I am interested in an Electric Ireland offer for insulating flat roofs

October 2013

Question

I am interested in having my flat roof insulated. Could you tell me the key things I need to consider? Also, I read about an Electric Ireland offer for insulating flat roofs — do you know what’s involved?

Answer

About 35 per cent of the home’s energy will be lost through the roof if it is not insulated, so it is very worthwhile to consider providing a decent level of insulation, particularly flat roofs, which are often overlooked as being too difficult by installers due to the lack of access and potential liabilities if they get it wrong.

Flat roofs are notorious for failings so installers are wary of them. However, if carried out correctly, additional or new insulation can help increase their lifespan and reduce maintenance bills too.

The key principle is to have a “vapour barrier” on the warm side of the insulation to ensure that condensation cannot form in the structure of the roof. By their nature, flat roof membranes prevent the passage of water vapour, which in the main is the cause of their downfall due to the condensation that forms underneath, in the roof structure because inevitably it’s on the cold side.

Building legislation calls for adequate ventilation within a flat roof to keep it healthy, but in practice this is very hard to properly achieve. I believe if it is made difficult for the builder then it probably won’t get done or will fail!

So, what to do to get that important insulation uplift? The Electric Ireland scheme suggests that ceilings are taken down and then insulation and a vapour control layer are installed. This can work if it is done well, if there are no “difficult” areas or if the installer is particularly rigorous at making important seals at junctions or wiring points to prevent passage of water vapour to that critical cold interface.

However, the ventilation still might not be sufficient to deal with the increased potential of condensation, even if the requisite 50mm air gap is left above the insulant layer and air is free to circulate. Other factors are the limited amount of insulation that can physically fit and the high level of internal disruption that will be created.

If the roof covering is in reasonable condition, if the ventilation is poor or can be sealed off or if the structure is in good health and capable of small additional loads, then external insulation might be considered. This involves placing special insulation over the roof area above the room, placing a perimeter strip and overlaying it all with a membrane and ballast in the form of either paving slabs or gravel.

This has the advantage of minimising disruption, creating fewer restrictions on the amount of insulation and ensuring the vapour barrier — as the roofing membrane — works. Additional benefits are the decreased maintenance liability of the roof, better health of roof timbers and the potential increased lifespan of the roof.

The Electric Ireland offer is designed to promote awareness of the possibilities of increasing insulation values and the savings made will be limited to the recovery of the €200 SEAI grant.

I am unsure whether the limitations of the internal insulation suggested by Electric Ireland will be appropriate in all cases, or whether the typical cost of the service stated is relevant to flat roofs.

You should also investigate grants available from SEAI. Search under “Better Energy Homes Scheme”.

All situations vary and the wrong choice can cause costly damage or actually reduce the performance of a flat roof, so it is important to get independent professional advice to ensure the job is done properly.

Fergus Merriman is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland.