Airtight Plaster for External Wall

wexred

Hi All,

Just doing external insualtion on a house, but I want to airtight it first from the external. I have read somewhere about an external plaster which will give airtightness. Maybe a clay based type or something.

Anybody know of any type?

Thanks

No6

The airtight layer is the inner face of the wall construction so if you have say a new blockwork wall with external insulation a wet plaster finish on the blockwork internally will form an airtight layer, you can then tape all around the window frames and plaster over it and the same of you are using a ceiling mambrane, run it down the wall and plaster over it, the idea is that is is continuous.

sas

I completely agree with everything No. 6 has stated.

To add a little more to this from my personal experience.

I also read the recommendation to plaster the blockwork externally for airtightness prior to externally insulating.

I asked my EWI company. They stated that their system was designed to bond directly to the new block work wall. If I decided to introduce an intermediate layer (i.e. the plaster), they would have to use additional mechanical fixings in order to compensate for any potential weaknesses in the plaster bond to the blocks. This incurred additional cost.

Better to keep your airtightness to the inside as described by No.6. You have to plaster that wall anyway.

Mellor

plus the mechanical fixings reduce the effectiveness of the airtight plaster slightly. Negating any benefit.

beyondpassive

Most EWI's use a primer and adhesive mortar to adhere the insulation to the substrate. Make sure the blockwork mortar is tidy, you also have to put a key over the DPC's.

sas

beyondpassive wrote: »

you also have to put a key over the DPC's.

Ah, this is an interesting one. Where do you envision that there are dpcs that would require a key on a typical new block build for EWI?

The reason being that I had a detail (my own invention) where I basically embedded 3ft dpc between the 2 and 3rd row of block on the flat. The idea being to create a stepped dpc to prevent moisture ingress at the junction of the EWI and my insulated raft.

I planned on using expanded mesh over this to provide a key for the EWI adhesive.

The technical manager for my EWI company told me to take it off because they weren't prepared to trust it. They speced a water proof adhesive at the same location which (hopefully) will do the same thing.

sas

beyondpassive wrote: »

Most EWI's use a primer and adhesive mortar to adhere the insulation to the substrate. Make sure the blockwork mortar is tidy, you also have to put a key over the DPC's.

I've never seen mention of a primer in an EWI system (NSAI certs etc) that is used between the insulation and the substrate.

beyondpassive

sas wrote: »

I've never seen mention of a primer in an EWI system (NSAI certs etc) that is used between the insulation and the substrate.

sorry SAS, I'm looking at Rockwool external insulation similar to
Attachment not found.

Also I will have a membrane outside the masonery substrate at the concrete wide slab first floor
GNH wide slab 7 July.JPG

and a membrane outside the quinnlite thermally broken rising wall, where the Quinnlite under DPC is wrapped to keep it dry, under the DPC bellcast I'm specifying XPS insulation.
GNH entrance DPC 7 July.JPG

sas

No need for sorry.

Was genuinely interested.

Very different detailing on what appears to commerical job

sinnerboy

beyondpassive wrote: »

and a membrane outside the quinnlite thermally broken rising wall, where the Quinnlite under DPC is wrapped to keep it dry, under the DPC bellcast I'm specifying XPS insulation.
GNH entrance DPC 7 July.JPG

So Bp - does the dark membrane here get turned up to be fixed through by the fixings used for the XPS layer below dpc ?

wexred

Hi All - Thanks for your replies.
I forgot to mention on my original post that it is a refurbishment upgrade porject. The client does not want to do any replastering on the inside - and the house is in goo condition and does not need such major works on the inside.

Does that make a difference to to advice?

Like SAS mentioned - the recommendation is to airtight the external walls with some plaster first before applying the EWIS, as the EWIS will not act as an airtight barrier - despite what the companies say!