Post tensioned concrete is used because it will have a thinner slab and/or beam and use less reinforcing steel than conventionally reinforeced concrete for a given span.    Depending on where you live, it may or may not be cheaper, but you can do things with it (such as long spans) that are not viable with conventional reinforcing.  It requires an experienced engineer and a skilled installer to do it correctly and I would not want to be the first customer of someone who has not done it before.  PT cabling and tensioning is often done by a specialist subcontractor.

It is VERY  important to keep the tendon pockets dry to prevent corrosion.  There have been lots of PT parking garages that have had this problem.

xexpat

OK - this is very helpful.

Since energy efficiency is near the top of my list of things (fire proof a very close second) it would seem to me that I would want regular old poured concrete (not pretensioned) because a thinner slab means LESS thermal mass (as long as I don't have any real long spans).

Todd

Doesn't really matter for inter-floor decks. In fact less mass is better so your hydronic system warms up faster. Thermal mass matters on exterior walls or those exposed to sunlight for passive heating.

I have been building steel and ICF shelters for about 6 years now in east TN and western NC (24 years experience total). We are currently looking at one for a client in the Asheville area and I personally would never build another home any other way than ICF envelope and light guage framing as well as metal truss roof. Look at www.metwood.com you willl be amazed at the quality of steel and ICF gives you. Good Luck if you need design assistance we can and would love to talk to you. Randy

I would differ with you on this.

First ICF insulates the wall mass and reduces the effectiveness of that mass for passive solar.

Second the point of a hydronic system in a large mass floor is not for quick heating and is usually set a one temp and in a well designed solar home will only cycle on during very cold nights and cloudy days.

Our SIP home with hydronic in concrete works this way. Cycles on during the night and not during sunny days( down to 2degrees last night and sunny during the day. The heat only came on at night.

Well, I've cited studies over in the SIP forum about the benefit of concrete's thermal mass in the walls. It exceeds the foam's insulative benefit, even as insulated by ICF.

And there is just no benefit to thicker concrete in floors (beyond the minimum necessary for no detectable flexure), and many disadvantages, including cost, weight, and thermal inertia. Slower warmup of floors is not beneficial, and thermal inertia here is no advantage since both sides of the floor are in the living space and the heat is generated. In fact, I say that thicker floor concrete uses more fuel; everyone wants to warm up fast, so will turn it up to try and get a quicker response.

"In fact, I say that thicker floor concrete uses more fuel; everyone wants to warm up fast, so will turn it up to try and get a quicker response."

You miss the point. A solar home is not about quick response. The mass should temper all quick changes in temp if operating correctly. This results in even and slow temp changes. The hydronic system is really operating as a back-up system.

Yes there will be those that just don't get it and turn up the heat expecting quicker response but it will only take short time to living in a properly working solar home to teach them that it will not work.

"Well, I've cited studies over in the SIP forum about the benefit of concrete's thermal mass in the walls. It exceeds the foam's insulative benefit, even as insulated by ICF"

Yes it "helps" somewhat with the mass but a properly sized concrete solar inter-floor will work more efficently because it is not impacted by the layer of insulation slowing down the thermal flywheel.

Throwing mass at the sitution is not the answer. Proper sizing and placement of the mass is.

You and I see the function of the floor differently. It's not that I am missing the point. I say the walls provide plenty of thermal mass for any structure; and the Sun will rarely hit the floors due to deep window returns.

People do not typically use their brains at home, frankly. We will just get pissed that it takes a long time to warm up the house when we get back from vacation, when the slab's too thick.

And again, be advised that the thermal mass of the walls has been measured to provide more thermal isolation than the insulative value of the ICF foam.

"And the Sun will rarely hit the floors due to deep window returns."

Not in a properly desigened solar home.

"You and I see the function of the floor differently.' Yes I guess we do.

"People do not typically use their brains at home, frankly. They will just get pissed that it takes a long time to warm up the house when they get back from vacation.'

Harsh and judgemental. Remember that the original poster is taking the time to research and plan a home that will work for them not your idea of what the masses might be living in.

I have just re-read the OP's posts. He does not mention solar so he may not be using this in his design. My mistake, I equate thermal mass with solar.

Jmagill:
You are very correct, properly designed passive solar will use different attributes of a structures' construction (such as thicker floors, windows) and can be a wonderful thing. Often it just takes a little bit of orientation and an open mind to gain great solar benefits. The question is, why WOULDN'T someone offer/investigate it as an option?

Because most of the time in Seattle, the only way to orient the long side of a home is E-W. This is because residential streets tend to run E-W. Many times a home is no more than 30' wide!

Anyway, I am dubious that the average homeowner will go out of their way to open blinds to capture a half degree of gain, particularly when they are at work during the day.

Oh no doubt, passive solar is beneficial. But I'm just saying, in actual practice...

Thanks for the thoughts and comments - as the original poster I'll key you in to where I am at:

- I do plan to have the sun hit the floors - I am working with a very green architect and we will be planning passive solar benefits into it. We are going to have house pointed north +/- 3-4 degrees and a great room with a 2 story bank of south facing windows with a patio/awning outside for shading in the summer. In the summer I open the awning every morning before work and close it when I get home. We are going to hand sew thermal blankets with magnetic seals for the upstairs windows at least and have R 5 or 7 triple cell shades downstairs.

- I do open all south blinds before leaving for work. Why block out free $$ coming in your window in the winter???

- As for the heating up slowly and blasting the heat when getting home from vacation - I will use digital programmable HVAC controls - I will usually program the whole house fan or AC or heat to come on a few hours before our plane lands.

thanks for the thoughts - any more input would always be appreciated.

Security, in the city.

Again - I appreciate all the help on this project. We have put a deposit down on a north facing lot...

I looked on the ORNL website and although they praise ICF they also state that concrete-foam-concrete (i.e. like T-mass from DOW) wall perform better.

this sounds like just a slight variation on the ICF theme -

does anyone have comments about how well this works or doesn't work?

Todd

I am afraid we are hijacking this thread.

Sandwich construction is common in far northern Europe.  My company built a townhouse complex in Moscow this way.  It is expensive, but very robust.  I was not involved directly with the project when I was there, but I know that the units were very comfortable.  I can find more info if you are interested.

xexpat

xexpat - thx for the info. I don't think we're hijacking the thread cuz I am the original poster.

Any info you have about sandwich construction would be useful. Is it more expensive than ICF because you have to put up and take down a frame in order to pour it? When I talked to the guy at Tmass he told me that you could get up to 9 inches of foam sandwiched there (i.e. a true R 45)

It seems like shotcrete systems are making some advances too for a concrete/insulation/concrete sandwich. Isn't the Tmass very expensive for a whole house construction method?