The difference between a damp proof membrane and a vapour barrier

In relation to sensitive (meaning they are directly effected by moisture) floor coverings like engineered wood and laminate floors that are installed on top of an underlay, there is a huge misconception about how to protect them from rising moisture from any sub-floor. Mainly solid sub-floors such as concrete. This article is going to make clear the difference between a damp proof membrane and a vapour barrier that are considered the best ways to deal with rising moisture and protect sensitive floor coverings from associated damaged. 

If you’re in the market for purchasing an engineered or laminate floor or any sensitive floating floor covering for that matter, you would no doubt have found information relating to underlays with built in moisture/vapour barriers. The misconception is that these moisture/vapour barrier underlays act as an effect damp proof membrane to prevent damage to your floor covering as moisture rises from the sub-floor. This is incorrect and widely misrepresented (unknowingly) by sales staff and even people deeper within the flooring trade.

Now let’s briefly look at the difference:

Damp proof membranes (DPM) 

In the context of this article, a DPM, as the name suggests, is typically a water proof barrier in the form of a thick plastic sheeting designed to prevent the passage of moisture through it.

These are normally used beneath a foundation concrete slab. As with most things in life, everything has it’s limits and subject to enough pounds per square inch (psi) of pressure, a DPM will allow moisture to pass. For example, should a low lying area be subjected to flooding, this will raise the psi on a dpm.

Therefore, as can be seen, the wording itself is slightly misleading. However, in normal conditions most DPM’s will prevent moisture from passing and do so effectively for many years, covering moisture contents up to 100% relative humidity.

Vapour Barrier

A vapour barrier that is attached to a wood or laminate floor underlay is designed to prevent the passage of moisture up to 75% relative humidity (RH).

A vapour barrier should never be confused with a dpm, but can be confidently used as an effective moisture barrier ONLY when the surface relative humidity is known and is within the tolerance above. The surface relative humidity can be found by using the correct testing equipment.

The surface relative humidity cannot be known simply by visually inspecting a sub-floor and coming to the conclusion it is dry by sight and/or touch. This off the cuff assessment has caught many people out.

Summary

Vapour barrier undelays are often blindly prescribed to effectively stop any moisture related problems arising from a damp sub-floor. However, now you understand that there are limits to how effective they can be.

When spending a great deal of money on a moisture sensitive flooring product and a great deal of time and upheaval installing one, understanding the limitation of an accompanying underlayment is absolutely crucial!

Underlay packaging information can be very appealing and seem simple with phrases like ‘moisture protection’ and ‘damp proof’. The emphasis here are the words moisture and damp, which when thought about, are extremely vague terms indeed. Now you know!!

If you are unsure and prefer to fully protect your new floating engineered or laminate floor, you can find out how to do that by reading my article ‘How to stop sub-floor moisture from damaging your new wood floor.

 

© Copyright 2013 Wes, All rights Reserved. Written For: Fitmywoodfloor
About

I am a Pergo trained professional installer of 20 years. I've been up close and personal with lots of floors and have the knobbly knees to show for it...Should you have any questions or comments please feel free to add them below. Thanks for taking the time to call by and I hope the information you've found has given you some insight!........................................................................................................................................................................“When you click on links to various merchants on this site and make a purchase, this can result in the site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network, amazon.com, amazon.co.uk, amazon.fr, amazon.de, amazon.it, amazon.ca, affiliate window network.” This statement is to comply with current internet regulations regarding transparency to consumers.

Posted in Engineered floor fitting, Installation and project guides, Laminate floor fitting, Sub-Floor preparation, Uncategorized, Underlays Tagged with: , , , , ,
  • Andy

    Hello. I purchased a laminate flooring and an upgraded Republic Flooring foil lined underlayment. The laminate flooring will be installed in a room with a raised foundation and plywood subflooring. Which direction should the underlayment be installed, foil side down or up? I have read conflicting articles. I would appreciate your expertise and any sites you may be able to reference. Thank you

  • Hi Andy,

    Typically, we find most manufacturers are advising foil side up with the seam taped with ‘vapour tape’.

    However, don’t take my word for that as some do differ. I would advise you to contact the supplier (Probably an obvious statement and you already have), or alternatively the manufacturer of the underlay. There should be installation guidelines in the underlay packaging, but it wouldn’t surprise me if there wasn’t.

    Sorry I couldn’t have been more help.

    Regards,

    Wes.