ICF - Insulated Concrete Forms

In Ireland, traditional build for years has been a 2 leaf cavity wall made from concrete blocks.  This consists of two walls of block separated from each other by a gap of at least 1 1/2".  The reason for the cavity is that masonary is pourous. and water will seep into it, through the outer leaf and into the cavity.  This is particularly true in wetter climates, and anyone who has spent a summer in Ireland can attest to that.  Popularity for this type of construction took off in the 1920's.  If you looked around Ireland's many building sites during the boom then you'd have seen a large number of cavity walls being constructed.

Above is a handy photo of such a wall from the internet.  More recently, these cavities have been packed with various insulation mechanisms, such as above, or blown-bead resin insulation.  While this has dramatically increased the insulation values of these walls, there are other problems.

ICF or insulated concrete forms is a different construction mechanism.  It is popular in other countries already, such as the US and Canda both whom have harsher climates (in su mmer and winter) than Ireland.  ICF houses are constructed from layering these forms on top of each other (like lego), and is a quicker and stronger mechanism than block.  Below is the anatomy of an ICF wall:

The ICF form is the polystyrene (like a coffee cup) on each side of the wall.  These are held together with a plastic tie that is hinged to allow it to flatten for packing.  The top and bottom of the form sides are crenellated and slot into each other.  They weigh virtually nothing (compared to the same volume of masonary block).  There is no glue or mortar to go between them, the polystyrene snuggles into the dips in the block below it.  In the plastic tie there are slots to lay steel rebar both horizontally and vertically to provide extra strength to the wall and help stop cracking.  Here is a rebar'ed setup

These are stacked up to a storey high, the openings for Windows and doors are cut out (its polystyrene so a knife/saw can do that) and shuttered with either timber or polystyrene to prevent the concrete escaping.  Then a concrete pump is used to pour concrete in the top opening, and left to cure.

All sounds good - you now have a highly insulated (say 0.2 w/m2K) wall thats very strong.  That alone might tempt you over the cavity wall, but - as they say on the shopping channel, that's not all.  You also get the following

1. Airtightness - no gaps in the wall/floor junction or between bricks
2. Strength - as described
3. Easier construction - less room for poor quality work.
4. Faster construction - foundation to roof plate for our build in perhaps 6-8 weeks
5. Great sound insulation

There is probably more, but this is what sold me.  It also allows us to use pre-cast hollow-core concrete floors for the 1st floor, so no creaky floorboards or TV noise from downstairs.

I will say that a well-constructed block house will provide every bit as good a structure as ICF - however you really need to be sure that the builder is skilled enough and pays enough attention to detail to compete.  This is the real difficulty.

Also a benifit of the ICF method is that the house has a much higher mass and density.  This leads to a thermal mass, i.e. a mass that will retain heat or stay cooler for longer and regulate the internal temperture more.  Think of it as a shock-absorber for temperture, smoothing out the bumps of hot and cold changes.  Coupled with under-floor heating these are both working in a similar way and complement each other well.

Finally about air-tightness.  Every house needs to breathe.  Traditional construction in Ireland has been to put holes in the walls and place a grate over them to provide this... so there are literallly big holes in the wall.  Imagine that when trying to stay warm in winter with a gale howling outside!  ICF houses are no different, but rather than punch big holes all over the house we'll install Mechanical Heat Recovery Ventilation.  Probably another post on that, but if you are going ICF it's worth the extra investment to install MHRV.