Insulating Grade beam with Rubble Trench?

[Sparticle]

We're building an earth sheltered house and after reading Rob Roy's "Earth Sheltered Houses" we want to insulate the Footings so that there won't be a condensation point.

We want to use a rubble trench foundation so actually the footer would be a Grade Beam of concrete. I have probably a dozen books on building alternative houses and they either have diagrams of insulated footers OR Rubble trenches with no insulation around the bottom of the grade beam.

So, I was thinking the insulation would be (for example) 2" along the southern wall, then what ever appropriate insulation for wrapping down the grade beam, under the grade beam with High compression strength blue board and back up the next side, then going under the floor. This would prevent thermal nose bleeds and a condensation point.

I wonder if there is something I don't understand because I can't find anything on insulating below the grade beam when using rubble trench foundations?

Here is a photo that will show the insulated footers and basically what we want to do with the grade beam.

[barn-apart]

Diagram 2 looks right to me. I think reflectec's insulation site may have some info on foundation insulating.May check the goverment site on insulation also.

[Sparticle]

I"m wondering why in every instance I find of rubble trench there is no mention of this. I'll go check the sites you mentioned.

[Rocky Fields]

I know shale was used in England to keep down the rising damp. It also was used for aquarium bottoms years ago.

[Harry Chickpea]

Rocks last a long time. Concrete lasts a long time. Insulation does not. High compression strength blue board is nowhere near the compression strength of concrete. What happens when a load is unbalanced and one area gets more weight than another? Why do you think that footer trenches are dug and placed into compacted soil rather than on a draining base? The concept of a footer is to provide the most stable foundation possible for further construction.

Although I wouldn't recommend it either, you would be better off placing the footers, then placing a simple thermal break on top. Heavily treated hickory would be an example of what might work. Hickory is dense and used in dams and pilings where moisture is a constant issue. Treating it would limit insect intrusion and damage.

[highlands]

I've been making foundations like this and others for over 20 years. I don't find it is necessary to get the harder to find blue board, the pink board works fine - it has more than enough compressive strength.

Good drainage is critical. Rubble, gravel, drain pipes, slope on compaction or ledge all work. Our foundation looks somewhat like the one on the right except we poured it all as one thing, not in multiple pours for footer and slab. The layers go:

Interior of cottage
Floor slab of steel reinforced concrete
Vapor barrier
Insulation
Gravel with drain pipes embedded
Mountain ledge

The edge of the foundation is shaped to thicken it to be a beam downward and upward to be kneewall on the south and west. All one mono-pour. We used block in other area for walls. There are a few small contact points between the beams of the foundation edge and the ledge to give firm support. This makes it so the house is not actually resting its weight on the insulation.

Our cottage is about 100,000 lbs of concrete, masonry, stone and brick completely within an insulating envelope on a 252 sq-ft foundation. It has a few anchor points to the ledge of the mountain, keying in around natural protrusions so we won't slide down the slope. It stays warm very easily, takes a minimum (3/4 cord) to heat and collects passive solar energy that soaks into the thermal mass.

Eventually we'll berm around the house and over the barrel vault concrete (ferro-cement) roof. A project for another year. That will further improve the thermal performance and lift the wind up off us more.

One of the beauties of the cottage is it is plumb, straight and level. Previously I've been fixing up old houses. A level surface is wonderful!

Cheers

-Walter
Sugar Mountain Farm
Pastured Pigs, Sheep & Kids
in the mountains of Vermont
Read about our on-farm butcher shop project:
http://SugarMtnFarm.com/butchershop
http://SugarMtnFarm.com/csa

[Sparticle]

Originally Posted by highlands

I've been making foundations like this and others for over 20 years. I don't find it is necessary to get the harder to find blue board, the pink board works fine - it has more than enough compressive strength.

Good drainage is critical. Rubble, gravel, drain pipes, slope on compaction or ledge all work. Our foundation looks somewhat like the one on the right except we poured it all as one thing, not in multiple pours for footer and slab. The layers go:

Interior of cottage
Floor slab of steel reinforced concrete
Vapor barrier
Insulation
Gravel with drain pipes embedded
Mountain ledge

The edge of the foundation is shaped to thicken it to be a beam downward and upward to be kneewall on the south and west. All one mono-pour. We used block in other area for walls. There are a few small contact points between the beams of the foundation edge and the ledge to give firm support. This makes it so the house is not actually resting its weight on the insulation.

So you are saying that it's OK to have insulation between the footers (or grade beam) and the rubble trench? Just want to make sure you are saying what I think you are saying. Sorry, this is new to me and I don't' want to misunderstand.

We are going to drain to daylight. Thinking 1/8" per foot and no more than an inch in 10 feet.

[highlands]

I actually built arches per say that transfer the load down to pillars at the points of the arches, corners of the slab, to the ledge. THat was the note about the edge detail and resting the weight on the ledge. Most (>95%) of the footer / grade beam / slab is on the insulation. Since our foot print is so small (252 sq-ft) I did this with just pillars down to the ledge at the corners. For a larger house I would likely engineer more pillars. Figure out your spans. This is an engineering issue that will vary with the structure size, soil strengths, etc. We're sitting on ledge.

We drained to daylight too. It is a good idea so you can cleanout if you need to. I used 4" perfed smooth drain pipes embedded at the bottom of the gravel under the pad.

Something I did was to build many models, such as animal sheds and other things, to test out ideas. Slow but worth it.

Note that where our house makes contact with the ledge directly there is definite heat leakage. I have digital thermal meters I have measured this with. The trick is lots of insulation outside and keeping the cross-section low to minimize this. In another new structure I'm building I went further with thermal breaks. See the link to the butcher shop above.

Have fun with your home building project.

Cheers,

-Walter

[kvr28]

sparticle,

my wife and I built ours this past summer, we didn't go totally underground because we cut robs plans down to 24x40 and the layout would have meant posts in the middle of our living/kitchen room combo, we went with this roof design instead for a good rain water catchment system




I'm slowly putting up a blog of the construction.

http://kvr28.blogspot.com/


I'm only up to the shell being up, posting 800 pictures and a comment on each one is taking longer than I thought.

Speedfunk also has a good thread on his drystacked home he's building on the alternative energy forum here.

[Sparticle]

We've been following speedfunk's thread! We haven't had anyone check our design for the engineering of the roof (we will), but I believe we'll only have one post and it won't be in the way. We have thermal mass interior walls that will bear some of the load as well. I didn't check your blog because we have dial up.

[texican]

I would definitely have a vapor barrier of some sort, regardless of whether you insulate or not. Plastic is cheap. Think of it as insurance in case something goes wrong.

[stanb999]

The picture on the right looks good except the insulation under the footer. I don't care how dense the foam is. For standard footing you want 1000 lbs per cu ft. of lift from the soil below. Foam isn't gonna give you that.


There are quite a bit of options for this issue tho.
Here is one suggestion. Over dig the footing by 1'. Put ultra-lightweight concrete in it as a base. You get lightweight concrete or even lighter a perlite added to the mix for even lighter weight. Get the Unit weight down to 60 or so. Talk to your redi-mix company They get requests for this sort of stuff for special projects. If they won't work with you go to the next guy. A good company should be able to do it for you. It won't have much compressive strength but it will be much higher than any foam and at least as insulated.