Economic analysis for the retrofit of a detached single family house to the EnerPHit standard
Author: Simon Camal, La Maison Passive
1. Envelope and Ventilation
This 320m² house was built in the late 70’s near Lyon. The heating demand of the existing site according to PHPP9 was 105kWh/(m².a). The cast concrete wall (already insulated internally by 10cm of mineral wool) has been externally insulated with 20cm of expanded polystyrene (lambda 0.036W/m.K), then fixed to the wall with mortar glue and thermal bridge free dowels, before being rendered with an acrylic render. The slab-on grade was already insulated by a 6cm layer of extruded polystyrene. Lower ceiling heights impeded further insulation directly on the slab, so buried walls have been insulated externally on the perimeter of the slab with 16cm of extruded polystyrene.
Windows have been replaced by Smartwin wood-aluminium frames (average installed U value 0.85W/(m².K)), installed flush on the outer face of the concrete wall and equipped with movable shutters. Existing shutter-boxes have been insulated with mineral wool and taped to be airtight. The ceiling in the unheated attic has already been insulated in the past with 40cm of cellulose fiber. The additional work involved creating an airtight layer. A Polyethylene membrane has been placed below the ceiling brickwork, interior bricks have then been cut on the external walls to tape the membrane to the concrete wall. The house was equipped with a mechanical supply-only ventilation unit installed around 1980, which fed fresh air into electrically heated inlets. The ventilation upgrade reused the functioning supply air ducts, and added a ventilation unit with heat recovery in the basement. New electric heaters have been installed in rooms and bathrooms, along with a wood log stove in the living room. See photos and construction details on http://europhit.eu/op20-house-st-cyr-au-mont-dor.
2. Renewal of Domestic Hot Water with integration of solar thermal panels
The existing electric 200 litre hot water tank will be replaced by a more efficient model by 2025, connected to solar thermal panels installed on the roof.
3. Comparison with a standard low-energy variant
The economic performance of Passive House projects is continuously compared to standard low-energy alternatives. Additional investment in Passive House quality in single family houses amounts to a range of 5 to 15% of the low-energy reference scenario (see study  in France, and the feasibility study on German projects available on Passipedia ). This article investigates the additional investment incurred for this particular retrofit, and whether it pays off compared to the operational costs.
The comparison will be based on a low-energy variant, defined by typical retrofit measures documented in the region for the same building typology (data gathered from www.observatoirebbc.org):
EnerPHit reduces primary energy consumption (renewable-based analysis, PER demand) by 4 in comparison to the existing situation, and by 2 compared to the low-energy alternative (Figure 1).
Real costs for the EnerPHit retrofit are available, and were extracted from detailed bills. Costs for the low-energy alternative have been derived from a French cost database. See cost table below:
Fixed costs are known for each measure, for example wall insulation: scaffolding, exterior acrylic render, and displacement of drainpipes sums up to €36/m².
The Comparison worksheet in PHPP 9 enables us to compare two project variants, including maintenance costs like filter replacement.
We’ll assume the following financial hypothesis:
- real interest rate 2%,
- financial assessment period = 30 years,
- no Residual Value,
- heating and hot water price = €0.15/kWh
The comparison between EnerPHit and low-energy variants, for Step 1 (walls, windows, ventilation, stove) gives a slightly lower total annual cost than the low-energy variant (see Figure 2) at €6500/a. The actualised payback period of the EnerPHit variant is 28 years. The saved kWh price is €0.14/kWh.
CO2 tax can be added to the operational cost. The price of CO2 is fixed according to a French law issued in 2015 : stepwise increases from €22/tCO2 for 2016-2019 to €100/tCO2 after 2030. The declared CO2 content of the electricity mix is 88gCO2/kWh in France , against 532gCO2/kWh according to GEMIS [PHPP9]. Heating and hot water are fully electric. Figure 3 indicates that a CO2 tax is not a strong enough lever to shift towards efficiency if the energy source has a low carbon intensity (CO2 tax is only 3% of the total cost for existing French prices and CO2 electricity content). On the other hand, EnerPHit grants a “CO2 bonus” of 3% on the total investment with German carbon intensity and prices.
The Comparison worksheet allows us to assess the economic efficiency of a single component. Application of this feature on a window type (casement windows, inward opening – 12m² gross area, 30 year investment period, 30 year product lifetime) gives a similar annual cost for triple glazing than double glazing (Figure 4). The actualised payback period for choosing triple glazing is 20 years, the saved kWh price is €0.10/kWh. Similar analysis on mechanical ventilation with heat recovery versus supply only gives a clear advantage to mechanical ventilation with heat recovery (saved kWh price = €0.08/kWh).
The following factors can explain moderate total costs:
- Reusing supply air ducts reduced the investment by €1830 (35m supply ducts at €36/ml: €1670, and 5 inlets €30/u = €150).
- No specific treatment of existing render (good strength) prior to exterior insulation (€30/m² wall avoided for scraping and refill of existing render)
- Good airtightness of concrete walls, which spared specific airtightness treatment on regular wall surfaces.
A few measures could be further optimized:
- The position of the ventilation unit and layout of extract ducts
- The Lambda value of wall insulation, avoid dowels to fix insulation
4. Detailed cost analysis with phEco
phEco is an economic assessment tool linked to PHPP9. It provides a more detailed view on operational and investment costs. Costs that would have occurred anyway have been considered for the existing building: windows had to be replaced, so the existing scenario includes new double glazed windows with minimal thermal performance and new low cost shutter-boxes. Considering a future-oriented price of electricity at €0.20/kWh, the EnerPHit variant shows the lowest total investment, expressed in terms of present value (Figure 5).
5. Sensitivity analysis: is the economic performance robust?
The energy efficiency market is subject to high price variability. We can use the phEco model to find out if investing in EnerPHit is robust, given uncertainty on energy tariffs and component prices (wall insulation, windows and ventilation). Investment robustness is defined here as the probability of a total cost lower than the low-energy alternative (€560/m²TFA present value). A sensitivity analysis is carried out with the Crystal Ball Excel Plug-In.
Realistic price distributions are set up and the following parameters are fixed constant:
- Financial evaluation period = 30 years
- Average residual value of components after financial evaluation period = 10 years.
- Thermal performance of components
- Log-Normal laws used for prices with observed stable minima: heat price (electric), wall insulation.
- Normal laws used for high added value components, for which prices have a higher reduction potential due to industrial scaling: windows and shutters, ventilation with heat recovery.
- Ventilation: standard deviation is chosen equal to the investment reduction reached by reusing the existing supply distribution system = €6/m²TFA.
Following a Monte Carlo simulation (1000 trials), the probability of a lower EnerPHit investment versus a low-energy alternative is 80% (Figure 6): for this case study, the EnerPHit concept is economically robust. Analysis of the variance shows in that case that the economic performance relies heavily on the price of heat (69% sensitivity, Figure 7).
6. Was there room for optimization: would higher efficiency pay off?
The ventilation layout could be improved: extract ducts were placed in the ceiling insulation to maximize room height, the ventilation unit is placed in the building core and does not have the highest efficiency on the market. We estimate a maximum achievable heat recovery rate of 95% instead of the current 79%.
The exterior insulation could have a lower lambda value for the same thickness, down to 0.021W/m.K. Windows could not be significantly improved under current market conditions.
The reference EnerPHit total cost with actual price is €535/m²TFA.
Two decision variables are investigated:
- Lambda insulation for walls (0.022 to 0.038 W/m.K), linear cost dependency (+8% cost for lowest lambda). Thermal bridge coefficients are kept constant.
- Ventilation heat recovery (79% to 95%), linear cost dependency (+20% cost for highest efficiency).
A stochastic simulation to find minimum total cost while respecting constraint on heating demand (<25 kWh/m².a) gives the following results:
On this case study, the investment in EnerPHit quality has proven to be efficient according to the economic simulations done with the sheets Variants, Comparison and phEco of PHPP9. However, deep retrofits are not automatically cheaper than low energy alternatives if no effort is made to simplify solutions and implement innovative products (certified products can be found here , fairs exhibit innovative products for EnerPHit retrofits , ). Designers should evaluate each retrofit measure in terms of total cost and try to make use of what the building offers (existing systems and geometry) to reduce investment and maintenance costs. Components with higher upfront costs like insulation with minimum lambda value can lead to a lower total cost. The concept of residual value has to be investigated closely when optimizing total cost, especially when dealing with step-by step retrofits.
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The EnerPHit Standard applied to large, complex existing buildings
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