Rocket Mass Heater Weekend Workshop

Last weekend we had our Rocket Mass Heater build workshop with Steve coming down all the way from Lackan Cottage Farm in Mourne, and an awesome bunch of people from all over the island.

The weekend kicked off with a short introductory session in the yurt, but by 11am we were up at the cowshed mocking up the burn chamber in reclaimed bricks. Once everyone had got a good look at how that was put together we cracked on and started the build with the aim of getting it to a point where it could be lit by the end of the weekend.

If you don’t know what a Rocket Mass Heater is, it’s a space heating system developed from the rocket stove, a type of efficient wood-burning stove, and the masonry heater. It has an insulated combustion chamber where wood is burned with high efficiency at extremely high temperatures, and rather than as with most stoves much of the heat exiting the building via the flue, we have a horizontal flue that is encased in thermal mass (in most cases cob, a mixture of clay, sand and straw) which is usually made into a bench that gets warmed when the stove is lit and retains the heat.

We followed the instructions outlined in the book “Rocket Mass Heaters” by Ianto Evans and Leslie Jackson closely with Steve telling us that there are all sorts of experiments and design tweaks that can be made, but it is best to start off following the rules, so that you know it’s going to work!

Personally, I was also keen to make the build as low impact as we possibly could and I managed to  salvage/scavenge most of the materials. The bricks and 8″ ducting that we used was saved from a skip in Dublin (thanks Sean!), the oil drum was found dumped by the side of the road one day in Swinford.  All of the clay that we used for cob came out of the ground here onsite, with some even being dug out of the floor of the cowshed where we put the stove which seemed a nice closed loop 🙂


On Saturday we concentrated on building the brick chamber, making sure the bricks were well bonded with lime mortar (old bricks are best as they have a higher clay content), creating the insulated combustion chamber with expanded metal plastering mesh and a cob and perlite mixture and getting the 8″ ducting in place propped up on more bricks and sealed with metal tape. On Sunday morning it was time for our first test lighting which went amazingly well. Great draw on the flue despite everything being wet still.  Then it was all hands on deck to gather the stone and make the cob for the bench. Great teamwork everyone!

So this week I’ve been lighting it for an hour every day to help dry it out before I get on with the next layers of finer cob and try and decide how I’m going to finish and sculpt it. It lights surprisingly easily, with just a few burps, which is great considering it is so cold and wet still. The wet cob warms and starts to steam so the whole thing looks alive! Maybe a dragon… Watch this space 🙂

Resources and further reading:
Leslie Jackson’s great website (and you can download the book) –
Ernie and Erica’s great website –
Ducting, connectors and fittings –
NHL 3.5 for bonding the bricks came from The Traditional Lime Company in Carlow –
ools and metal tape –


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