I am building a strawbale, passive solar home in Boise Idaho. We have an concrete slab (4 inches) insulated below and on the sides. We have access to donated oak wood flooring (1 inch) which we can adhere directly to the slab using a new adhesive (Taylor Products Meta-Tec MS-Plus which is 100% solid adhesive). I would like to have a discussion about using the oak floor over the slab and how this would effect the thermal mass and heat storage properties of the floor/slab. I think it is a matter of how well the oak can store and conduct this passive solar heat to the concrete slab?

Oak has an Rvalue of 1 per inch. It should not have too much effect on the mass.

Changing moisture and temperature can wreak havoc with the gaps between the boards in tongue and groove flooring.  Ideally, the moisture and temperature should stay in a very narrow range if you want the gaps (cracks) to stay tight.  Of course, this assumes that the oak floooring was properly acclimated before the install.

The solar gain that the concrete absorbs is dependent on the surface being non-reflective, which is a combination of the surface itself (concrete is less reflective than wood) and the color of the surface (black is less reflective than white.) This is why dark concrete floors (or asphalt, for that matter) work well at absorbing the radiation.

If you cover the concrete with wood, you'll disadvantage yourself in 3 (that I can see) ways:
1. The wood won't absorb all of the radiation, and will instead reflect much of it. I don't know how much less it will absorb, but my guess is that the difference will be quite significant.
2. The wood will need to get rid of the heat that it does absorb, since it can't hold all of it, and since it is insulative (with that R-value of 1, compared to concrete, which has an R-value of about 0.1 per inch), it will tend to radiate itself or conduct or convect more of the heat right back out into the air rather than down through the insulative wood to the concrete below. Note that wood floors never really feel hot because they tend to give up their heat as quickly as they take it in.
3. Because wood is an insulator, your concrete will be less responsive to giving up the heat to the air than if it were bare, though I doubt that it will have much heat to give up.

I think that from a passive floor perspective, this is a (really) bad idea.

Jeff

So what is a good passive solar flooring that is more comfortable then concrete. I'm guessing carpeting is really bad.

Tile, slate, etc. But I'm not sure if they are more comfortable than concrete. Carpet is not good. I'm going with stained concrete on my basement floors and ceramic tile in my first floor kitchen dining area which has lots of south facing windows.

I would look at the options that are available from your local concrete suppliers and finishers. Here is a site that might give you some ideas: http://www.concretenetwork.com/ . I would also consider a 6 inch slab, if it is exposed to direct sunlight. Of course the coverings and exposure will mean less if you place a lot of furniture in the way of the direct sunlight.

Let's say you have 1000 sq ft of concrete floor that gets hit by the sun. That's 333 cu ft of concrete that at 10F above ambient, will store 100,000 btu. Probably enough to get through the evening. If you want longer term storage or more even temperatures, I think you need to look at active systems (fans, rocks, water, etc).

I agree with Jeff and his point number 1. The other item not mentioned yet is any hard covering over concrete applied with a mastic or mortar mix or thin set will have trouble getting 100% area covered with no air gaps under the material. Those areas will not transfer heat by conduction.
Ray[

The sunlight on 1" wood flooring will not heat the slab significantly. I suppose this is hindsight, but if the insulation is buried under the fill, you can achieve higher thermal mass than just the concrete slab alone. If you can install thermo-siphon panels downslope and/or below the altitude of the concrete slab, you can achieve radiant floor heating even with oak wood flooring. (You would install tubing before laying the oak). This can be done with air convection if air ducts are installed below the slab. If you are willing to invest in a solar-electric pump(fan) some panels can be higher but would not have passive thermo-siphon. Solar-electric could also improve the flow rate over thermo-siphon alone.