Building with cobb

GreatOaktree

Hi all, Myself and my girlfriend are in the process of buying a house. The issue is that there is no bathroom and i plan on self building on a small extension in cobb so i'm just wondering if anyone on here has any experience with building with cobb in the Irish climate. My main question is, is it cold in the winter ???

Vexorg

Hi GreatOaktree

Cob works well in Ireland. The main thing to bear in mind is keeping as much rain off the walls as possible, so a good over hang on the roof and stem wall to take the brunt of rain splash. If you are not limited to where you can build the extension, try choosing a spot the is protected from prevailing winds thereby reducing its exposure to driven rain and south facing to let the sun heat the walls so making use of the thermal mass.

Internally, as it is a bath room you will want to protect areas around sinks, baths and showers from water splash and leaks, you do not want the cob to be in regular contact with water.

Is it cold, well yes, cob has little insulation value, but its is no colder than any old house be it stone, brick or concrete. If you insulate the floors and roof well and have a heat source in the room, it can be quite cosy. You could put a couple of inches or 50mm (or more) of hemp lime on the walls for additional insulation.

Dont forget to sink "deadmen" (if you are familiar with cob you should know this term) in the walls while building, you will want them anywhere you want to fix cabinets, sinks, windows etc so have a good think about where "stuff" will go before you start building.

Goodluck with the build and keep us up to date if you decide to go for it.

Ps, while cob and hemp lime are hygroscopic materials consider having good ventilation and a humidity controlled extractor fan in the bathroom.

GreatOaktree

Thanks for the reply Vexorg. I have no experience of building with cobb but i have been doing some research and I am confident that it would be a good choice as a building material. I plan on using some timber uprights for extra support of the roof and for areas that i want to "deadman". I plan on putting in concrete foundations and using standard concrete blocks (on their flat) as a foundation wall. Is using a mixer to mix the cobb against the rules?? Is building a 90degree corner a bad idea ?? should the corners be round ??
Thanks again

Vexorg

Using timber uprights in that way is a good idea and will work just as well as deadmen. Using a mixer is not against the rules however I believe that they do not work terribly well. The standard concrete mixer with the blades joined to the sides of the barrel, tends to roll the cob instead of mix it through you also want your cob to be dryish not wet and sloppy like a cement mix, too wet and it's will slump. A paddle mixer would produce better results. Neither will work effectively if you are adding straw to your mix. Now I have never used a mixer so you may well find it produces a mix that works for you.

If you have access to any form of digger, mini digger, job etc you could mix enough cob by lifting, dropping and pressing the cob.

Cob just happens to lend itself to building organic shapes and can be just as easily used to build in straight lines, just round over your external angles as the cob is brittle so will break off easily if you bring it to a point.

What is your soil like, is there much clay in it?

If you near West Cork, there is a one day cob course in early october. If it is close to you it would be worth a look. if you are interested let me know and I will dig out the info and send it on to you.

Keep asking questions, happy to share any info I can.

GreatOaktree

We got the keys a few weeks ago. The chimney needs reflasing and every thing else you can imagine with a rubble stone house. I've decided to build onto the back of the house. It's 9.5 meters wide and i haven't decided how far back i should build but i want a bathroom, utility room, a sitting room and a master bedroom above all that. So basically I want, say, a 10mx12m hipped roof dormer built with cobb. I got two books on the subject "the hand sculpted house" and "building with cobb". The house is in leitrim and the soil is poor, the water seems to be lying on the top soil and the top soil is very shallow. Directly below that the clay was bone dry, stony, gravely, coarse, grey and very sticky when a small amount of water is added. So i guess I'm going to need a lorry load of bright clay ?? I want to use as much local material as possible. I think i should go with a concrete foundation rather than a rubble one. My girlfriends father has a mature forestry of spruce and pine. He said i can use as much timber as I want. Will pine be suitable for rafters and roofing timbers ?? if so i'll be welding a chain saw mill.