Rebuilding stone house with external insulation ("outsulation")

Some friends recently had a catastrophic fire (caused by stupid tenants) which completely gutted their stone-walled house here in N Spain.

They want to do the rebuild on a limited budget using natural materials where possible.

My experience is in cob and straw bale building, & I want to help them with the design to achieve a healthy and energy-efficient house.

One key element, I think, will be external insulation. Even though the house is right be the sea and average January temps are only about 10°C, it's still important to outsulate in order to make the most of the thermal mass of the stone. (Also I will be recommending they open more windows in the south wall.)

Does anyone have experience with applying a natural outsulation layer to existing stone walls? The industrial method would be to hang prefabricated insulation panels anchored to the walls. But what about, for instance, using a lime-hemp or lime-straw plaster? How would that work?

For the interior, I'll probably suggest they use a cob floor with expanded clay pellet base for drainage and insulation; and for the inside of the stone walls, earth and/or gypsum plaster.

Anyone got other suggestions or have experience with this sort of renovation work?

Thanks a lot

Robert

I've seen wood chip clay used on a stone barn wall. I haven't done it.

A layer of regular cob could cover the insulating layer. Footings may need to be enlarged.

Hi Dale

I would probably avoid cob in the exterior because of proximity to the sea and severe weather in this very rainy area. Hence lime-hemp or lime-straw.
Thanks for the reply... Any other suggestions?

Robert

Hi Robert,

I have been following along, and Dale made the same suggestions I was thinking of...

I am not sure if proximity to the ocean and rain are a reason not us use either a "lite" or "heavy" cob material on the stone. Lite cobb with a lime render is going to add more "insulation" than a heavy cob would.

Cobs in general are found all over the world from the tropics (with over 3 metres of rain a year) to places like Japan that have earthquakes and tsunami force wind/rain.

I like your idea of lime materials yet these are not going to be as "cost effective" (which seems part of this project challenges) as are just clay based encapsulation systems...

I would probably go with 50 mm of mineral wool board, and air space and wood siding before I would choice a lime/hemp since the goal is "insulation" and not just a "cladding system."

My 2¢ for now......

Robert, check this method, tho it's in french!

http://www.areso.asso.fr/spip.php?article508

Thanks all for your replies!

The problem I see with most of these methods is driving rain. True, a solid cob wall is capable of standing up to driving rain (if well plastered on an annual basis) but its insulation value is poor.

The rockwool/siding method would be most comparable to a standard "industrial" ventilated facade system. Even then I would be very concerned about exposed wood siding. Straw bale or light cob would have the same issues.

Here's a picture of what happened to a (standard) cob plaster over straw bales on my own house (www.abrazohouse.org) which is 10km from the sea in the same climate. In a wet winter driving rain penetrated the lime plaster and saturated the cob, which began to fall off. (This is the south facade but west facing due to "organic shaped" house.) i had to redo both cob and lime plaster, hoping it would stand up better. On the main west wall I have used clay roof tiles -- a traditional method around here on stone buildings!

You might get away with straw bales or light cob on the north and east walls. South wall doesn't really need it anyway.

I suppose I'm really just arguing for the answer I first thought of -- but anyone have more suggestions?

Another, important question: how much insulation value should we be going for to get a reasonable level of indoor comfort?

All the best

Robert

Robert, there must be a way.

May be strawbales, then battens, to make an airspace, and a wood latice to hold the "plaster" I would be inclined to use "hydraulic" lime in this case https://en.wikipedia.org/wiki/Hydraulic_lime Then both sides of the plaster would be able to dry, and so the outside of the strawbales.

I realy think you wouldn't get away with the airspace. With any of the techniques availlable. You could use stuff like "fermacell powerpannel HD" as a backing board. Ok, it's nowhere near green. But it works!