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.

Moving our lives

Well, we sure have been busy. There has been substantial progress Phase 1 of our project. We finally finished Roger’s bungalow, down to the silicone sealant around the shower tray. And for the last three weeks, we have been moving two households between three houses by means of the Land Rover and a 3m long trailer. We had a deadline to meet as well, as the new tenants are moving into the house in Harrow today.

Finishing ‘the bung’ took so much longer than we had hoped. We missed all the deadlines that we ever tried to put on it. But finally, by the end of February, the last lick of paint was on and it was ready for Roger to move into.

Loading the trailer in the rain

And he has. Several trips with the Tardis and trailer moved all the furniture he needs, and 80 years of possessions and memories packed away in boxes. He is now having a happy time settling in and sorting out.

Meanwhile, Alex and Hilde have so far taken 5 trips up and down the M40 to go and fetch their own lifetime of belongings. Several boxes had already made it to Westacre, but much more had to be fetched, including all our furniture. We worked non stop for two weeks packing, carrying, loading and unloading. For the heaviest items we had help from our friend Matt, and we couldn’t have done it without him.

Less than two weeks ago, we heard that tenants had been found for us, and that they wanted to move in today, 16th March. This made our time scale a litter tighter than we had hoped. As well as all the packing and moving, we spent a day giving our empty house a once-over with fresh paint. It has been professionally cleaned and looks sparkling. Hopefully our tenants will like living there.

Today, Alex and Hilde are having a little break, and are nesting at Westacre. Alex is setting up the new telephone system, and Hilde feels the need to clean things. But we will also have time to just sit around doing not much, and for dinner with friends this evening.

Next on the list is making our bedsit in the big living room, but we’re optimistic about how much space there is. We hope to make it into four separate areas: a bedroom, a living room, a kitchen and a little office corner.

We finally are the occupiers as well as the owners of Westacre. It’s been a long haul. Now the real adventure can begin.

Can we do it in two weeks?

Renovating Roger’s bungalow has been great experience for us. We have both attempted things we’ve never done before and we have learned a lot about plastering, tiling, grouting, plumbing and the rest. All useful stuff that we’ll be able to apply directly to the Westacre project.

On the other hand, the bungalow work has taken much, much longer than we ever expected. Now, finally, we are able to see the end of it, as the bungalow slowly starts to look less like a building site and more like a place where someone could live.

This week, we have made great progress. The kitchen is virtually complete, and the rooms are emptying of building materials and bits of furniture. Walls are being painted and shower screens installed.

Alex hiding evidence of Roger’s minor surgery in the new bungalow kitchen.

We have made an estimate of how much longer we think it will take before Roger can move in, and we think we can do it in two more weeks from today. That’s by 1st February. The last big jobs are the en suite and the floors. And quite a bit more paint.

By the end of the month, we won’t have a perfect show bungalow, but we will hopefully have a home for Roger that is liveable-in, with a few touches of DIY yet to do.

Whatever happens, we are now at the very satisfying stage where you can see very clear changes every day. Keep your fingers crossed for us – we need your moral support.


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.