I received a quote from my contractor for our ICF house. He gives us the option between a Hambro System (metal trusses) with concrete slab on top or Open Joist (wooden trusses) with 5/8 plywood on top. The Hambro system is twice the price. I guess we can use a thinslab on top of the wooden joists. Is the thinslab as good as a full size concrete slab for radiant. I saw a few examples of the Hambro system and I like but it is a bit steep. Just worried that a thinslab wont perform equally. Any thoughts

no, thinslab doesn't typically perform as well as a regular slab. that said, it's usually pretty good.

We have radiant in a thin slab on wood floor system. It works great for us. I don't know what more benefit the full slab would have other than additional mass.

I'm not sure if it's formulation of the concrete or what, but full slabs are fairly well documented in their performance and it's typically a significant improvement. Whether that "significant improvement" is actually important or not, varies project to project. and, the benefit has nothing to do with mass.

Man, is there ever a lot of info on this site. I think we will go for the thin slab over wooden joists. Is 1.5" the "normal" thickness for a thinslab? I guess it might be beneficial to beef that up a bit. I will check with my contractor how much concrete we can pour over that. If needed could we increase the number of joists to increase the floor`s load capacity? I`ve read an older post talking about "Gigacrete", went on the website but the "Gigafloor" system is listed as a future project. Anybody heard from that kind of thing, add volume but not mass.

We increased the number of joists and went with deeper ones. We did a 1.5" pour and acid stained the floor. We used regular concrete. We did the acid stained ourselves. All in all it was a very cost efficent way of doing the floors. We would do it again.

Well the only thing I can see that would effect performance is the mass and r value. Thicker slab slower response times but longer coasting on the heating cycle. Thinner slab quicker( but not fast) response and maybe a simpler system( no need for all the extra in slab temp monitors and reset systems)

when I say "performance", I mean water temperature requirements. this would be adversely affected by slab thickness (increased R-value) increasing, and is not affected by mass at all. thinslab can't conduct laterally as much before it gets to the surface, but this should be counterbalanced by the higher temperature at the points directly over the tubing.

Therefore, since full thickness slabs do outperform thin pours, I have to think it's due to the formulation of the concrete used in the two applications. for example, lightweight concrete is less dense, therefor its R per inch must be higher.

Cycling performance I do not discuss with emitters much because you can address cycling with any emitter by using a buffer tank. water temperature requirements, however, are not very malleable.

Jmagill,

what joist did you use (spacing and size) and when you say regular concrete, do you mean same as for a regular slab. That sound interesting and easy, kinda. We tought about leaving it uncovered and acid staining.

Rob,

I remember when my parents renovated their house they poured a kind of concrete with beads in it, I don`t know what they are called but they were aerated (bit like Aero chocolate bar) and you see them sometimes in flower beds. It was light weight and was used to bring to the old section of the house to the same level of the new part. This was in belgium, all brick and concrete construction.

I guess that would beef it up but also increase the R Value.

I will see how much it would cost to increase the joist load capacity and maybe try to go for 2 in overpour with regular concrete. Don`t want to go to far otherwise I might as well go for the Hambro.

We have a very simple floor system and small home. We were able to go from 16" on center to 12" on center and increase the depth from 9" to 11". Our spans are 12'. It was relatively easy. We also installed our own radiant. We did lots of research and reading and of course read every post on this site. All the ideas and different opinions on this site may have your head spinning at first but it will be the best time you have ever spent researching. The back and forth and exchange of info makes you look at the good and bad will save you much hassle down the road.

Our house is 12' spans, 16" on center, w. 2x12 joists. The heating is a thinslab (1.5"). The floor was designed for the load by an architect and confirmed by an engineer; it works fine (but the slab does make it stiff!) I have no complaints about the heat; it is not super-fast to respond, but once it is up to temp, it stays there (we have a slab sensor feeding into the thermostat in the main room, which appears to help a lot; that room is the most stable in the house).

Well I did a radiant slab in the major part of remodelling my home. The original house was 2x12 @16 cc. Two sections, one 14ft and one 12 ft span. I resupported the  14 into  two 7 ft spans and stayed w/ the 12 ft.  The conc is 2 5/8 over a 7/8" floor. Using the existing plates made screeding simple. The 7 ft spans are very solid. I would like the 12 ft to have less deflection but it has not been a major problem. With tile on top my floor is 18 lb/sf.

I used pea gravel as I felt any lightweight aggregate would be at the expense of thermal conductivity and it was not damaging to the PEX.

Mine has a layer of 8 mil poly over the wood then metal lathe. The idea was/is to minimize bonding to the wood to not have wood creak/noise as the conc temp changed. The lathe is to minimize cracking. It was attached w/ just a few staples. Be sure to design in and install Expansion Joints into the slab.

Whether you have thick or thin slab the effectiveness is in the insulation on the underside. Heat won't naturally go up, it will just take the path of least resistance. My choice is foam around the perimeter and then poly wrapped batts. When I added staple-up plates under the bath floors it was simple to pull and reuse the batts.

It looks like we will go for a thinslab. Leaves me more time to prepare it myself. I asked my contractor for a quote for a 1.5" slab, with a 1/2" size tubing that still leaves me an inch. One extra question though. Does a thin slab also take wire mesh for reinforcement and if yes does it go on top or under the pex?

not usually, if you did use mesh it would only be to tie down the tubing. typically you staple to the subfloor.

If you are using the slab as a finish floor concrete acid wash or the like to minimize cracks I would suggest a 2 1/2 inch pour.
This would leave room for wire. Flat road mat 6x6 type in 7 1/2 foot by 20 foot pieces placed on top of pipe, pay attention to the sharp edges of the wire that they are not over pipe in a way to poke a hole in your pipe.
Alternatively if this is just a heat mass substrate, pour 1 1/2 cement with a mix pea gravel, 7 1/2 sack portland, and FIBER GLASS strand, 1 bag per yard.
You can do this in the thicker slab as well in place of the wire but your finish surface will have small bumps or imperfections.
Dan