# Internal plastering for airtightness



## boots (10 Jan 2011)

Hi All,

We are a few weeks away from putting in windows to our new build, (concrete & block construction with UFH & MVHR).
As part of achieving a decent level of airtightness, I am researching putting a coat of plaster on the internal face of the external walls.
Does anyone have experience of this - if its worth while and any pro or cons?
An architect mentioned to me that he had heard of a specialised plastering product which would be even better than conventional plaster, does anyone know this product?
Thanks in advance
Boots


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## onq (10 Jan 2011)

Nope, but at Plan Expo 2009 one of the contributors confirmed that a pressure test had been done on a 1950's internal wet plastered house and it was up to current standards of air-tightness with the doors and windows taped.

In other words, a standard wet plaster finish, properly detailed and applied, matched what was required in the building regulations.
Obviously you'd need to replace the old doors and windows to achieve current standards without the tape.

Now just remember its apples and oranges when you're comparing this with drywall construction.
Wet plaster means chases in walls for services and no hidden voids behind the plaster.

That's why its so airtight - its not a timber frame construction.
Its solid with all the holes totally filled with plaster.

Trouble is, you need a good plasterer for this.
Not many of that rare breed around now.


ONQ.

[broken link removed]

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## Sconhome (11 Jan 2011)

AS onq says, its wet work.

After chasing and positioning cables, service pipe etc, photograph their locations.

The blockwork has to get a scud coat of sand cement & lime to provide a key for the scratch coat render. It is this scratch coat in conjunction with the tops skim coat of plaster that achieves the air tightness.

The down side is the amount of time to allow the layers to dry correctly. As you are drying into the rooms as opposed to drying off plasterboard it will take additional time, also the additional wet work of scud and scratch coat layers increase the overall time.

Depending on your construction method air tight membrance, sealants and tapes under plasterboard with tape and filled joints may suit. The benefit here is the minimal amount of wet work.

A note on wet work render and plaster is that it has to be done in mild weather as frost will cause glazing of the finish. In colder weather it also takes ages to dry out as there is nothing to draw the humidity. So watch the forecast.

The point on photos is important as reference for 6 months later when the rooms are painted and you are looking to hang pictures . . .


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## boots (11 Jan 2011)

Thanks for that Guys,
some food for thought.

I should have added that these walls will also be getting covered with insulated plasterboard, which will then be plastered - either tape & joint or skim, not sure if that makes a difference.

_Depending on your construction method air tight membrance, sealants and tapes under plasterboard with tape and filled joints may suit. The benefit here is the minimal amount of wet work.
_Maybe this is what I should consider instead, I didn't know that air tight membranes could be used over blockwork...

_The blockwork has to get a scud coat of sand cement & lime to provide a key for the scratch coat render. It is this scratch coat in conjunction with the tops skim coat of plaster that achieves the air tightness.
_

My builder has given me a quote of € 17 / m2 to carry out one coat only, approx 10-12mm.

Points about the extra drying time and the need to take the photos are also well made, thanks.

Boots


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## fclauson (15 Feb 2011)

*Dew point*

Make sure you check where the Dew point falls before you use insulated plasterboard

it can cause problems 

Francis


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