Casa AYA/

The House

Casa AYA is the first straw bale home built to PassivHaus standards in Latin America and the first PassivHaus in Uruguay. It was designed by Martin Comas from Arquitectura Regenerativa, with PassivHaus consultancy services provided by Oliver Style from Praxis Resilient Buildings, and the ‘as built’ embodied carbon assessment post-construction conducted by Bethania Lanzaro.

From the outset, this project aimed to be built in the most sustainable way possible.

The use of concrete was reduced by 93% compared to conventional construction by using raised foundations. This technique allowed the ground profile to remain unaltered, respecting the existing land. As a result, rainwater flows under the house just as it did before construction, and it will continue to do so even after the house is gone. No heavy machinery was used on the ground, as there was no need to move soil or add aggregates due to the minimal use of concrete.

Once elevated from the ground, the floor structure was constructed entirely from locally sourced timber. The suspended floor, as well as the exterior and interior walls, were made from prefabricated, compressed wheat straw panels manufactured by a local company, BioFraming. This technique offers exceptional insulation and acoustic properties. Locally sourced timber and blown cellulose insulation were used for the roof. The floor, wall, and roof details were designed to create a continuous, thermal bridge-free envelope with at least 20 cm of thermal insulation. The walls were plastered with clay sourced directly from the site.

Most materials used in AYA are natural and locally sourced, with approximately 80% obtained within a 150 km radius. All the timber is national, the straw is harvested a few kilometers from the site, the straw panels are assembled 5 km from the site, and the clay for the renders is extracted from the site itself. As natural materials, they not only have great embodied carbon benefits in comparison to other conventional materials and are able to be composted on site at the end of the building’s life, but they also help regulate the building’s indoor hydrothermal conditions and reduce internally generated pollution (lower VOCs). This contributes to significantly better indoor air quality and comfort.

A mechanical ventilation system with heat recovery was installed, ensuring air renewal every 3 hours without losing heat. This is a crucial element of a PassivHaus building, as it maintains extremely low heat demand and high levels of air renewal to ensure good indoor air quality. The house does not have fireplaces or combustion kitchen stoves, sources of indoor air pollution which are commonly used in most Uruguayan homes.

The Blower Door airtightness test conducted for this house resulted in a value of 1.2, which is very close to the PassivHaus standard and significantly better than most international regulations. To achieve PassivHaus certification, the test must result in a maximum of 1.14. Therefore, the house is not PassivHaus certified. However, since the value is so close to the limit, the International PassivHaus Project Database still included it in their records.

In addition to the benefits mentioned above, this house was built faster and at a lower cost than the average house in Uruguay.

We are extremely proud of this house. Four years ago, when we decided to build this house, it was an ambitious project. Creating an environmentally responsible, high-performance home using local labor and materials, while also being faster and more economical, was a significant challenge. We want this house to serve as an example of what is possible. If we could accomplish this in Uruguay, it can be done anywhere. There are no excuses!

Embodied Carbon Assessment

A lifecycle embodied carbon assessment was conducted for this house to evaluate the global warming potential of the materials used in AYA. The assessment revealed that the total embodied carbon emitted up to the completion of construction (lifecycle stages A1-A5) is 291 kgCO2e/m² of gross internal floor area. This figure does not account for the benefits of biogenic carbon sequestration stored in the natural materials, which amount to -448 kgCO2e/m². The net balance between the upfront embodied carbon emissions and the carbon sequestration storage is -158 kgCO2e/m². According to some frameworks, this would classify the house as a ‘negative carbon building,’ meaning it stores more carbon than it emits by the time construction is completed.

A baseline model (named ‘AYA(HA)’) was created for comparison, featuring an equivalent house with the same geometry and dimensions as AYA, but built using conventional methods and a reinforced concrete structure. AYA significantly outperformed this baseline, achieving a 54% reduction in upfront embodied carbon (excluding biogenic carbon benefits) and storing 4.2 times more biogenic carbon. The -107 kgCO2e/m2 for the biogenic carbon stored in AYA(HA) relates entirely to an exterior wooden deck that was retained in the design.

Focusing on the individual material carbon contributions, the 5 components in AYA that contribute the most to embodied carbon emissions are:

1. The timber used for the structure (Eucalyptus Grandis) has emitted a total of 8,157 kgCO2e for the entire building but has stored -35,173 kgCO2e, storing 4.3 times more carbon than it emits.
2. Plywood, which is used extensively, emitted 7,476 kgCO2e and stored -18,332 kgCO2e through biogenic sequestration, storing 2.5 times the carbon it emits.
3. Concrete, despite its minimal use, has a significant embodied carbon contribution (6,388 kgCO2e emitted) and no associated carbon sequestration. Cement replacement in concrete is not common in Uruguay.
4. Hot-dip galvanized steel sheet, used as strapping to stiffen the walls, contributed 5,946 kgCO2e with no carbon sequestration.
5. Reinforcement steel, modeled with 0% recycled content due to lack of evidence of recycled material, contributed 5,286 kgCO2e with no carbon sequestration.

Ranked 13th is the cellulose insulation, one of the lowest carbon-emitting types of thermal insulation. In AYA, it emitted only 406 kgCO2e and sequestered -2,557 kgCO2e through biogenic benefits, storing 6.3 times more carbon than it emits.

Ranked 18th in contributions is straw. Despite its extensive use throughout the building, its extremely low industrialization process in harvesting, manufacturing, and transportation (harvested 50 km from the site and panels assembled 10 km from the site) resulted in an upfront embodied carbon impact of only 243 kgCO2e for the entire building. Meanwhile, it stored -23,707 kgCO2e within its mass, storing 97.6 times the carbon it emits.

A full report of the embodied carbon assessment is available as per request.

The carbon benefits described in this analysis only account for upfront embodied carbon, which is associated with the materials used during the construction. Additionally, the AYA house is Passivhaus certified, meaning it requires minimal energy to stay warm in winter and cool in summer due to its highly thermally efficient envelope. This results in significant carbon savings throughout the building’s entire in-use stage, potentially far exceeding the embodied carbon when compared to a conventional equivalent over a long lifespan. As mentioned earlier, PassivHaus not only reduces carbon emissions but also provides top levels of comfort and healthy indoor air quality.

Embodied carbon emissions (kgCO2e/m2): 291 kgCO2e/m2 (A1-A5), -448 kgCO2e/m2 biogenic carbon stored.

• Architect: Martin Comas
• PassivHaus consultancy services: Oliver Style from Praxis Resilient Buildings
• Embodied carbon assessment post-construction: Bethania Lanzaro.
• Photography: Martin Comas