How much ETICS makes sense? Revisited again!

I have found that my previous attempt at optimising the EWI depth was flawed.

I noticed when I looked at a different angle and considered that I had considered the cost of constructing the underlying wall as a given, but had considered the common cost elements of installing ETICS in the analysis, which may be inconsistent when the comparison is assuming the use of ETICS.

When I removed the fixed cost - then the optimum depth went to 0. Also, the depth was not affected by either the total energy requirement or the cost of the fuel used.

I built a spreadsheet implementation of the model and indeed it behaves the same way: it is very unintuitive. The depth that gives minimum for the repayment time is calculated for the given assumptions, but this is based on the relationship between the cost of adding more insulation and the initial fixing cost. The repayment time itself is heavily dependent on the cost of fuel and the number of heating hours, but the depth that minimises it in each scenario is not.

So its an optimisation - but not a very useful one. I was tricked by an equation and the possibility to find a minimum. And, I should have tested it before I did.

So, a new model is created. This time:

• specify a time horizon in which we want to identify a savings
• assume ETICS and that the common installation cost parts are a sunk cost as part of the structure
• compute a simplified nett savings ignoring cost of funding and fuel inflation
• identify the maximum savings

The new model is available as an Excel sheet with no macros. It works on Open Office 5. The sheet also includes the old model on a separate tab.

This time we do see much more intuitive behaviour and that the result is dependent on:

• the base fuel cost
• the number of heating hours
• the incremental cost of installing the insulation (insulation and longer fixings)
• the thermal conductivity of the insulation
• the time horizon
• the base thermal resistance of the wall.

As expected we see that there is an optimum depth: additional insulation drives up the cost linearly but has a smaller and smaller effect as it is added - the total thermal resistance is increasing linearly, but the value of the fuel savings is based on the change in U value.

In the case of my current assumptions:

• 28000 degree hours of demand
• thermal conductivity 0.035
• base thermal resistance 0.8
• cost of fuel €0.25 per kWHr
• cost of insulation and fixings €230/m
• 10 year time horizon

Then it seems that the optimum depth is 76mm and the total savings (per square metre of wall) is €46.65.

That figure is somewhat misleading - remember that there is a base installation cost that I have regarded as a sunk cost, and an optimistic estimate is that it is just €25 per square metre. On Leroy Martin today a 15kg sack of pellets is €3.69, and conventionally we regard a kg as having 4.8kWHr and a pellet boiler to be 85% efficient. So the cost per kWHr is about €0.06, when buying individual sacks.

If I use €0.06 as the cost of energy then the optimum depth is 24mm and the total savings is just €4.20. In principle I am €20 a square metre better off by not adding any insulation at all! I would have to extend the payback horizon to 27 years to see any benefit. Given that we are unlikely to retire before we are 60, that's a problem.

In fact the model does break down somewhat because:

• we do already have a sufficiently powerful oil combi boiler, and replacing it with a hot water cylinder system, pellet boiler,and large pellet hopper would be expensive
• however the remodelling we are interested in doing will require the boiler to move so the cost of replacing it is somewhat offset, and the expected final layout of kitchen and bathrooms is not favourable for using a combi compared to a number of smaller high high recovery tanks placed near demand centres
• pellets (and wood) have low energy density and replenishing the hopper and reloading a wood boiler is problematic to get the output required in winter.
• insulating and using smaller point energy inputs yields a more comfortable environment

Clearly a rational decision will require a number of scenarios to be investigated.

Unfortunately some are easy to eliminate. A simple calculation based on Weber Aislone suggests that - even with a lowered base installation cost - the cost of adding insulation is less than the fuel savings unless the time horizon is also extended. Other insulating mortars and renders are unlikely to be better - Aislone has a reasonable cost for its thermal perfomance.

In the end, it is likely to be a question of user convenience and limiting the exposure to things that can go wrong: simpler is better and automatic is better - which is somewhat conflicting.