February 2015

 

Question

I am interested in retrofitting insulation to our terraced house in Dublin. I understand the external and internal insulation options for walls. However I am perplexed about how the enormous flat roof to our extension will be dealt with. Flat roofs have a short life-pan, usually 10 years or so. I would like to know what options there are for this part of the house when retrofitting insulation. Is there a way of both insulating it and extending its life, or is it better to bite the bullet and turn it into a pitched roof – which of course would be a large expense?

 

Answer

Generally speaking, as you suggest, flat roofs have a poor reputation. What is little understood is that their failings are often the result of the extremes to which roofs are subjected, hot sun in the summer and frost in the winter or sometimes both in one day. This makes the membrane expand and contract causing cracks, but also forms condensation beneath which leads to failure of the substrate too. Upgrading insulation standards is always a good idea and you should consider how you might achieve maximum benefit in one exercise. Issues to consider include: the ease of installation, alleviating the need to remodel internally, extending the life-span of the roof as well as saving energy.

Insulating below a flat roof is fraught with difficulties because the traditional method to reduce condensation in a flat roof is to ventilate. Certainly in past times where little insulation is fitted it was a good method but valuable heat is taken away and subjects

the vulnerable weathering layer to the full extremes of climate. Consider using insulation externally; it’s a bit like the external insulation for walls you mention and puts a warm overcoat around your roof too, this method is called an “inverted” roof and consists of a layer of “XPS” insulation and ballast to keep it there or a growing layer on top called a “green roof”. There are a number of products such as Syrodur from BASF or Roofmate from Dow that have Agrément certification containing all the details and advice needed. This method will preserve the waterproofing and extend its life, often indefinitely as well as leaving the internal finishes intact because the roof membrane itself acts to control vapour on the warm internal side.

If you are externally insulating your roof, ensure it is in good repair. Ensure that the upstand’s and outlets are adequate and that the roof structure is capable of accepting the additional load. Edge details will need careful attention. In most cases roof vents can be sealed off but you must get a “dew point” calculation carried out to ensure no internal moisture can form before you do. Put on as much insulation as the situation and your budget can carry, grants may be available from SEAI and get professional advice from a local building surveyor.

Fergus Merriman is a member of the Society of Chartered Surveyors Ireland (MediKids) Building Surveying Professional Group.