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Knowing Your DPM From Your DPC

Martin Cummins on knowing your DPM from your DPC

THE world appears to have gone crazy about using acronyms nowadays. Although I can’t claim to know what ROFL, LOL or OMG mean, we’re all familiar with examples such as the RSPCA and NASA, and know what these organisations do even if we don’t know their full names!
However, when it comes to acronyms in the flooring industry I have to confess that we may not always be as clear as we could be. This is especially true for products used to manage moisture levels in floors. Is it DPM or DPC? Will an MVS help me with my MVTR? This month I will consider these terms and hopefully help to shed some light on the benefits different products offer to flooring professionals.
Firstly it’s important to make the distinction between and DPC and DPM:
DPC stands for damp proof course whilst DPM stands for damp proof membrane.
A DPC is a material placed between courses of brickwork to stop the rise of water up the walls.
The types of products used can include polythene materials, pitch polymer combinations or even materials such as slate. These have minimal impact on what goes on in a floor.
The design of a DPM is to limit the movement of moisture between two areas. Structural (or base) DPMs are incorporated into most modern (from the 1970s on) buildings and provide a thin sheet or pliable barrier such as visqueen (a type of polythene sheeting) to prevent the water in the ground rising into the building.
It’s important to note that DPMs do not stop moisture entirely and a very small amount in the form of vapour will pass through. This is measured by another acronym – MVTR – which stands for moisture vapour transmission rate.
MVTR is a very useful measure for the ability of materials to slow down the movement of moisture vapour. For example, a carpet will allow a great deal more moisture vapour through than a vinyl product and therefore has a higher MVTR, whilst a thicker vinyl will have a lower rate than an equivalent thinner vinyl. In the case of visqueen the MVTR is very low and gives confidence that moisture from the ground is not an issue.
Moisture movement from the subground in buildings without a structural DPM needs to be carefully controlled. Similarly, a new screed or concrete floor contains a large excess of moisture, referred to as residual moisture, which needs to be controlled otherwise we will have problems when installing smoothing compounds, adhesives and floorcoverings.
We control the ‘problematic’ moisture vapour by applying a layer of material which has a lower MVTR than the materials placed above, which means that the moisture levels due to vapour will not get high enough to affect the flooring system.
Traditionally we have referred to such products as surface applied DPMs. Most manufacturers have history and experience of what their own DPM can achieve, and some will vary the coverage rate required to give a thicker layer, for example if a thick rubber flooring is being used, if underfloor heating is present (this accelerates movement of moisture vapour) or when the relative humidity levels are high.
Historically the use of thin film epoxy DPMs has been the most effective and even enables use on buildings where a base DPM is absent (provided hydrostatic pressure is not significant). Not only do they have extremely good MVTR per application thickness but epoxy materials work by chemical reaction and cure rather than dry out. This results in an extremely strong bond to the substrate giving added security.
Recent advances in technology have enabled manufacturers to look at alternative systems such as single-component dispersion products, which are often classed as MVS (moisture vapour suppressant) rather than DPM. This is a good example of how we manufacturers need to be clearer in explaining which products do what.
The nature of MVS products mean they are best used in dealing specifically with residual moisture present from new screeds and concretes but they basically perform the same function as a ‘traditional’ DPM.
As with smoothing compounds or adhesives it’s about understanding the specific demands of a project and matching this to a particular product designed for that application. So in this sense the term MVS is confusing and is arguably redundant.
My opinion is that we are better off keeping to the one phrase (DPM) for this type of product and making sure that packaging and technical support are working together to help guys on the floor get the right product for the job.
So, in summary, DPM or MVS are materials reducing the rate of transfer of moisture vapour and have their place in flooring projects. All products have an MVTR, the lower the MVTR then the more resistant it is to the passage of moisture vapour. DPCs are not DPMs. Oh, and I’m afraid NASA doesn’t stand for Now Another Scintillating Article!
Martin Cummins is UK technical support manager at Bostik. T: 01785 272625

This article has been reproduced from the Contract Flooring Journal. You can find them at