Tuesday, June 14, 2011

Why ICF - part 2

One of the reasons we are building our dream house with ICF - besides quiet and comfortable - is the energy efficiency it provides. When it comes to rating energy efficiency, the most well-known measurement is the resistence to heat value of the product; the R-value.

So why do certain people (mostly builders) keep saying that an ICF wall R-value might be R25 but it performs as an R40-60 wall?

If you only look at that R-value you are missing a couple of other key factors that factor into the overall energy efficiency of ICFs and building envelop it creates;
  • inertia / thermal mass
  • amplitude suppression
  • phase displacement

Thermal mass and Amplitude supression
The first two kind of go together and I'll try to explain this without requiring a deep dive back into your math books.

Because ICF is dense, the concrete takes a long time to respond to temperature changes, so from morning to night on a hot Summer day the outside temperature may fluctuate by 30 degrees, while indoors there is hardly any change in an ICF home.

This density is an area where ICF walls perform better than SIPs and other walls. Not only do ICF walls offer the traditional insulative resistance of the EPS foam that covers it on each side, also the thermal inertia of concrete very nicely smoothes out those outside temperature fluctuations throughout the day, keeping the home comfortable and thus requiring less energy to keep the temperature level.

Lightweight insulation alone, which has a shorter (aka worse) thermal inertia but still a good resistance or R-value, means a highly insulated lightweight building built of such materials would conceivably still be subject to overheating on a hot summer day or cool down significantly on a cold winter night, whereas an ICF building would be far less affected (with the same R-value...).

For example, in a study done by the TNO University in my native Holland in Summer ’97, two identical houses in Delft, Holland were insulated to the same rated R-27 value, but with a different density of insulating materials.

One house used fiberglass insulation (1.25 lb/ft3 density) and the other house had cellulose insulation (4.37 lb/ft3 density). As the outdoor temperature fluctuated by 30 degrees during testing, TNO's measurements showed the house with fiberglass insulation had its indoor temperature fluctuate by an average of 13 degrees, the house insulated with cellulose insulation fluctuated by only 3 degrees during the testing period.

low supression - high(er) temperature fluctuation indoors

high supression - almost no temperature fluctuation indoors

This illustrates the benefit of using heat resistance insulation that also has 'thermal mass', i.e. the ability to moderate daily temperature fluctuations, also known as the 'Amplitude suppression'.

You can simply calculate the suppression factor by dividing the temperature swing outside by the resulting swing inside to get the AS factor. You want a product with a factor of 10 or higher.

The product that combines that high R-value with a high amplitude supression will perform far better in real-life than a similar high R-value product with a lower amplitude suppression.

Phase displacement
The other item that factors into the effectiveness of a building structure is to bridge the time span between the highest external temperature and the highest interior temperature. 12 hours is about ideal (between 2:00pm and 2:00am). And so 12 hours in this case is the Phase Displacement. This means that the 2pm heat will 'arrive' inside the house around 2am when the outside temperature is much lower so you simply ventilate it back out either using ventilation methods or the walls allowing the build-up heat to flow back out (thermal dynamics).

ICFs is one of the products that posesses an excellent mix of thermal resistance (foam), thermal inertia leading to amplitude supression (concrete) and phase displacement. Thus combining the high R-value with a high AS factor and phase displacement you can argue that an ICF wall performs as a much higher R-value if you only (want to / able to) compare on resistance.

Combine that with a near absence of air leakage leads to your much-coveted lower overall energy consumption and ICFs providing a safe (4hr fire rated), quiet (sound rating of 48), and comfortable environment year-round make it our building material of choice..

(Thanks to icf-dev.com and TNO Delft)
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