Final Render Coat - is this breathable or not?

BigGeorge · 10-10-2011 05:41PM #1

May need to apply an acrylic render to the outside of my timber frame build & have been told that the final layer had a water vapour permeability of ' <1m equivalent air thickness'; the later in question will be 1mm thick. Have not heard of this unit of measurement before.

Basic question - is a 1mm think render with this water vapour permeability breathable or not

sydthebeat · 10-10-2011 05:48PM

BigGeorge wrote: »

May need to apply an acrylic render to the outside of my timber frame build & have been told that the final layer had a water vapour permeability of ' <1m equivalent air thickness'; the later in question will be 1mm thick. Have not heard of this unit of measurement before.

Basic question - is a 1mm think render with this water vapour permeability breathable or not

what is this acrylic render being applied on? Is it EWI outside a Timber frame?

acrylic renders are highly vapour resistant.

In the stanard timber frame, where there is a weathering leaf, the timber frame should be vented in the cavity behind the weathering face.
Breathability is important to allow vapour to travel from the internal to this cavity, and vice versa. The vented cavity then exhausts this vapour away.

BigGeorge · 10-10-2011 06:03PM

Absolutely fair questions & its a long story as to why this is its now needed. Dont wish to go over that here.

The wall buildup is very breathable with a ventilated cavity on the exterior almost everywhere. This will be applied on the outside of the existing render layer - long story again. Bottom line is I need to know if the material of these characteristics is really breathable or not, its just a unit of measure I;ve not experienced previously & have no idea how it compares to other materials

BryanF · 10-10-2011 07:13PM

BigGeorge wrote: »

Absolutely fair questions & its a long story as to why this is its now needed. Dont wish to go over that here.

The wall buildup is very breathable with a ventilated cavity on the exterior almost everywhere. This will be applied on the outside of the existing render layer - long story again. Bottom line is I need to know if the material of these characteristics is really breathable or not, its just a unit of measure I;ve not experienced previously & have no idea how it compares to other materials

is it a rainscreen?

acrylic render is generally used in EWI of older homes where mineral wool is being used, there has been some debate on its actual breathablity, i have my doubts.

these guys seem to have gone for lime on their EWI http://constructireland.ie/Articles/Passive-Housing/19th-century-ruin-renovated-with-passive-house-aims.html
maybe an option?

sydthebeat · 11-10-2011 10:17AM

BigGeorge wrote: »

Absolutely fair questions & its a long story as to why this is its now needed. Dont wish to go over that here.

The wall buildup is very breathable with a ventilated cavity on the exterior almost everywhere. This will be applied on the outside of the existing render layer - long story again. Bottom line is I need to know if the material of these characteristics is really breathable or not, its just a unit of measure I;ve not experienced previously & have no idea how it compares to other materials

Acrylics are not breathable, in my understanding anyway.

As bryan says lime renders are by far the most breathable external finish.

Breathability is usually measured in sd value, and called water vapour transmission (tested to EN ISO 7783-2). Sd-value is the vapour diffusion-equivalent air layer thickness. The lower the sd value the better.

Your product has a sd value of <1, which is about standard for acrylics i would think.
There are silicone based renders which have better sd value (<0.4 to <0.35)

Final Render Coat - is this breathable or not?

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