Building Advice: Start on a good footing

Is there a significant difference between a passive foundation raft and traditional slab footing type foundations as regards cost? Would you go for underfloor heating with a passive foundation approach? Colin, Louth Hi ColinThanks for this wonderful question. Good to see you don’t suffer from bias from the recent spell of unseasonably warm weather as you look at the potential need for underfloor heating in winter in a Passive House! So, let’s look at what a passive slab is, what a traditional footing and slab foundation is, how they differ and how they differ in terms of cost.In the past, there was only one type of foundation. You built footings (a line of big rocks) under your walls and you hoped for the best. Your walls were built of stone and held together with clay and lime mortar (if any existed at all). Any movement in the foundations (which was highly likely) was absorbed by the density, depth, and somewhat flexibility of the thick stone walls above. A passive house in Oldchapel, Bandon. Picture: Janice O'Connell/F22 Photography As building technology grew, along came the invention and use of portland cement. This cement enabled the use of concrete for footings, cement mortar for blockwork, and concrete blocks. Now, everything became more regular but also more brittle. These building processes need to become better engineered to avoid cracking when the building loads create tension in the structure. Along with these more advanced footings came the advent of the concrete slab. This slab was originally just a layer of concrete above hardcore but then insulation and damp proof (and now radon) membranes were introduced.This form of footings and concrete slab is now extremely popular and is the default choice for most new build homes in this country.Now, as we all know there are always the extremists among us. These people were the guys who insisted on having an Apple Mac before anyone believed Apple would survive the 1990s and bought a Tesla (well, until recently anyway). These folk are termed ‘early adopters’ by marketeers.They are researchers, they spend time in libraries (remember those), they read books and, more importantly, specs. These folk opt for passive House design. The passivhaus was developed in Germany in the 1990s, a house construction approach that dramatically reduces the energy requirements of the house.How low the energy requirement of the house gets depends on how exact the house construction is. You need to eliminate cold spots using very thick insulation and eliminate any cold bridging (uninsulated elements).  You need to use the house’s orientations to reduce solar gain in summer while taking advantage of it in winter. You need to minimise drafts using airtightness and maintain air quality and indoor heat using heat recovery ventilation. This purist’s approach brought on the invention of the passive foundation. It is essentially a foundation which is wholly wrapped in a load-bearing (structural) insulation. This eliminates any fear of cold bridging from the ground underneath, which is always cold.Now that we know where these two approaches originated from, which one to choose and which costs more? For me, this comes down to the individual. As I mentioned above, I feel the passive house aficionados are a particular club. You are either a member or you aren’t. For most people, a modern A-rated home is most of the way along this low energy path.A passive house, Riverside, Carrigaline, Co Cork. Picture: Denis ScannellModern traditional footing and slab foundations use a combination of ‘thermal’ blocks in the foundations, which have an insulating capacity and reduce cold bridging. Concrete slabs also have insulation underneath and an edge insulation at the sides to again reduce heat loss. In truth, these measures have closed the gap between this traditional foundation approach and the passive foundations, particularly in terms of cold bridging.When it comes to costs, most quantity surveyors will tell you the passive foundation is more expensive but if you speak to contractors, it gets a little more cloudy. They tend to prefer (and offer better prices) for the method they are more experienced with.Lastly, to your question on underfloor heating: I guess this comes down to your faith in passive house technology and your building crew who put it all together, this and the fact you will be betting on this defence against an unseasonably bitter Irish winter (which is reasonably rare).I still think if you felt the need for heat in these instances, retrofitting a full underfloor heating system and heat pump is a big step, and by the time you start to get your head around the figures, the cold snap will have passed. Kieran McCarthy is a building engineer and director of KMC Homes bespoke A-rated new home builder, serving Cork and Limerick.  He is also a co-presenter of the RTÉ property show Cheap Irish Homes. Check out KMC Homes’ brand new website kmchomes.ie Follow Kieran on Instagram @kierankmc for more home building information, tips and Q&A advice. You can also follow Kieran on the Built Around You Youtube channel and @kierankmc on TikTok

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