Acrylic wall paint on an internal surface

[james bertram]

Good morning (best wishes for the New Year)

I have been reading "Moisture buffering effects on indoor air quality - Experiments and simulation results" and would appreciate some clarification on how to best model acrylic paint as an interior surface coating on a wall.

The paper states "...paint on the surface (2 x acrylic) acted almost as a vapour barrier and effectively limited moisture transfer between the structure and the indoor air..." and also that the "surface transfer coefficient" probably has a bigger effect on reducing vapour flow than permeance.

Given the above, can I use "Surface transfer coef / Interior surface / Sd value" to replicate the likely affect of a primer (alkyd) + 2 x coats of acrylic paint on the interior surface, and if so, what setting would allow for the combined affect of the above factors?

Thank you

[fostertom]

Good and important question. I often wonder what happens to 'breatheable' constructions when a few yrs later it gets redecorated with typical warehouse emulsion!

Perhaps a supplementary question is - what are the vapour permeabilities of typical commercial paints, or common-language paint types - which are 'safe' and which should ring warning bells!

[james bertram]

G'day Tom - good to hear from you.

The referenced paper suggests paint permeability data might not be a useful because "surface transfer coefficient" appears to inhibit vapour flow, resulting in "painted plasterboard" having virtually zero vapour (room-to-wall and wall-to-room).

Hence the questions as to the settings needed to replicate the likely "real" outcome.

[antretterf]

Hello James,

you can use the "Surface transfer coefficient / Interior surface / Sd value" to account for the additional resistance for the water vapor flow due to the paint on the interior surface. The value highly depends on the used kind of paint and manufacturer. Therefore I recommend to ask the paint manufacturer for appropriate values.
The water vapor surface transfer coefficient is modelled in WUFI automatically. Since the ambient conditions affect both boundary layers in analogous ways, there are similarity relations between water vapor transfer and heat transfer. The water vapor transfer coefficient is just the convective component of the heat transfer coefficient multiplied by 10 by minus 9. A detailed description is found in the WUFI help under "Water Vapor Transfer Coefficients".

So to model the combined effect you just need to adjust the inner Sd-value to reflect the permeability of your paint (and primer). The water vapor surface transfer coefficient is already accounted for.

Florian

[james bertram]

Good morning Florian - thank you for the clarification.

[olivir]

Hi everybody,

Do you have some mu example for paintings ? There's nothing in wufi databases, isn't it ?

[james bertram]

G'day Olivir - I found information a bit hard to find.

This paper (http://en.ktu.lt/content/impact-hydro-p ... neral-wool) focusses on external paint applied to a permeable render...and has some good data on the likely Sd values of several paint types.

Good luck

[ALC]

Hi James,

The link you posted doesn't work anymore, do you have an updated one please?