Dew Point - What is it and why does it matter

[Dreadnaught]

A warm welcome back @SteamyTea!

[SteamyTea]

Thanks @Dreadnaught

[newhome]

7 minutes ago, Dreadnaught said:

A warm welcome back @SteamyTea!

 

Indeed! Where have you been? We thought you’d been abducted by aliens! ? ?

 

 

[SteamyTea]

@newhomeStill at the other end of the country from you.

[Tennentslager]

Welcome back old boy @SteamyTea

Missed your dodgy innuendo and bad taste jokes?

[Gone West]

Good to hear from you again @SteamyTea, thought you had gone for good .

[Ed Davies]

2 hours ago, SteamyTea said:

Do a statistical model, some things just cannot be sorted out with a relatively simple equation.

 

Hi Steamy ?

 

I only partly agree. For some renovations where you're stuck with materials not selected with further insulation in mind then it's probably true that you need to play with WUFI or the like to get much confidence. With a new build, though, a simple equation should be enough to see if the build up is robust, that is if any liquid water gets in in some way then it's likely to evaporate and disappear reasonably quickly.

 

In the absence of any further input from @Triassic the particular rabbit hole @TerryE and I went down was whether or not the straight lines in @A_L 's March 11th graph was reasonable. It might be worth going back to attempt to address @Triassic's original question, though.

[Triassic]

I asked the original question from a point of ignorance of how this whole moisture thing works. I’ve got a membrane on the outside of the timber frame, with the frame filled with a plastic insulation and a waterproof membrane on the inside. Will moisture even get inside this plastic bag if it’s sealed up?

[Ian]

8 minutes ago, Triassic said:

I asked the original question from a point of ignorance of how this whole moisture thing works. I’ve got a membrane on the outside of the timber frame, with the frame filled with a plastic insulation and a waterproof membrane on the inside. Will moisture even get inside this plastic bag if it’s sealed up?

Vapour pressure will generally be from inside toward the outside so best practice guidance is that the materials on the warm side should have a total vapour resistance of at least twice the sum of the vapour resistances on the cold side (BS 5250:2011). IE in practice if you are using membranes, then the outside one would be a breather membrane and the inner one should be impermeable.

[Triassic]

@Ian you describe exactly what I am doing in my build. So no need to worry about Dew point problems!

Edited April 29, 2019 by Triassic