Damp-proof membranes

Protecting yourself against the perfect storm.

By Neil Roberts Market manager Tremco.

C LIMATE change is rarely out of the news these days. Following another wet UK winter punctuated by the ‘beast from the east’ and the subsequent ‘mini beast’, not only are hundreds of contractors struggling to make up for lost time onsite, but developers are also faced with building on land where water tables remain at unprecedented levels and specifiers must take into consideration the long term effects of moisture before, during and after the construction process.

This ‘perfect storm’ means building professionals must confront an even greater challenge than normal in dealing with damp substrates and their potential to jeopardise the long-term performance of interior finishes, especially floorcoverings.

Of all building elements, flooring installations can be some of the most problematic, with callbacks all too common. In fact, technical information published by the construction industry research and information association (CIRIA), suggests screeds and flooring have one of the highest rates of failure of any construction material with the key culprit being the effects of moisture.

The main issues to be considered are:
The effects of moisture
Moisture in concrete
Testing methods
Selection and application of a damp proof membrane

Effects of moisture
Moisture damage creates an unhealthy environment and can be a major contributor to so-called sick building syndrome. If moisture is allowed to penetrate a building unchecked it can rapidly lead to damage of floorcoverings as well as the less easy to quantify effects on the occupants of buildings; whether they be tenants or employees.

Excessive moisture may cause vinyl floorcoverings to blister and plasticiser migration may be encouraged (this can result in shrinkage of the floorcovering and/or degradation of underlayments and adhesives).

Loss of plasticiser can also cause the vinyl to become brittle and thus reduce its life expectancy. Water borne adhesives are also more prone to emulsify in damp conditions. Meanwhile, parquet or timber flooring is quite likely to warp. In some cases, there could be mould growth and even odour.

When things start to go wrong the costs of rectification can be enormous – often up to six times as much as the original contract value.

Key factors include downtime of an area, perhaps even an entire building; loss of use, possibly trading and of course revenue; relocation of people; and the always worrying health and safety implications.

So, what are the early warning signs? One easy to spot sign of moisture ingress is what’s known as surface measles – small bumps in the floor surface. These look like small hillocks in a flat plain.

Cracks may also start to appear in poorly prepared screeds. Corners and edges of floorcoverings may begin to peel back as the adhesive emulsifies.

Mould can start to grow in the adhesive residue, reducing the effectiveness of the bond and eventually breaking it down completely.

Moisture in concrete
Although concrete and screeds are often subject to foot and other traffic within days of being laid, in practice concrete can take a surprisingly long time to dry. Even under good drying conditions, concrete bases of 150mm or more often take a year to dry if there is only one surface exposed to air.

Testing methods
The most common testing methods for damp in concrete include:
Surface hygrometer
Humidity box
Humidity sleeve

The advantages of a surface hygrometer are that it’s non-invasive (no need to make holes into the screed) and it’s also the test method favoured by British Standards. But taking a reading from a hygrometer is a long, drawn-out process. With a sand/cement screed it can often take up to four days. With power-floated concrete it could be 10-14 days before a meaningful reading can be recorded.

Moisture measurement systems (MMS) that use a humidity sleeve are also invasive, but they’re quick and reliable.

The humidity box with MMS doesn’t require holes to be made in the floor and it complies with BS 8201, BS 8203 as well as BS 5325 codes of practice. But if this system is used it has been found there’s a need for frequent re-testing and the humidity box can easily get damaged onsite.

Selection and application of a damp-proof membrane
A surface DPM, which bonds to the surface of the substrate, minimises moisture vapour transmission. It therefore prevents moisture build-up beneath vinyl or other floorcoverings.

DPMs can be epoxy-based (two-part), water-based (single-part) or sheet membranes.

To determine the most suitable and cost effective surface DPM for any application, it’s essential to determine the relative humidity and the moisture vapour transmission rate of the substrate and floorcovering.

For two-part DPMS, correct mixing is critical if the product is to perform correctly. It’s also vital to have skilled contractors who’ll be able to ensure the DPM is correctly applied.

Leading manufacturers including tremco illbruck have made substantial investment in recent years to bring new generation, water-based moisture vapour suppressants to the market that set new standards in sustainability.

Top performing DPMs should be BREEAM-accredited as well as ECI compliant, produce very low levels of VOCs while being ideally suited to suppressing residual moisture in cementitious sub-floors including power-floated concrete and sand/cement screeds.

They can be used where RH readings are up to 95% (ideal for refurbishment situations where a DPM already exists). Significantly, such products facilitate the rapid installation of new floorcoverings in situations such as social housing where tenants need to have access restored to kitchens or communal corridors.

Single component membranes mean there’s no mixing required which makes onsite application easier, while fast drying times reduce walk-on time and speed up the all-important laying of floorcoverings as well as follow on trades in other areas of the building.

An additional advantage is, as a single component product, any fluid remaining can be resealed and saved for another job, offering further cost benefits to the contractor while cutting waste and thus further improving environmental credentials.

The finishing touch
Smoothing compounds used to finish the flooring installation include a latex mix (suitable for medium and heavy foot traffic) that bonds directly to the DPM and a water mix (offering high compressive strength and self-leveling) but with this type, primers are usually required.

Adhesives are many and varied, with special grades available according to substrate, floorcovering and application use. Water-based adhesives can be trowel or spray applied.

There are also two-part epoxy/PU and solvent-based adhesives. A wide array of product types are available and it’s important to ensure the right adhesive is selected for the precise requirements of the finished application.

Additional issues to be considered include:
Is there an underfloor heating system?
Is it a calcium sulphate subfloor?
Is it a concrete or sand/cement screeds?

The popularity of underfloor heating systems – preferred for their compatibility with modern high- performance condensing boilers and all types of heat pumps – has created several issues for specifiers and flooring contractors, in terms of sequencing and long-term performance.

In refurbishment situations, underfloor heating loops or electrical heating cables, are often incorporated into shallow screeds or dry panel overlay systems. Meanwhile in newbuild installations, the pipework is normally laid within a deep section screed.

This ensures efficient heat transfer and the warmth can help a concrete/screed sub-floor to dry out far more quickly than it would naturally. The critical consideration for the flooring contractor, however, is the relative humidity at the time the floorcovering is laid and the material’s vapour permeability.

With calcium sulphate screeds, there’s no option but to wait for them to dry out to a point where testing indicates an RH of 75%, although some manufacturers now permit application of a surface- applied damp-proof membrane at 87% RH or below.

Force drying can shorten this process but is fraught with practical and programme difficulties. Calcium sulphate screeds containing underfloor heating systems must be allowed to dry out naturally in all cases.

Concrete or sand/cement screeds still predominate in the industry and overlaying them with an epoxy damp-proof membrane may be an effective way of dealing with residual damp. However, only a limited number of reputable manufacturers have developed and tested epoxy systems able to deal with RH readings up to 97%; and written specifications should always be obtained to ensure adherence to manufacturers’ guidelines.

Correct specification and application of surface DPMs and moisture vapour suppressants contributes immensely to the performance and longevity of the flooring system, regardless of the effect of ‘beasts from the east’ during the construction programme.

It’s essential for any installation contract to be based on an agreed specification. The writing of this, along with the identification of suitable sub-contractors for the tender stage, is something with which any reputable manufacturer will be able to assist.

This type of design service should also be backed up by the availability of well-trained technical representatives able to offer site guidance and supervision in theory (through CPD seminars, for example) and in practice onsite.

Possibly the most important point to consider for the specifier, conscious of claims for professional liability is the cost of rectifying any failure – both in direct cost and interruption to the use of the building – inevitably far outweighs any percentage difference in the initial pricing of competing systems.

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