Sheep’s wool vs. phenolic board: How we made the choice

Our concern about climate change have led us to an ambitious project. We intend to move our 1930s house into the 21st century by massively reducing its need for active heating.

The main part of this project is improving the insulation. We plan to envelop the entire house in a layer of external wall insulation that keeps the heat inside. For this job, we will use 10 cm thick phenolic insulation board.

You may ask: if you are that concerned about the environment, why are you using a man made material? Why aren’t you going for something natural, like sheep’s wool or straw?

Does it make sense to use a material that ultimately derives from crude oil? Well, for us it does.

Our main restraint is that this project is a retrofit. We are adapting an old house for the future. If we had the means to start from scratch, we probably would choose different materials. As it is, we have a 1930s house that loses heat through its solid brick walls. We need to find a solution that works for us.

The speed at which heat is lost through particular materials is measured in U-values. On average, a solid wall (which is how the main part of the house is built), has a U-value of 2.1. Newly built house walls nowadays have a U-value of 0.3. We are aiming to go better than that, to about 0.23.

What would we have to do to get to that value for different materials?
- phenolic insulation board
- sheep’s wool
– straw bales

1. Sheep’s wool

Wool works best layered between structural elements. (image from

Wool works best layered between structural elements. (image from

I love the idea of wrapping my house in a nice warm woollen blanket. Natural materials certainly have an appeal, and for some projects, wool would be the right choice.

For retrofit external wall insulation, however, sheep’s wool isn’t practical. This material works best when it is sandwiched between two structural walls, like the two parts of a cavity wall. We would have to somehow attach the wool to the outside of the existing wall, and then build another structural wall outside that.

The U value of 10 cm of wool is 0.42. In combination with our solid brick wall at a U value of 2.11, that brings us to a total of 0.35, which is above the value we’re aiming for. We would have to make the insulation twice as thick to come to our ideal value.*

2. Straw bales

The advantage of straw bales is that they are structural. In other words, you can build a wall with them and it will stay up. It is a great material to build new structures with, as it also has a high level of air tightness. If we were to start from scratch, this would probably be one of our materials of choice.

However, to get to our ideal U value of about 0.2, we’d need to wrap the house in straw to a thickness of 40 cm. That is rather a lot of straw and would give us very thick walls. We would have to extend the eves of our roof quite a long way to cover this.

3. Phenolic insulation board

Phenolic insulation board is light and easy to work with

Phenolic insulation board is light and easy to work with (image from

Phenolic board is extremely light and easy to work with. So much so, that we are intending to do this part of the installation ourselves. Covered by a mesh and a special render, it is also very durable and flexible.

We will only need 10 cm of phenolic board to come to our overall U value of 0.23. This is a better standard than modern newly built houses.

We calculate that this will be enough to cut out gas central heating all together. With luck, we’ll be able to install the insulation, and the new windows, this year. We need to live with it through the winter to find out how much heating we will actually need.

Here’s hoping that solar hot water, a wood burner, and phenolic insulation board will keep us cosy even in freezing temperatures.

Have you made similar choices? What worked for you? Please share your experiences in the comments below.

*To find out how U values are calculated, click here.

The Westacre Adventure – how you can get involved

It’s Earth Day today. One day every year, millions of people all around the world get together to raise awareness of environmental issues. This year, the emphasis is on the effects of climate change already visible around the world.

At Westacre, we are building a home and a way of life that uses a minimum of resources and is as self-sufficient as we can make it. Our vision is to create a place of peace and beauty where we can live lightly on the Earth.

Our garden was once a fruit orchard, which retains many of the old apple and damson trees. Right now, everything is bursting into life. We have primroses, cowslips and lungwort, and the damson trees are just about to come into flower.

Westacre’s cowslips in bloom

We would love to share this beautiful place with you. If you support our aim of living more lightly on the Earth and reducing our contribution to climate change, there are things you can do to help us.

1) Share our posts
We aim to reach as many people as possible, so that others can learn from what we are doing and get inspired to start their own projects, large or small. You can help us do that by retweeting, sharing, or linking to any interesting post you see.

2) Tell us about your own efforts
Share your own adventures in working for the good of the Earth, great or small. Comment on our blog posts or on the Westacre Facebook page, or tag us in a tweet. The more conversations we can generate, the more people will hear about us.

3) Come and pitch in
We’ll welcome you with open arms at Westacre.  We’d be grateful for any help you can give us, from mowing the lawn to helping us remodel the roof. There’s always plenty to do.  And there is plenty of time to just hang out, or explore the surrounding countryside.

Rather than being just two people doing an ambitious renovation project, we’d like to grow a community of people supporting each other to do the right thing for our planet. Do you fancy coming along for the ride?

Heating Westacre

The problem of how exactly to provide Westacre with heating and hot water is not an easy one to solve. The main difficulty is that there are many unknowns.

Our first strategy for making the house environmentally friendly and cheap to heat is to insulate it as well as we can. We will be installing high spec windows and external wall insulation.

The trouble is that, once all that is in place, it’s hard to know how much heating we’ll actually need. How much heat will we lose on an average day? We won’t really know until we’ve tried it.

Initially, we thought we would have a wood burner with a back boiler to provide us with most of our heating and hot water. The problem with that is that either
1) you have to light fires in the boiler to get hot water, even if it’s warm in the house, and if we don’t need much heating, the room will soon get too hot.
2) you have to install a sophisticated high efficiency burner that will put 90% of its heat in to the boiler. Trouble with those is that we don’t like the sparse aesthetics of them.

In order to give ourselves the rustic looking wood burner we want, as well as a steady temperature in the house, we need to think of some alternatives.

The current idea is to have more solar hot water panels on the roof, so that most of our hot water and heating comes from that source. And for insurance against many consecutive days without sun, we’ll need a bigger heat store than we originally planned.

Heat stores work like this:

We were originally thinking of having a heat store with a 300 litre capacity. Now we’re considering 1000 litres.

These things are big. I’d even say huge. At 2.1m high and with 1.05m diametre, they take up a lot of space.

So where in the house can we lose one of those?

The thinking continues.

Westacre Phase 1 – Settling in

We have great plans for Westacre.

The house will be getting a complete makeover. We intend to make it as energy efficient as we can. This will involve lots of insulation and a new heating and hot water system based around a wood burner.

As the years go by, our garden will provide more of our food. Slowly but surely, a mostly ornamental garden will turn into a productive permaculture system, incorporating the fruit trees of the orchard the house was built in.

We would like to share all of this with you. You can follow our adventure online, and soon you will be able to get involved in more practical ways as well.


The first phase of the project revolves around settling people into their new homes:

– Roger into his bungalow.
– Alex and Hilde into one room at Westacre.
– new tenants in the house in Harrow.

This is less straightforward than it sounds. We’ve basically got our own little property chain here, and nothing will move unless we make it. The process will involve the following six steps,not necessarily in order:

1) Finish renovating the bungalow

Roger’s bungalow

We had hoped that Roger would be able to move before Christmas. Clearly, this is not the case, and there is still a lot of work to do. We are in the decorating phase now, so hopefully it won’t be too long into the New Year before this renovation is complete.

2) Move Roger’s furniture and belongings into the bungalow
With the Tardis (our Land Rover Defender – it’s a big blue box and it growls) and the big trailer, we should be able to do this ourselves. The greatest difficulty will be for Roger to decide what to take and what to leave behind in the big house. Downsizing isn’t easy.

3) Find new homes for things that are no longer wanted
Thankfully, we have our friend Eva to help with that. She used to do logistics for MSF, so if anyone can move stuff, she can. There will be a number of items from Westacre and Harrow that will be looking for a new home. We will see how much we can sell and give away.

4) Turning the Westacre living room into a bedsit
We will be living at Westacre while we’re doing the renovation. We need to set up one part of the house so that we can live relatively undisturbed by the building work. Westacre’s current living room is ideal for that. It is large, and already has a toilet and shower just next to it. We will also plumb in a sink and put in a cooker. It will be our little refuge for the first year.

5) Move Alex and Hilde’s furniture and belongings from Harrow to Westacre
This process is already on the way. Every time we travel between Harrow and Westacre, several boxes of our possessions come along with us. They are being stacked in an unused room for now. We hope that the Tardis will be man enough for the main move. We envisage a whole week of driving up and down the M40, loading and unloading, and doing a bit of DIY while we’re not driving.

6) Let the house in Harrow
Again, we have done some of this already. The garden has had its winter tidying done and new double glazing has been installed. We still need to do a few paint jobs in there, and a thorough cleaning job, before any tenants can move in.

Once the bulk of that is done, we can move on to Phase 2: insulation. That’s where it gets interesting.

How to get our latest news:
– On the Westacre web site, you can read weekly articles about what we are doing and why. Hilde’s weekly blog of her spiritual journey is also published there.
– On Westacre’s Facebook page, daily highlights will be published.
– On Twitter, you can find a blow by blow account of what is going on. We hope to tweet several times a day.

Video Blog Episode 3 – Ground work

Some of the floor laying and demolition work we’ve been doing in Roger’s bungalow. The ground work before you see real progress.

Laying an insulated floor on top of concrete:
– Put down a damp proof membrane (DPM)
– Screw and nail down laths
– Cut Celotex to size and lay between laths
– Glue tongue and grooved chipboard to laths
– Hammer into place
– Screw down
Going around corners is the tricky bit.