So we're building a passive solar home, rectangle with long side facing south, rooms oriented north/south. I was thinking of having polished concrete floors to provide thermal mass, but there are some downsides. It's hard on the feet and I spend much of my time in the home so I worry about it on my joints. I also happen to love the look of hardwood floors, but I know that they would be very poor as thermal mass, blocking the underlying slab from getting much heat at all from the sun. So I had an idea to use concrete on the southern ends of the rooms, say the 10 feet closest to the south wall, and then have the rest of the floor (slab) covered with engineered hardwood (because of the layout, this could actually work without being weird-looking). My thinking is that where the sun actually shines onto the floor would be concrete, and the heat soaked up there could travel underneath the hardwood as the heat migrates northwards through the flooring. Does this make sense?

what is the size of the rectangle?

10' is really not enough but without know the size

Good point. its ~ 28 feet wide, so let's assume 10 feet of concrete at the south end and about 15 - 18 would be covered by hardwood at the north end. Of course this could be adjusted as needed. I can also do the calculations with solar angles and windows to see the farthest point at which the sun would touch the floor and make sure that is concrete....

But in theory, would heat from the concrete-in-the-sun travel towards the north end of the room under the hardwood flooring?

I think you're cutting off too much of the thermal mass to radiate and heat the area.

U need a sun calc and window area heights etc planned out to get a better idea.

Just a guess 18' uncovered & the north 10' with wood.

where is the build?

With the small delta T you will have, figure that significant heat will travel horizontally some number of inches, not feet.

I can do the calculations to see how far in the sun shines in winter.

But I wasn't thinking about the fact that the heat needs to radiate back out again, and I guess if I have more than half my floor covered that isn't going to work too well.

Oh well, I guess concrete it is! Thanks everyone!

walleygirl-

It is a personal thing, but I don't like dancing on concrete, running on concrete, or walking around a home with concrete floors. Concrete is hard on the feet, shins, and joints. Lived in a place with concrete slab floors covered in a carpet pad and carpeting for many years, and glad to leave it behind. My current house has a conditioned crawl space with 55 tons of gravel on top of rigid foam that provides some thermal storage, but most of all, it allows the use of hardwood floors on floor joists with plenty of give. The hardwood floors look nice, and feel good for walking or dance practice. I would sacrifice some thermal performance for a more comfortable house.

Latitude and window design are key, but at 40 degrees (my location) and high windows the sun at its lowest reaches well beyond 18 feet. There's no need to guess. Make a crude model of you house using sketchup. Link it with google earth and you will see exactly how the sun will hit your floors through the year.

Note that proper overhangs will limit insolation to six months or so, so feel free to break out the area rugs in april. Use rubber pads where you stand a lot. Gotta say based on my experience with tile over slab downstairs and hardwood upstairs, Lee Dodge's tootsies must be much more sensitive than mine.

Better than a model go to a pro.  Chris did mine.

He can do the overhangs and all the other info you're looking for shading & penetration design

His fee is VERY reasonable.

If Chris knows his stuff, HE is using sketchup/google earth. Will you need sunglasses to cook dinner? Set Earth at 5 pm and run it through the winter months. Evergreens in the south yard? No problemo. How cold is it on the spring day the overhangs put the house in full shade?

Floors are a minor issue in passive solar IMO, but sunlight is not. It cooks your noggin, fades your furniture, renders Days of Our Lives unwatchable and overheats and underheats depending on the calendar. A pro won't deal with the multitude of tradeoffs because the bill would involve heart attack numbers. What's more, the best time to take light into account is during the planning stage (i.e. adapting the house to the windows.) There is no end of free tools. Using them is work but not as much as retrofixes.
http://www.builditsolar.com/References/SunChartRS.htm

I'm pretty handy with a calculator, and have already looked up the solar angles at my latitude, so I can easily do the calculations myself to see how far into the room the sun shines throughout the year.

When discussing the overhangs with my builder, I noted how frustrating it was that choosing a cutoff angle (which determines the length of overhangs), I had to choose between blocking out sun in the spring when it is desperately wanted or ending up with full exposure in the late summer when its still really darned hot (if only Jun 21 corresponded to the hottest time of year!)...he suggested we come up with an adjustable overhang over the windows and said that he and his partner would enjoy coming up with a design for that. The idea is it would be of fixed length (or width, more accurately) but could be propped up at a higher angle, or down to a lower angle, to expore or shade more of the window. We haven't gotten any further than that yet, but it is a neat idea I think!

You may be able to use a fixed angle with the slats in the shader slanted to allow in winter and spring sun. The summer sun being higher in the sky would be deflected by the slanted slats. If you decide to use a movable shader, then make it so that it can be adjusted for different angles for the different seasons.

You'll have a step between the wood and concrete. Wood flooring or engineered will be 3/4 thick. Even engineered at 1/2 will need thin padding underneath. You'll be stubbing your toes all the time.

gokite, that is a very good point.

I found a site that says you could lay 3/8" laminated prefinished hardwood strip flooring installed with mastic and it would not have sufficient R to block the heat from getting into the slab....what do you all think about that?

I read about a builder that suggested 3/8" was possible,
if it does work, the problem I see is using adhesive that is based on the moisture content of the concrete.

The moisture must be tested and the glue matched to that %.

I think you'll see that glue is expensive and hard to work with given that no fasteners are used.

Now what do you do if the home doesn't get to the desired temps?

here is Mr Radiant John Siegenthaler saying that even thin wood flooring interferes with heat capture. http://www.jlconline.com/radiant-floors/q-a--passive-solar-flooring.aspx

But no reason you can't mix tile and wood in a pattern that ups heat storage and puts wood where you want it. If your plan is open architecture you'll want the floor to demark functional areas anyway. https://www.flickr.com/photos/58061641@N06/14921217367/in/datetaken/ Or use a tile that looks like wood. Google wood grain tile.

If you can tolerate work in (slow) progress, leave the slab bare through a winter to decide how much solar gain you can give up. As a bonus, the longer the slab cures, the less fussy gluing becomes.








https://www.flickr.com/photos/58061641@N06/14921217367/in/datetaken/

I agree Todd is spot on.

If you used wood it also would need to be a dark color, with wood stained dark may as well have concrete.

Personally I think the OP is taking this wood idea too far, skip it or don't bother building passive solar

Todd, that is an excellent suggestion. The interior finishing is likely going to take some time due to a limited budget and us wanting to put most of it into the envelope. So yeah, I would be totally into that experiment.

I could even lay some plywood over spots to see how that affects things. Cool!

gosolar: I see your point. honestly it's the one downside of going passive solar. I love hardwood floors (the look). I really dislike tiled flooring. Polished concrete looks lovely, but I'm pushing 50 yrs old and my joints are starting to talk to me!

I'm looking at a similar design and asking the same questions. One thought I had was using the rubber interlatching tile for the kitchen. It can be pulled up in the dead of winter if you need every extra BTU, and then replaced for comfort most the year. Not exactly glamorous finish, but it's really comfy. http://www.greatmats.com/rubber-flooring/



Also, area rugs.




In general, for a passive solar house I figure on leaving it adaptive. If it needs more thermal mass, add concrete planters or brick up a choice wall. I'm looking at phase change material insulation in walls with access panels that you can slip more or less in, or redistribute it. Theory one is to use two separate temperature PCMs lower and higher in the house. I'm guessing higher phase change temp lower in the house and higher temp, higher location. I could be wrong, so I'd like to make them swappable.




My objective is to get an inspection worthy house as cheap as possible and then renovate where I see fit. I want to try the polished concrete floor and decide then if it's too hard or slippery.

Radiant tubing in the floor and a small pump will distribute the heat for about a dollar a square foot installed cost. 50 watt pump, easy to run off small PV panel. No on going costs. This will solve the heat distribution issue.

Brian