Straw house and Minergie label: analysis of thermal calculation

For our straw house project in Broye Fribourgeoise, we commissioned the engineering office Perenzia to carry out the thermal study necessary to obtain the building permit. The official file concludes that there was non-compliance with the Minergie criterion.

The main point concerns ventilation modeling: according to the normative calculation, the overall energy index remains above the label’s admission limit.

Since this study, our project has further evolved. We have moved from a classic wooden frame technique filled with straw to an innovative carrying straw system. This evolution, developed specifically for this project, makes it possible to do without a massive wooden structure, thus maximizing the proportion of natural insulation in the thickness of the wall.

The objective of this article is to verify the figures in the report and to evaluate the effect of a lower ventilation assumption, already documented in the calculation annexes.

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1. The official report: 42.6 kWh/m², what does that mean?

We base our analysis on the official thermal report (SIA 380/1) and the energy form (EN-101b) established by Perenzia in October 2023 [^1]. Although our construction system has since evolved towards carrying straw, these figures remain a reliable and conservative order of magnitude.

According to this report:

• Calculated heating requirement (Q_H): 42.6 kWh/m² per year.
• Legislation (MoPEC): validated. The legal limit is 65.3 kWh/m². Our house largely complies with current Swiss law.
• Minergie Label: not achieved. The overall energy index, called MKZ (Minergie-Kennzahl), reaches 78.1 kWh/m², while the admission limit for our building is 63.6 kWh/m².

Technical note: MoPEC vs MKZ

• The MoPEC is the harmonized cantonal legal framework. It sets a single limit (here 65.3) based mainly on heating.
• The MKZ is the Minergie label index. It is more strict and specific to each building: it adds heating, hot water and electricity for auxiliaries, all weighted by the energy source. The limit of 63.6 kWh/m² is tailor-made according to the compactness of our house. For the higher Minergie-P standard, this limit would further drop to around 45 kWh/m².

The report highlights a very penalizing item: ventilation. The project is modeled without installing comfort ventilation (main ventilation by opening windows), with a normative air renewal rate. However, the technical file mentions occasional extraction vents in the WCs. In the SIA 380/1 calculation, the air renewal hypothesis retained remains high and without heat recovery, which greatly increases ventilation losses and, in turn, the MKZ index.

The point to check is therefore the difference between this normative scenario and a scenario with reduced air flow.

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2. Distinguish QH and MKZ clearly

To avoid confusion, two different indicators must be separated:

• Q_H (SIA 380/1): heat requirement for heating alone (kWh/m²·year).
• MKZ (Minergie): overall label index (heating, DHW and auxiliaries, with weightings).

The report simultaneously confirms:

• Q_H = 42.6 kWh/m²·year (basic calculation).
• MKZ = 78.1 kWh/m² year, for a Minergie admission limit of 63.6 kWh/m² year.In other words: the project respects the MoPEC legal limit on heating, but does not respect the MKZ criterion of the Minergie label.

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3. Effect of air flow in the ratio

The same report contains two SIA 380/1 calculation scenarios, with identical geometry and envelope, which only differ in the thermally active air flow (qth):

• Basic scenario: qth = 0.7 m³/(h·m²)
• Report variant: qth = 0.3 m³/(h·m²)

The results indicated in the balance sheets are as follows [^1]:

Heating indicator (SIA 380/1)	qth = 0.7	qth = 0.3
Losses by transmission Q_T	58.0	57.5
Ventilation losses Q_V	21.1	9.0
Contributions (Q_i + Q_s)	~51.0	~50.1
Heating requirement Q_H	42.6	32.1

The figure 57.5 kWh/m² therefore comes directly from the “total” line of the “heat balance with thermally active air flow (Qh,eff)” table for the qth = 0.3 variant.

The SIA 380/1 calculation used is:

[ Q_H = (Q_T + Q_V) - \eta_g \cdot (Q_i + Q_s) ]

where η_g is the contribution utilization factor (share of internal and solar contributions actually used to reduce heating). This factor is not 100%: it depends on the monthly heat balance, the season and the level of losses.

Using the values from the report:

• Case qth = 0.7:
  42.6 = (58.0 + 21.1) - η_g × 51.0
  hence η_g ≈ (79.1 - 42.6) / 51.0 ≈ 0.716 (consistent with ~0.71–0.73 displayed depending on the zones).
• Case qth = 0.3:
  32.1 = (57.5 + 9.0) - η_g × 50.1
  hence η_g ≈ (66.5 - 32.1) / 50.1 ≈ 0.687 (consistent with ~0.68–0.70 displayed depending on the zones).

This explains why the drop in Q_H is not strictly equal to the drop in Q_V: when the air flow changes, the software also recalculates the effective use of the supplies via η_g.

What we can clearly conclude:

• Going from qth = 0.7 to qth = 0.3 greatly reduces ventilation losses (Q_V) and reduces Q_H by 10.5 kWh/m²·year.
• This conclusion concerns the need for heating (Q_H), not directly the MKZ.
• The small differences on Q_T and Q_i+Q_s between the two tables mainly come from rounding and the aggregation mode displayed by the software.

3.1 Estimation of MKZ with qth = 0.3 (indicative)

The file does not provide an official MKZ recalculated for qth = 0.3, but we can make a consistent estimate from the EN-101b data:

• Q_H goes from 42.6 to 32.1 (i.e. -10.5 kWh/m²·year).
• Q_ww (hot water) remains 13.9.
• Heating share in (Q_H + Q_ww) in the base case: 42.6 / (42.6 + 13.9) = 75.4%.
• Basic MKZ: 78.1.

By applying the reduction of 10.5 kWh/m² only to the heating component of the MKZ, the estimate gives an MKZ around 67–68 kWh/m²·year.

This value is indicative (not a certifiable value), but it allows orders of magnitude to be compared.

Concerning Minergie-P:

• With mainly manual ventilation, the project does not fall within the certification framework expected by Minergie/Minergie-P ¹.
• The relevant question here is therefore the performance comparison, not obtaining the label.
• On heating only (Q_H), the variant qth = 0.3 gives 32.1 kWh/m²·year, i.e. a low order of magnitude and comparable to very efficient buildings.- On the global Minergie index (MKZ), the estimate at qth = 0.3 remains around 67–68 kWh/m²·year, therefore above the Minergie limit (63.6) and above a Minergie-P level typically close to 45.

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4. Beyond heating: the real ecological balance

Even with a margin of uncertainty on the exact heating figures, the broadening of the view confirms the relevance of the approach.

Criterion	Standard construction / “classic” Minergie	Our hamlet in load-bearing straw
Energy approach	High performance via systems (PAC, double flow VMC)	High performance by the envelope (thickness, inertia)
Energy production	Often weak or just sufficient	Positive (32 PV panels + thermal solar)
Carbon footprint (construction)	Positive (concrete, steel, industrial insulation emissions) 2	Negative (carbon sink: straw + local wood, no concrete)
Technical systems	Complexes (maintenance, filters, dedicated electricity)	Simple (100% manual, without machines)
Humidity management	Mechanical (pullers required)	Natural (breathing walls + manual ventilation)
Air quality	Filtered (be careful of dryness and duct maintenance)	Natural, hygroregulated, 0 VOC

A classic Minergie approach sometimes compensates for “standard” insulation with very efficient technical systems (high-tech). Our approach is low-tech: we focus on thickness and material. By using a lot of straw (local and renewable resource) and wood, we achieve high thermal performance while minimizing embodied energy. In addition, our supporting straw technique removes the secondary wood structure, further optimizing the carbon footprint.

The absence of concrete (foundations on steel screw piles) is a decisive point: concrete being one of the main sources of CO₂ emissions in construction, its removal allows straw and wood to fully play their role as carbon sinks, making the overall balance of construction negative.

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5. Conclusion

Labels like Minergie remain a useful reference for objectiveizing the energy performance of buildings.

However, today they are showing their limits for certain low-tech projects. In our case, the calculation is very sensitive to the air flow assumption: the transition from qth=0.7 to qth=0.3 significantly reduces the heating requirement, which the report clearly highlights.

The file shows that the same building can display significantly different results depending on the ventilation hypothesis used.

• A high-performance envelope confirmed by the file (low U-shaped walls, controlled transmission losses).
• A heating requirement which can go from 42.6 to 32.1 kWh/m² depending on the air flow retained in the model.
• A negative carbon footprint (thanks to the absence of concrete and straw/wood storage).
• Optimal health (natural humidity management, no VOCs, no mechanical drafts).
• Total resilience (no dependence on electricity for ventilation, simple and durable systems).

This point does not replace certification: it allows a coherent technical comparison of heating performance with reasoned ventilation.

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References and sources[^1]: Perenzia Ingénieurs Sàrl. SIA 380/1 thermal justification and form EN-101b – Family hamlet project, Surpierre. October 2023. private document (unpublished). File values: Q_H = 42.6 kWh/m²·year (basic calculation, qth=0.7), Q_V = 21.1 kWh/m²·year, Q_H,eff = 32.1 kWh/m²·year (variant qth=0.3, with Q_V = 9.0), MKZ = 78.1 kWh/m² year, admission limit 63.6 kWh/m² year.

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Analysis based on the SIA 380/1 thermal report established by Perenzia Ingénieurs (October 2023) for the building permit application. The project has since evolved into an innovative carrying straw technique, confirming the trend of high energy performance and reduction of the carbon footprint.

Footnotes

1. Minergie — Les standards Minergie 2023 (FR) ; Application guide to the Minergie / Minergie-P / Minergie-A standards, version 2023.1 (PDF) (German, reference document on the MKZ, the requirements per standard and ventilation). Our comparison on the paper (MKZ, thresholds) and the role of ventilation for certification is based on these sources, not on a measurement of real Minergie-P consumption for our site. ↩

2. KBOB — Eco-balance data in construction (Swiss Confederation). Basis for comparing emissions (including GHG) of materials over life cycle — useful for contextualizing the impact of concrete or industrial insulation in relation to biosourced materials mentioned in § 4. ↩