaster. Rather than attempting to remove all the plaster, which might take much of the blockwork with it ~ can I instead punch holes in the plaster as say 300mm centres to allow the wall to breathe, prior to affixing the internal wood-fibre boards?

ReplyQuote
Posted : 30/11/2021 5:00 pm
Chris Brookman
(@chris)
Member Admin

@cognis1 Hi. Gypsum plasters are perfectly breathable but they do tend to hold moisture and so if they are in the zone where condensation will form (which they would be in your case if you had standard concrete block) they will tend to retain more moisture than you'd want. However, because you have lightweight blocks on your inner leaf you could retain the gypsum plaster as it will not be near the dew point in the wall. You should still use a layer of the Baumit Viton Fein Plaster on the back of the wood fibre boards first though as the wood fibre would be best kept away from the gypsum plaster.

Sorry, slightly long winded but hope that helps.

ReplyQuote
Topic starter Posted : 01/12/2021 11:16 am
cognis1
(@cognis1)
Member

@chris Good information, thank you.

ReplyQuote
Posted : 01/12/2021 2:17 pm
Share:

CPD Training

We offer CPD training for architects practices across Devon and Cornwall.

Software Modelling

We offer dynamic heat and moisture modelling as well as condensation risk analyses.

Building Plan Review

If you have some plans and would like our opinion on which materials would be the best for you, contact us.

Overview

Back to Earth is a natural building material supplier. We're not your typical builders'​ merchant though and instead work closely with our clients during the planning, designing and building phases of projects.

Back to Earth is a proud signatory of the Anti-Greenwash Charter:

© 2018 Back to Earth