#8 Building Details

After my previous entries on water, heating and power, I think it's time to go back to the house and reflect on some of the details involved in the building process. I already discussed the foundation and some of the design decisions, but in this entry I want to go over things like insulation, roof design, and other aspects that are crucial for this project. At the end, I also briefly discuss some updates to the power system.

The Roof

The roof construction is more involved than one might think. For one, the eaves have to be far enough out from the wall to help protect the wall from water. But there is much more: it needs to hold the insulation, it needs the ability to dry out in case of moisture, it needs to support the snow load, and of course it has to be waterproof. For these reasons, a typical roof construction over here looks, with possibly a few other material choices, like this:

Let me go over these details and explain why I made these material choices compared to others.  Going from the outside to the inside, we first have the actual roofing material. Typical choices here are bitumen, but also metal. I went with a bitumen roof (and underlay, not on the diagram) because it's more silent when it's raining compared to a metal roof. The material can also be easily recycled at the end of it's life, as is a metal roof, so both materials are good choices. Things like tile are not that common here, and wouldn't be my choice due to the weight - it makes the construction much more complicated since the snow load comes in addition to the already heavy roof. Speaking about snow load, the roof is designed with a ground snow load of 2kN/m² in mind -  strong enough to hold most of the regular winter snowfall. The ground snow load represents the snow load that has a 2% probability of being exceeded in any given year.

Below the bitumen and underlay, there is a full wooden roof supported by the rafters. Before the insulation however, an air gap runs over the entire roof, with ventilation pipes connecting this air-gap to the outside on top of the roof. The reason for this air-gap and pipes is to provide ventilation: any moisture that could accumulate, dries up this way by means of the natural draft. High density fibreboard separates the insulation from the air gap. This fibreboard, together with the actual insulation and the air barrier create a moisture control system. In typical approaches, you use a sealed air/vapor barrier, usually some kind of plastic, between the insulation and the room inside. In the case of cellulose insulation however, you use a moisture permeable air barrier instead. The reason for this is that due to the hygroscopic nature of cellulose insulation, it manages to transfer moisture from areas of greater to lesser concentrations. In other words, it prevents damaging amounts of moisture from accumulating - if this moisture is not blocked by a vapor barrier. 

Cellulose insulation by the way is made with recycled newspaper (pictured above), and is thus also a carbon sink (just like the logs) for the lifetime of the house. It is treated with a flame retardant (boric acid) which also acts as a bug repellent. Using material like newspaper means the energy used to make this kind of insulation is also much lower compared to for example fiberglass insulation. The insulation is approximately 45cm thick, which corresponds to a U value of 0.09W/(m² K), an RSI value of around 11 or an imperial R value of around 63 ft².°F.h/BTU (sigh... ;) for my American friends.

The Floor

I've briefly talked about the floor in both the heating and foundation entries, but I thought it would be a good idea to give a little more detail here. 

The picture above shows what it looked like right before the concrete floor was poured. There are a couple of things I want to emphasize. For one, the floor is floating - I think I mentioned this before. It's not connected to the foundation to prevent thermal bridging. It also sits on top of insulation: about 200mm of EPS, providing an insulation U value of 0.16 W/(m² K) or an R value of 35. 

The piping for the floor heating is clearly visible, but there are two areas where you can see a cut-out in the insulation. Those are reinforced areas for a) the masonry fireplace and b) one of the two structural pillars in the house to increase snow load performance. Besides those two pillars, there are no structural internal walls, so it gave me the freedom to divide the entire space as I saw fit. Also in the middle of one of the cut-outs: the fresh air intake pipe for the masonry fireplace. I talked about this in the heating entry. This is where the combustion air enters the fireplace so that inside air is not used for combustion. Without it, there would be draft issues and it would also mean sending already warmed up air out the chimney.

What if no one is home?

This is a question that I get asked all the time: what happens in winter should you not be there? Your water pipes will freeze and burst, the tank will freeze and break, etc. These are of course valid concerns, but have of course been taking into account when I planned this project. For one, the floor heating system, coupled with the wood burner and large hot water tank form a closed system. That is, domestic hot water is generated with a heat exchanger, and is not an actual tank of water you take it from and refill. The liquid in the floor heating and hot water tank is composed of about 40% propylene glycol based antifreeze and 60% water (and some corrosion inhibitor). This prevents it from freezing at least down to -20C. However, this is also the freezing point, not the bursting point, which means that the liquid might turn into a slush beyond that point, but it won't turn into a solid, expand, and cause any pipes to bust. That would only happen below -40C. 

The reason to go with propylene glycol is that it's safe, non toxic. Because of the domestic hot water heat exchange coil, I wanted to make sure that even if something catastrophically goes wrong and the antifreeze mixture ends up in the domestic hot water supply, this would not be dangerous. Ethylene glycol is often still used in systems where there is no risk of mixing with domestic water, usually because it's much cheaper. Propylene glycol is also readily biodegradable, which is why I can also use this to prevent water traps (sink, toilet, drains, etc.) from freezing without impacting the waste water processing system.

There are of course a few disadvantage of adding an antifreeze to the system. For one, it lowers the amount of energy you can store. The specific heat capacity of water is 4.2 kJ/kg°C while the glycol mixture is closer to 3.75 kJ/kg°C. That means I lose around 11% of the energy storage capacity in the 3000L tank. In this climate however, it's something I have to live with. 

The other disadvantage is that a 40% propylene glycol mixture (with the other part being demineralised water) is not cheap. Pure propylene glycol in these quantities goes for around 4€ per kg here - so you can imagine it makes a bit of a dent in the budget.

As for the regular water pipes: I can drain all of them. A valve is installed where a compressor can be hooked up, and this in turn can flush all the pipes of water. The filters have to be done by hand, but otherwise this is just a matter of connecting the compressor (at 3 bar), and then opening the water taps until there is no more water coming out. Should there be any water left in the main pipe connecting the house, this is not an issue since it sits well below the frost line and is also insulated. The 'summer' hot water tank (resistive electric boiler) can also be just drained, and usually gets already drained before it even starts getting too cold. The pipe going to the pump does not get drained - it's also installed below the frost line with added insulation. Likewise, the wastewater treatment plant is also installed below the frost line and insulated on top. Appliances such as the washing machine and the dishwasher are also drained. Then, using a rinse cycle on the washing machine or short wash cycle on the dishwasher (both interrupted early) while having some antifreeze in their respective dispenser and basin, I make sure that the internal tubing is safe from frost. 

Making sure the house can handle the cold when no one is around goes a bit further still. The bathroom for example needs a waterproofing layer under the tile that can handle freezing and thawing cycles. For this, two component cement-based waterproofing is used. This layer remains flexible in order to handle the stresses caused by temperature fluctuations. Good thing people here are very familiar with that kind of requirement, due to all the cottages in this country that remain cold over winter. The floor material should also be chosen to deal with the cold. In principle a wooden floor would be good, but this is not optimal in combination with underfloor heating since it insulates too much. Vinyl is another option, but I'm not a fan. I went with a laminate, which handles the cold just fine and works perfectly with underfloor heating. 

System Upgrades

I've made some upgrades to the power system in the mean time. First, I doubled my battery storage capacity to 56kWh. This was done by adding an additional two 16x280Ah cell battery packs. The additional storage will allow me to use more of the potential power generated in summer which I don't capture yet, and increase my days of autonomy. I bought the 32 cells from Docan, shipped from China for a cost of USD $4184 all in. The low Euro compared to the dollar and shortages/COVID/increased shipping cost/inflation/etc. made this order about USD $500 more expensive than my original 32 cells almost three years ago. Still well below anything available off-the-shelf. 

Additionally, I replaced the older Victron Phoenix 'winter' inverter with a 3kVA Victron Multiplus II. This gives me some more flexibility, adds another battery charger (from generator, in winter as a back-up), and because the Multiplus II is a later version of the technology, consumes even less when idle while improving efficiency compared to the older Phoenix. It also allows me to dedicate the large 6kW inverter in summer purely to charge an EV (with the help of the battery upgrade) - which hopefully should be something I'm getting sometime next year or so, depending... Cost of the inverter was €1150 including shipping from a store here in Finland; the cheapest available right before prices went up everywhere again.

Winter is coming, and I'm looking forward to it!