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Why not to use DPM over existing floorboards.

We recently received the following question


I'm planning on laying a wooden floor throughout the ground floor of my house (a 1930's terrace), on top of the existing suspended floor boards. I've read your guide and now feel much better prepared!

That said, I do have one question. You state (quite clearly) that when laying a new wooden floor on top of existing floorboards, you should not use a DPM because it might cause condensation between it and the underfloor. But if I don't use a DPM then surely this moisture would just pass into the base of the new wooden floor, potentially causing problems there instead? I'm a little confused on this point! Any chance you could clear this up for me?

Many thanks


Non-concrete/screed Floors

Thank you for your question.

The reason for not using a DPM when installing onto existing floorboards is that this would block the normal ventilation that should exist underneath the existing floorboards (the void which should have air gaps in the walls). Any moisture that normally evaporates harmlessly through the boards into the air will be trapped against the DPM and might cause rotting of the existing floorboards and even your joists.
Using a normal foam underlayment in these cases makes sure the moisture - every home has more or less moisture during the seasons - will now still be able to evaporate through the whole construction.

When your underfloor is level a 2-3mm foam is sufficient. If you need more sound-insulation it's best to use a thicker underlay (brown roll in picture, also available with DPM attached to it for concrete floors).

Whatever kind of underlayment you need, don't cheapskate with the materials you use: it'll cost you in the end (inferior materials = more time correcting plus the end result of your natural real wooden floor could sound like a 'plastic' Melamine Laminated floor).

Constantly updating our guides


It is thanks to serous DIY'ers like Richard we are able to constantly update our guides when they feel/think information is missing or we could have explain items better. Being professional installers of wooden flooring does sometimes mean we overlook the fact that some terms or methods are daily tasks for us but not for our clients.

We love your input on these matters, so feel free to help us help you.

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Jon Black

You mention air bricks for ventilating the sub-floor void. Does that mean that the embargo on using DPM over a wooden underfloor does not apply to the upper floors of a house? I'm about to lay an engineered oak floor in a bedroom which has an underfloor of ordinary pine floorboards. I already have a roll of heavy foam underlay which is bonded to a foil sheet so I assume this will act as a DPM.

Karin H.

Hi Jon

Thank you for your question. Even upstairs it could block the normal air flow, depending on the rest of the construction.
The foil could indeed be a DPM so you're taking a slight risk using it.

Kind Regards
Wood You Like Ltd
Karin Hermans

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