The Risks of Condensation & the effects of a Vapour Control Layer 

One of the most common problems now found with flat roofs is with the risk of condensation. This is where, in the past, many have blamed EPDM, Felt, GRP etc on their roofs leaking and this has earned flat roofs a bad name with some people and insurance companies too, but if constructed properly they can outlast many other roofing systems including tiled roofs.  A well constructed flat roof should last as long or longer than the BBA certificate it's been tested for so long as the manufacturers guidelines are followed. (For more information on BBA certificates on EPDMS please see our blog post here)

In this blog post I am going to explain the importance of a vapour control layer and how installing it incorrectly or not at all will result in a failed roof.  And let me also point out this can and does happen in walls and pitched roofs as well.

Firstly let's look at the technical aspects of it
Condensation in a roof, or any structure for that matter, happens where the hot air from inside the building meets the cold air from outside at the dew point (the same as you see on grass in the morning). Another example would be if you were to blow warm air onto a single glazed window such as in your car in winter, condensation forms on the inside of the window and drips down the window, fortunately your car can be ventilated so that moisture that forms warms up and evaporates.  If we do the same to a double glazed panel this no longer happens because the air is sucked out of the air gap between the two pieces of glass and sealed. If there is no moisture in that gap then condensation can't form at the dew point. When the seal between the two panels fails moisture can get in and condensation will form.

Dew on Grass   Condensation on a window 

This is the same for a building and the dew point is usually found within the insulated layer. This is why it's so essential to have a vapour control layer that is well sealed as this is the part of the roof build up that prevents moisture from forming in that insulated space between the vapour control layer and the waterproofing membrane. So even though the dew point still occurs within the insulated zone of the roof, if there is no moisture at that dew point then no condensation can form.

In essence, if there is no vapour control layer or its incorrectly installed then you end up with condensation and moisture forming at the dew point in the roof space which can then result in the roof failing through rot. Therefore to have a correctly fitted and sealed vapour control layer is just as important as having a good quality waterproofing membrane on top.

Warm roof build up   Cold Roof

To give you a couple of examples here are some of our findings on jobs that we have helped on in the past.

On a recent consultation we were asked to investigate a rotten roof that had only been installed 7 years ago.

The build up of the roof was a cold roof (mentioned in this article) with the vapour control layer nailed to the underside of the roof joists prior to installing the plasterboard. The joists were filled entirely with rock wool, the structural deck was nailed to the joists and an EPDM membrane was fully adhered to the deck. Then there was a decorative hardwood timber tile to form an accessible deck so that the customer could use the space as a balcony.

The customer noticed the hardwood tiles were making a strange cracking noise which turned out to be the structural decking giving way underneath.

We first checked the EPDM for any sign of damage and to make sure it had been installed to the manufacturers guidelines. There was no damage to the membrane and it was a good installation.

We then checked to see which type of structural decking had been used, it was an exterior grade plywood which is the correct type to be used in flat roofing.

Next we cut 2 investigation holes in the plasterboard ceiling to check the integrity of the vapour control layer. It was a recognised and approved by the roofing manufacturer for this type of installation.

Upon further investigation of the roof outside we noticed that the worst rot appeared to be at the front of the roof so we cut one more hole in the ceiling and found the cause.

The vapour control layer hadn’t been sealed against the building structure with the manufacturers tape. This small gap allowed the warm moist air from inside the building to meet the cold air from outside and the result was condensation and a rotten deck. So even though a vapour control layer had been used the fact that it hadn't been fully sealed as to the manufacturers guidelines it still allowed warm air with moisture into the roof space allowing condensation to form at the dew point (i.e. the decking) and rot the timber deck.

On another investigation we were tasked with finding the cause of a rotten ceiling in a kitchen.  We first asked for a set of drawings and found there was a vapour control layer specified again at ceiling height.

When we cut into the rotten plasterboard ceiling we found no vapour control layer had ever been installed. After explaining to the client about the effect of vapour control layers and the problems that are caused by not having one the roof had to be re installed due to a large amount of damage to the timbers, decking and plasterboards.

Hints & Tips

  • A very handy tip that we got taught many years ago is that the vapour control layer should go on the warm side of the insulation.
  • Always make sure that the vapour control layer is completely sealed around everything and anything. (i.e. walls, openings, wires etc.).
  • If there are any punctures in the vapour control layer use the manufactures guideline to seal it back up, whether its tape or a patch.
  • Our favourite and we personally think the easiest method of avoiding problems is to put the insulation board on top of the roof and use a self adhesive vapour control layer such as ALUTRIX onto the decking. This installation method also gives the best U-values as the whole roof is then insulated and not just between the rafters and roof joists.
  • Also remember that if you are using insulation boards try to stagger them on the roof for the best results

.Alutrix in a Warm Roof Build Up