Insulating the house (part 3)

For those of you that have followed our blog from the start, you may be surprised to see that we are still in the process of “wrapping up” the house. Parts 1 (see post here) and 2 (see post here) on this topic showed how we insulated the foundations, slab and walls of the house to prevent heat loss. Yet, over the last few weeks, we have insulated the ceiling and the roof and added more insulation to the outside of the walls in order to meet our energy consumption goal of less than 15 kWh/m2/year, based on the Passivhaus methodology.

Walls represent the largest surface area of a house, and after the roof, they are one of the elements where significant heat loss occurs. Part of the formula used to calculate the energy consumption of the house in the Passivhaus and CEV methodologies is the u-value of the walls, which refers to how much heat is lost through the material. The u-value measures how effective a material is as an insulator. The better-insulated a structure is, the lower the U-value will be. To reach our goal we need to achieve a u-value of 0,23 or lower in the walls, whereas a “normal” house with our characteristics (wood panels) has a u-value of approximately 1,9.

To prevent this heat loss and to eliminate thermal bridges, the final width of our exterior walls will be 26cm of which 21cm are insulation. This insulation consists of 2cm of cellulose (shredded paper) between the panel SIP and the interior wall, plus 9,2cm of expanded polystyrene in the panels (15kg/m3 density) and a further 10cm of expanded polystyrene (20kg/m3 density) on the outside of the walls after the application of the hermetic membrane (see post here). The current Chilean building code requires a mere 2,5cm of insulation in the wall. We have almost 8 times more insulation than the requirements!

Those of you who know about SIP Panels will know that they come in different widths. We chose the thinnest panels instead of the thickest which at the end of day would have resulted in the same amount of insulation as we have now (21cm). There are a few reasons for this decision, but mainly it has to do with the concept of thermal bridges. A thermal bridge is a specific area in the outer layer of the house where heat can easily be lost due to differences in the thermal resistance and because at this exact point there is no insulation. In a SIP house, in order to join each of the panels together, they are attached to wooden upright poles between each panel. These connections are only wood and have no insulation, thus they are thermal bridges where heat can be lost. For this reason, by putting extra insulation on the outside of the house, we have eliminated these heat loss points and improved the u-value of our walls.  

The cellulose was first considered only for the interior walls dividing the rooms as it is one of the best acoustic insulations available. But we then decided to add it to the inside of the exterior walls as well.

The main challenge we faced in this process was how to attach the insulation to the outside walls with as few screws as possible perforating the hermetic membrane and then how to attach the final siding of the house to the insulated walls. Based on lots of research that Rodrigo has done over the last few years, we decided to place 2×4 inch poles at 1m distances in order to be able to place the polystyrene panels between the poles without having to screw them onto the wall. Then, after the windows have been installed, we will attach the siding (wooden panels) to the upright poles, thus securing the insulation and making sure that our walls are nicely wrapped up. 

Installing the poles to hold the external insulation in place
External insulation installed. The parts around the windows can only be finished once the windows have been installed. We will also finish the airtight membrane and siding around the concrete slab soon.

The roof insulation was another challenge due to the same issues such as thermal bridges. In the end we decided on zinc panels with the same polystyrene insulation between them. In this way, it was easy to assemble the panels on the roof and ensure that there are no thermal bridges.

The zinc and polystyrene roof panels being installed

Between the ceiling and the roof, the original idea was to use the same polystyrene panels that we have used in the rest of the house, but unfortunately a design flaw of ours with the roof trusses so close together made it impossible to be able to place the panels on the ceiling. Thus, we opted for the easiest plan B which was the typical “glass wool” insulation. Not the best option, but at the end of the day, factoring in the different material makes no difference to our overall energy consumption calculation.

Stay tuned as we will be installing our long awaited windows next week and this is a whole other factor to take into consideration in an energy efficient house. I had no idea that there is so much more to consider than just double glazing…

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