SBMS Posted June 25 Share Posted June 25 (edited) Possibly a stupid question that demonstrates my lack of understanding but… is a VCL required or recommended on a masonry cavity wall, and if not why not? Speaking mainly about an EPS bead full fill cavity. I sense not, and my guess is down to the wall makeup versus timber frame where the vapour permeability goes from low to high to encourage ‘drying out’.. does this principle not apply to masonry? Or just not to the same amount? Edited June 25 by SBMS Link to comment Share on other sites More sharing options...
nod Posted June 26 Share Posted June 26 Difficulty of installing Link to comment Share on other sites More sharing options...
SBMS Posted June 26 Author Share Posted June 26 36 minutes ago, nod said: Difficulty of installing So not required? Link to comment Share on other sites More sharing options...
nod Posted June 26 Share Posted June 26 1 hour ago, SBMS said: So not required? NO You can get the same effect by using a foil backed T&G cavity board and making sure all the joints are tight and taped Link to comment Share on other sites More sharing options...
SBMS Posted June 26 Author Share Posted June 26 10 minutes ago, nod said: NO You can get the same effect by using a foil backed T&G cavity board and making sure all the joints are tight and taped Thanks nod - but we're using blown in beads, not cavity board. Anybody had to look at VCL with a cavity and blown beads - or a cavity with a full fill rockwool for example? Link to comment Share on other sites More sharing options...
Mr Punter Posted June 26 Share Posted June 26 I am using blown EPS beads on a project. The only issue is that fairfaced brickwork may require a coat of Stormdry to prevent moisture ingress. No need for a VCL as the dew point is near the outer leaf. Link to comment Share on other sites More sharing options...
SBMS Posted June 26 Author Share Posted June 26 1 hour ago, Mr Punter said: I am using blown EPS beads on a project. The only issue is that fairfaced brickwork may require a coat of Stormdry to prevent moisture ingress. No need for a VCL as the dew point is near the outer leaf. That makes sense - so does the dew point move further toward the outer leaf based on the size of the cavity? Does this make denser full fill (like cavity batts) higher risk (I understand there's a problem of wicking across the cavity, but am I right in thinking interstitial condensation is moisture travelling from the inside out? Link to comment Share on other sites More sharing options...
SteamyTea Posted June 26 Share Posted June 26 4 hours ago, SBMS said: so does the dew point move further toward the outer leaf based on the size of the cavity Yes and no. The dew point is just somewhere at the right temperature, humidity and air pressure for water vapour to condense. Take an extreme example, low humidity and high air pressure, and a badly insulated wall. The dew point may be outside the wall i.e. on the cold face. The counter is a colder, well insulated wall, high humidity and low air pressure. The dew point may be on the inside. Those examples are regardless of walk/insulation thickness. Just the thermal conductivity. In reality, a VCL is fitted internally (in the UK climate), and the living area has controlled ventilation. Controlled ventilation replaces warm, humid air with less humid air (less mass of water overall). Link to comment Share on other sites More sharing options...
SBMS Posted June 26 Author Share Posted June 26 19 minutes ago, SteamyTea said: In reality, a VCL is fitted internally (in the UK climate) @SteamyTea is this correct - that a cavity masonry should have a VCL then? Link to comment Share on other sites More sharing options...
SteamyTea Posted June 26 Share Posted June 26 1 minute ago, SBMS said: @SteamyTea is this correct - that a cavity masonry should have a VCL then? Not the cavity as such. Put a VLC on the warm side i.e. inside the house. Make sure the cavities are properly ventilated, but not draughty, and all will be fine. The outer leaf should act similar to a wind tight, but moisture open layer, but that does depend on choice of materials and quality of build. Link to comment Share on other sites More sharing options...
SBMS Posted June 26 Author Share Posted June 26 18 minutes ago, SteamyTea said: Not the cavity as such. Put a VLC on the warm side i.e. inside the house. Make sure the cavities are properly ventilated, but not draughty, and all will be fine. The outer leaf should act similar to a wind tight, but moisture open layer, but that does depend on choice of materials and quality of build. Interesting. Any others out there putting a VCL on inside leaf of masonry? Will chat through this with architect, see what they think. Link to comment Share on other sites More sharing options...
Mr Punter Posted June 26 Share Posted June 26 1 hour ago, SBMS said: Interesting. Any others out there putting a VCL on inside leaf of masonry? Will chat through this with architect, see what they think. There is not much point if there is no risk of interstitial condensation. If you had a this layer of Rockwool between battens on the inside you could staple on some polythene before you plasterboard. Paint can be fairly vapour impermeable. Most insulated lofts have no VCL and no issues and they are on the horizontal plane which is more vulnerable. 1 Link to comment Share on other sites More sharing options...
SteamyTea Posted June 26 Share Posted June 26 19 minutes ago, Mr Punter said: Most insulated lofts have no VCL and no issues and they are on the horizontal plane which is more vulnerable Not so sure about that. Google Search Link to comment Share on other sites More sharing options...
ADLIan Posted June 26 Share Posted June 26 No need for AVCL in a cavity wall, full or part fill - not sure how it would be installed! Wall may show slight condensation risk but will still comply with BS5250 on condensation & moisture in buildings. Stop worrying about it. 1 1 Link to comment Share on other sites More sharing options...
JackofAll Posted June 26 Share Posted June 26 Can only add that we didn't add a vcl on the inside of a full fill mineral wool cavity, also didn't ventilate the cavity. 1 Link to comment Share on other sites More sharing options...
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