Installing Junckers floorboards on ground floors with suspended joints

Should I use a vapour check?

We are regularly asked if a vapour check should be used above floor joists where the joists are suspended above a ventilated void (ie with air bricks and sleeper walls) at ground floor level.

The answer to this question is; yes, you may need one, but it is important to be clear about the reasons why, so I hope the following will help to make your decision easier.

The vapour check can be Junckers SylvaThene or similar and it will need to be fixed to the tops of the joists prior to nailing the floorboards in place. It is important to understand that this is a vapour check/ VCL and not a damp proof membrane or protection from rising moisture from below.

The vapour check layer WILL -
- prevent condensation beneath the floorboards by stopping warmer moisture-laden air from the room above migrating down through the floor and causing condensation where it meets colder air in the void. Condensation in the floor void can rot the joists.

However it will NOT -
- prevent floor failure if there is a damp problem or unusually high humidity in the floor void. Problems like this may be a result of rising ground moisture or poor ventilation for example. If such a situation exists the vapour check may offer some limited short term protection to the hardwood floor boards-, but it will not prevent the joists from rotting, and may in fact even accelerate the process.  The solution in this scenario is to tackle the underlying problem eg by increasing ventilation or using appropriate damp proofing measures.