We’re very used to central heating that responds fast when we turn up the temperature. We like our rooms to heat up on demand. With underfloor heating running on cooler water, things are a little different.
Underfloor heating uses water of much lower temperatures than a conventional central heating system. High temperature heating systems run at about 70 degrees C. The water in underfloor heating and low temperature radiators can be as cool as 40 degrees C.
That’s part of the point, though. The concrete is a solid thermal mass that will hold on to the heat even when the underfloor heating isn’t on. That means the house will maintain a relatively stable temperature throughout the day and night.
Our heating system won’t be very responsive. If we want to warm up quickly, we’ll need to use our wood burning stove. If we want to cool down, we’ll need to open a window.
The advantage is that it doesn’t take much energy to maintain an even temperature when a lot of heat is stored in the fabric of the house. We’ll be able to keep warm on very little – hopefully just thermal solar topped up with a small amount of wood in winter.
Our heating and hot water will be completely carbon neutral. And that is just very cool.
We hope that we won’t have to heat the Westacre house at all. And if we do, the thermal solar panels should cover most of it.
If all our insulation efforts, and the money we spent on high quality triple glazed windows, work out as intended, our house should be snug throughout the year.
A 16 cm layer of insulation on the outside of the walls, combined with the air tightness we hope to achieve, should cut out any draughts. That should keep all the warmth we generate from cooking, running appliances, and our body heat, inside the house.
Air tightness is great for that, but of course we also need to breathe. We will have to pump fresh air into the house and duct it to different rooms. The air will go through a heat exchanger, sharing the heat of the stale air going out with the cold air coming in. Our heat loss overall should be minimal.
It’s no guarantee that we’ll be warm enough though. We’ll need to live in the completely insulated house over one winter to work out exactly how much heating is needed.
If we do need to heat the living areas, we intend to do that with hot water from thermal solar panels on the roof. The hot water they generate will go into a heat store, basically a big insulated tank of water that can be pumped around the house when needed. We intend to use it for washing and bathing. And for any heating we need on cold days.
The tricky part, of course, is that when you need heating most, on cold winter days, the solar hot water panels are going to be the least effective. If we find ourselves feeling cold in winter, we’ll use a wood burning stove as extra heating. A stove with a back boiler will also be able to generate more hot water if we need it.
Consequently, we’re building a bit of redundancy into the house. We’re putting pipes for under floor heating into the concrete layer of our downstairs floor in case we need them. We’ll wait and see if we need a wood stove with a back boiler, and how big it needs to be.
And we’re keeping our eye out for free wood, like the dead alder trees Alex’s dad had removed from behind his house. It took us a fair while to chop them up, transport our share to Westacre, and pile them under the leylandii where they will have to season for a couple of years. We are also growing hazel trees for coppicing in parts of the garden.
Wood is a carbon neutral fuel, but if everyone started heating their uninsulated draughty houses with wood, there would soon be a shortage of trees. Insulating the building as much as possible first ensures that we don’t use more than our fair share of trees.
Insulation is the main ingredient in our efforts to minimise Westacre’s carbon footprint. If we can retain as much heat as possible within the house, we may not have to heat it at all. To get there, we need to insulate thoroughly, including above the ceilings in the loft and below the concrete floor.
We are adding a substantial layer of concrete to the floors downstairs. They will function as a heat store, helping to keep the house warm and the temperature even.
Of course, if you want your concrete floor to be a heat store, it needs to be insulated from the actual cold and damp earth. Consequently, we have to install a lasagne of insulation layers under the floor. When all of it is done, it will look something like this:
The starting layer is the site concrete. In the part of the house we are working on, that’s nearly 90 years old now, and very lumpy. We had to even it out with sand to protect the first layer of our lasagne.
First of all, we put down sturdy black plastic as a damp proof layer. It is glued to the actual damp proof course of the house. And of course, it’s important this is not perforated anywhere. Hence the sand.
On top of the plastic are three layers of polystyrene, to a total of about 22cm. All of that is edged with thin Celotex insulation boards going a bit higher up the walls. It’s all been foamed in at the edges to keep everything in place.
Next comes another layer of clear plastic for protection.
The concrete layer is next. But we’re adding a few more features to that.
A lot of electrical and date wiring will run through conduits under the concrete. And we are putting in the pipes for an underfloor heating system. As we don’t know how much heating we’re going to need, we intend to be prepared for everything.
So, on top of the insulation, raised on some bricks, we will be placing a mesh of concrete reinforcement bar (rebar). The underfloor heating pipes are coiled on top of that.
And then we order in the cement lorry. When that day comes, I’ll be sure to get the video camera out. We are hoping for a concrete layer of about 15cm.
The whole thing is levelled off with screed, and eventually we’ll tile on top.
We are investing in staying warm for the long run. When the renovation is finished, we hope to only have to use the underfloor heating on very cold days. It will be fuelled by solar hot water panels, backed up by a back boiler on our wood stove. We’ll only know how well that has worked when the whole project is finished and we’re living in it.