Blocks, concrete or timber? What should we begin to construct with?

Hi Dylan Thanks for your question. Yes indeed there are many different building frame options available. There are even a few that are not on your list above but they wouldn’t be as popular and only for very specific uses.

When you consider traditional blockwork, ICF (Insulated concrete forms) or timber frame solutions, the question is what are the pros and cons of each, when are they best employed and which do I think would work best for you?

The first thing I need to make you aware of here is that all frame types need to comply with the same building regulations so they will all be energy-efficient and structurally sound. I also need to assume that you are self-building direct labour and that you have a relatively flat site.

Building a building using concrete blocks is still probably the most widely used practice in Ireland. It is one of the oldest trades (masonry) and believe it or not it is generally the most cost-effective, as most QS’s will tell you. It tends to be the default new home building option.

Blockwork is great in that it is a well-established practice and most architects and engineers will be very familiar with it. As you may be aware it is literally buying all the component parts (concrete blocks, mortar, wall ties, insulation, damp proof courses and steel) and your team of masons will assemble them on-site according to your construction drawings. The benefits of blockwork tend to be the availability of materials (once you have masons you can start straight away). It is very adaptable (if you decide to make a change to your house it is straightforward to cut blockwork and install steel etc).

From a thermal perspective, blockwork is a heat store, meaning that when you heat your new home, the internal facing blockwork takes longer to heat but once warm, retains the heat for longer so that when you turn off your heating, the blockwork still emits heat for some time.

The downside of blockwork is that it can be slow, particularly in winter. It is also complex. There are many details you need to get right. Damp-proof membranes, clean cavities, plumb walls, cold bridging, airtightness, setting-out, lead trays; the list goes on. Whereas you will (hopefully) have a team of experienced masons on site, you will need to have experience and know-how in all these areas and be able to stay a few steps ahead of them to problem-solve each detail before you build it. Yes your engineer will help too but I have yet to be on a project where we didn’t have to upgrade construction drawings here and there on site during a build.

The next thing to be aware of is the reality of running the blockwork phase of a new home build. The masons will require the blocks to be lifted to a height so that they can move them a few metres here and there to suit. They will not accept that they need to move them more than that. You may need a loadall forklift and driver to assist here and you will need to ensure you have adequate scaffolding and hop-ups available when needed. If you have a complex build in terms of design, site characteristics or access, then blockwork may become unfeasible.

ICF is very different. You essentially have insulated hollow blocks (insulation on the outside and inside) and you pour concrete and fix steel in the middle. These blocks are lightweight so they are easy to transport and move around on site. They are relatively quick to install (once using trained installers) but the insulation mould walls need to be carefully propped before you pour the concrete.

Once the walls are in place, you thread the steel through them and you hire a concrete pump to add the concrete. I have used this method to build houses in difficult-to-reach sites as the concrete pump will reach almost anywhere. The other benefit of ICF is that the (reinforced concrete) walls are very strong so they can in fact be designed to act as retaining walls (this may be of great help if you have a sloped site).

The finished ICF wall is airtight due to its construction and the insulation outside means your dew point is outside of the building thereby eliminating condensation being trapped in your building fabric. The downside of ICF is that it tends to cost a little more than blockwork for a flat site, where you cannot take advantage of the extra strength of the walls, and the ease of access means the characteristics of building with ICF are less valuable. ICF is also less adaptable to change as cutting reinforced concrete walls is costly and very time consuming.

Timber frame buildings are a little different again. This is a full wall, floor and roof kit manufactured off-site and assembled on-site. Essentially the timber frame company takes your planning drawings and creates their own set of working frame drawings. They will also detail the insulation, airtightness and anti cold bridging details for the entire house.

The benefits of a timber frame build is that it is quick and relatively straightforward. Once you order the frame (months in advance of when you need it) the timber frame company will install the walls, the floors, the roof structure, battens and felt in about two weeks. This means that at this stage you have a dry building to which you can fit your windows (again order well in advance) and begin your plumbing, electrics and ventilation.

The frame company can even add plasterboard to your floors for ease of installation by you later. The advantages are the speed and the fact that so much is detailed and constructed by one company. The downside is that it tends to cost a little more than blockwork and because it is installed using a crane you need somewhere for a crane to sit and operate with access to the site (watch out for overhead wires etc). This may not suit a built-up area.

So, in reality, it comes down to your site, your access, your schedule, your appetite for managing a complex build and of course your budget.

All frames work and work well but there are significantly different characteristics to each as I have outlined. I would also speak to friends who live in these different house fabrics to see how they have performed over time and what the living environment is like because ultimately, that is what matters most.

Kieran McCarthy is a building engineer and director of KMC Homes bespoke A-rated new home builder, serving Cork and Limerick. He is also co-presenter of the RTÉ property show Cheap Irish Homes.

• Follow Kieran on instagram @kierankmc for more home-building information, tips and Q&A advice and his website kmchomes.ie.
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