Type of underfloor is type of underlayment

Based on the 'floating' installation method it depends on the type of underfloor you have what kind of underlayment you need for your wooden floor. (Floating means: wooden floorboards not fixed to the underfloor with adhesive or nailed onto joists/on existing floorboards or sheet material, and where the T&G is glued with PVAC wood glue.)

Concrete floors = DPM plus sound-insulation (DPM stands for Damp Proof Membrane). More and more sold as a 1 in 2 product with handy self-adhesive overlapping strip of DPM. (Blue roll in picture)

The DPM layer prevents any residue moist from the concrete getting into the wood. Not really needed on concrete underfloors on second floors, but it won't hurt.
The attached foam layer acts as an sound-insulation preventing your real wooden floor 'sounding' like a Melamine Laminate floor and has the ability to 'fill' some minor unevenness in the underfloor.

In existing floorboards, plywood or chipboard underfloors DON'T us DPM.
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.
Note: when you have vinyl (or lino, marley tiles) as floorcovering and you do not/can not remove this before installing a new wood floor, you're better off using an underlayment that does contain a DPM - see above - to prevent any "sweating" of the original floorcovering effecting your new wood floor.

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).

Question received
Hi
We are thinking of laying a wood laminate floor in our rented house. At the moment it is carpeted, with underlay, on top of concrete. The carpet is quite thin. If we give the carpet a good shampoo, is it ok to put the floor on top of the carpet, to all intents using this as an underlay for the floor.

Our answer:

Using a carpet underlay or a carpet as underlayment for wooden flooring is asking for trouble I'm afraid.
Cleaning a carpet will never get rid of all the dust, dirt (and bugs) it has gathered over its lifetime.

Best is to remove it, including the carpet underlayment and start 'a fresh'.

A wide selection of natural wooden flooring: From Basic Oak to Bespoke can be found in our secure webshop

Buy the 160 page Wood Floor Installation Manual created by Wood You Like. Filled with tricks of the trade to install your own floor like a professional!

Paperback £ 17.97 (plus £4.25 p&p) (secure Paypal payment)

E-version available too! See here for more information and all three purchase options