Vapour Control Layer Behind Moisture Resistant MDF

Hi

I am building a timber framed construction :

100mm outer block - 50mm cavity - 9mm sterling board with breather paper - 140mm timber stud with glass wool insulation.

We are going to finish the inside with MR MDF and was wondering if we still needed to fit a vapour control layer as the MDF is moisture resistant ?

Cheers

Hi

Moisture resistance is not the same as vapour resistance, moisture equates more with water whereas vapour equates more with air, and its the vapour phase of water that causes the majority of problems with both surface and interstitial condensation, in your particular circumstances it is the vapour pressure of the air penetrating the fabric of a building that will cause the problem with interstitial condensation.
This is a winter problem: the warm air will eventually meet the cold external zone/face of the insulation, the air will then reach its saturation point and moisture will be deposited within the affected zone of the insulation which will significantly reduce the effectiveness/U value of the insulation.

The vapour barrier must be on the warm/room side of the insulation as the wall is unlikely to ever be sufficiently cold enough for interstitial condensation to occur (in normal conditions).

Not sure why you are using MR MDF rather than plasterboard - plasterboard being a far more inert material than mdf, also, you are unlikely to get movement cracks with plasterboard - if you are dead set on using mdf it should be of the fire resistant variety!!

You may also want to consider the sound absorption characteristics of the two materials - there would be nothing worse than living in a 'tin drum'!

Regards

Many thanks.