frost line insulation

hey all!

i have a question... i tried to answer it on internet but could not find the information :/

i am wondering what is the best way to insulate the ground below a structure (house or greenhouse) if you hit bedrock before frostline? because i am guessing insulating perimeter walls only wont do it... so what options there is? do you need to insulate flat on top of the rock? or maybe a combination of perimeter walls and skirt? Let me know please!

The only thing that comes to my mind that might work for you is cellular concrete or aircrete. It's concrete full of air bubbles. All insulation gains its insulative (is that a word?) ability thru tiny air pockets. Foam, fiberglass, cellulose etc. all work on the same principle. I don't know what sort of R value potential cellular concrete has but that will vary on thickness. I am not an engineer, and aircrete may not be applicable in your situation. I don't know what sort of tensile strengths it may have over traditional concrete or if it can bear the same point loads. Just a thought, hope this helps!

You can do a skirt of insulation. If you do a google search for frost-protected shallow foundation you will get a lot of information and a few different ways to do it. Check out the diagram.

thanks for taking the time to reply guys!

the one thing i dont understand is how can this (shallow foundation type of insulation) prevent that freezing cold rock from sucking all the heat of the floor? If you were to build a concrete slab i am pretty sure the slab would be extra cold unless you built in a warming system or unless you use this 'aircrete' mentioned above. But still you would have to constantly fight against the cold and have to put in a hughe amount of energy.

The thing is i am not interested into concrete slabs for the moment...I am thinking about a wood planks floor with an airspace beneath. So i am guessing the air beneath is going to be extra cold from the cold rock even with side insulation...is it not?

i know i am in passive solar forum and i am talking about a non-massive floor :S my appologies. But still even with concrete it is still a concern...

You will want to insulate under your floor or you are right and the earth will constantly feed off of the heat from your house. Get as much insulation under that floor as possible. You don't need to do insulative concrete, you can use rigid foam board or anything else you might prefer. If you have easy and cheap access to crushed pumice or lava rock that will work as a nice natural option. Also, roxul makes a rigid insulation that is made from spun metal fines and volcanic rock. Perlite stabilized with clay will also insulate well.

The link i posted below has a chart for the recommended r value for floors depending on your zone.

http://www.greatdayimprovements.com/insulation-r-value-chart.aspx


The insulation skirt around the house is only to prevent freezing in your foundation and keep it from heaving. It wont do much to insulate your house so you will need proper insulation under the floor as well.

When we first built our school in Ladakh 20+ years ago we didn't use any modern insulation materials in our passive solar heated buildings. In many rooms, especially bedrooms and rooms where we'd be sitting on the floor, we got sawdust and shavings from the local lumber yards, and laid that down under floor mats. It was a nice low-tech method, and did make a soft layer. Over the years, it proved to become lumpy and bumpy, and eventually hosted various bugs that eat wood, paper and jute mats, so we've gradually removed it (and added it to the composting toilets). We also had a lot of dust, but can't agree on whether that is coming up from the ground or coming in from outdoors. Some sawdust floors, we replaced with wood plank flooring with gravel underneath. In some rooms where we sit on the floor, and have slate or earth under the mats, we lay closed-cell foam that we can buy in big rolls under the mats, but after a few years it becomes a big piece of garbage. We used crumpled waste paper in a ceiling for insulation, and the mice just LOVE living in it.

We have excellent solar gain all winter so we can afford to have imperfect insulation. Our rammed earth walls provide enough thermal mass so we don't depend on the floor for heat storage.

The point of the horizontal skirt insulation is that the heat moves through the solid ground at a slow pace, so even if the insulation is horizontal, it will protect the heat in your subfloor somewhat. But you'll still be connected to the deep earth temperature, which is probably lower than you want in your house; in our area it's supposedly 5C.

If you are going to make a wooden plank floor, then the insulation doesn't have to bear any weight, so you can use any insulating material that you can get. I recommend that you think about how it might get eaten by or host bugs or mice, and whether it might decay or settle. For example, wool, discarded clothing, and waste paper are good free or cheap materials but may host bugs or mice. Mineral insulation avoids those problems but isn't cheap.

thank you for the information, very appreciated. so many options ill figure it out when time comes.