My opinion, in this situation, just go with the duct system.

Can you please elaborate on the reasons for your suggestion? TIA.

I'm not clear on a few points. It seems like you have an existing slab. Are you going to jackhammer that out and pour a new one, this time with insulation underneath or do you plan on adding to the existing?

Keep in mind that the slab acts as a thermal mass and so it isn't as responsive to temperature swings, meaning that your house is not going to cool down as quickly overnight and not heat up quickly like a forced air system would.

The hot floor effect associated with hydronic systems is most pronounced in poorly insulated homes - the more efficient your air sealing and insulation, the less hot floor effect you feel.

Regarding underslab insulation, XPS is usually used but there is debate regarding the cost premium in that it outgases over the years and this reduces the R-value so that by X years, the xps works to the equivalent of cheaper eps which comes with a lower initial r-value.

Your 2nd floor will have what kind of floor system, a poured thin concrete/gypcrete layer, a warmboard system on top of the subfloor with aluminum spreader plates or an underfloor staple up? If you go with the thincrete, then your floor needs to be able to carry the additional mass. This system will be more energy efficient than the other alternatives but will come with the lag time issue, but not as severely as a slab. The other systems will have quicker response time but will have to operate at higher temperatures but won't require a floor to carry significant additional mass.

As to which system or combo makes sense, well that depends on a number of factors. If you have to pour a new slab, then you should be insulating under the slab no matter if you go with radiant or not - it pays to insulate and it increases comfort. If you're pouring a new slab, it might be worthwhile to lay out the radiant tubing and spend the minor amount of money for that tubing, and this then keeps your options open, and same for a future buyer.

Keep in mind that radiant doesn't have to be restricted to floors, you can put it inside walls, or use interior panels, it can even go inside the ceiling and radiate downwards. The positive side here is quicker response time but this comes at the cost of higher temps and loss of warm floor.

You didn't make clear whether your 1st floor will be concrete or whether you will cover the floor with other material. Carpets have insulative value, wood floors less so, tile doesn't, and acid or polished concrete is most efficient. Here's a pic of stained concrete.

http://www.stainedbydesign.com/images/AcidStainDualScore1.jpg

Thank you, Alex House. To answer some questions: we will use our existing slab foundation and build on the top (rest house gutted). There is a tiny portion of the foundation that will be built from scratch, this portion is attached to main original foundation, but will be part of "addition" at ground level.

First floor will be either tile or wood. I currently have wood over concrete slab, separated by a thin insulation, and it's not enough. It is uncomfortable to the point where I have to be in the house with socks on. Thus, my main motivation for radiant is to make it comfortable for my feet. Thus the heating will go in the flooring, not the ceiling.

Regarding second floor: it will not be concrete, it may be ply with possibly metal support but not sure. It will not be concrete for sure, thus I do not anticipate any coldness that I experience on the first floor currently.

Thanks again, look forward to more discussion

Without insulation under your slab, you're going to be pumping heat into the ground. Even if you put dricore down, and run radiant underneath, you still have the uninsulated slab to deal with. If you can re-do the slab and put insulation underneath, you'll increase your comfort. What do your professionals tell you to do?

You are a poor candidate for radiant. Your geographical area is too temperate. Your slab is uninsulated. And the front end costs would be more than double the cost of the minimal forced air system that can handle your mild climate.

Thank you, Sophie, Alex and bry911. I hear you. I see that your point is regarding cost effectiveness/cost efficiency. What if I decide to ignore that part and focus on pure comfort? I ask this not because I am rich, but because currently I am living in a house that has wood floor over slab, with an insulation in between the slab and wood. Winters are miserable with heater blasting non-stop, and it's hard to get motivated to do chores around the house with cold feet.

That said, I hear you that cost-wise it will never pay me back. Now regarding effectiveness: will the fact that the slab is not insulated underneath (Alex: no, we will work with existing slab and have no option of building new slab for several reasons) suck up so much electricity that the effect of radiant be negligible over what it is right now? In other words, will the slab heat in decent amount of time and provide at least some comfort? Any experiences of doing this in SF Bay Area?

homey_bird, as others have said it's probably not a good place for radiant heat. Radiant is slow to respond, and here we can have large daily temp swings. When we were looking at houses a few years ago, we saw one relatively new house with radiant. They had almost all the windows open in December because the house was too hot in the daytime with the radiant heat (even though the heat was off, slab takes a while to cool down).

Radiant is nice in a leaky Eichler but I think you will be better off building a tight house. We are in the Bay Area too and don't wear shoes in the house either. My folks have a radiant electric mat in their bathroom and they rarely use it now because it tends to overheat their entire wing.

Good luck!

Oaktown, thanks. Which type of house do you have, concrete slab or raised foundation? The house we live in is old, so a lot of my problems may be due to its losing warmth rather than cold coming from below.

And, then to solve this problem without heat, are there any thicker insulations that can be laid on the top of concrete to separate cold slab from floors?

Homey_bird, maybe also try over at the greenbuildingadvisor site. A quick search turned up a thread on "insulating floors over concrete slab". On my phone and can't link, sorry.

I mentioned DriCore in my earlier comment, perhaps you should consider a layer, or two, of insulation topped with dricore.

We have a radiant warmed concrete floor for the entire first floor of the house. It is currently 35 degrees outside and it is running enough to warming the house up to 68 degrees and the floor is not hot to the touch - it is cold like concrete in the shade on a summer day. I love our radiant heat, but if your in a warm climate and are looking at radiant just for toe warmers, I would agree that you would be happier with carpet.

neonweb - thanks for sharing. You mentioned "concrete floor" -- is this concrete floor over concrete slab or concrete floor over a foundation with a subfloor underneath? I ask, because as I understand, the concrete slab foundation would delay heating up of the floor (according to some posts here).

Regarding your comment on just wanting toe warmers, what was your principal reason for choosing radiant? Just curious.

The short story is we chose radiant because it worked the best with the floorplan / material choices we made.
The long story is how we got there. We had the choice of a basement, crawl space, or slab. Basements are common in my area because of the required 3' footers - it doesn't cost much to go deeper and have a basement. Financially they make sense, but I don't care for them. They are cold, dark, and low ceilings feel oppressive.

Crawl spaces are nice to allow access, but the floors were crazy cold in winter and the amount of humidity could also turn into a mold problem.

We chose a slab foundation before we had land, and it worked out since the land we built on would not have supported a basement. Our slab has xps foam board under and along the edges for insulation.

Our floor plan is completely custom that I did in autocad myself. All the common areas are on the south side and open. The bedrooms and bathrooms are on the north side. With an open floor plan (some of it 17' tall rooms) and concrete floors it makes forced air ducting more difficult. My experience with forced air is that it blows dust and curtains, feels like a cold draft across your neck when it first kicks on and forces the cold air out before the heated air, and is noisy enough to have to raise the TV or conversation volume when its on.

Choosing radiant floor heat with separate zones for the common and private areas seemed like the perfect choice for heating. But we do use our air conditioner for 6-8 weeks and the floorplan layout was critical in being able to use a wall unit to comfortably cool the whole house.

I was planning on finishing the concrete with coloring and waxing of some kind, but my husband became friends with a person who polishes commercial concrete floors and we had him polish our floors. It looks wonderful, and is easy to clean.

The radiant heat is quiet and even heating with about a 2 hour lag time. I also thought it would be warm on my toes, but I learned that it's not - even in the middle of winter, and want to make sure others are aware of that so they are not disappointed. It's not cold, but its not warm either. Our upstairs is a quality pad and carpeted which feels nicer to my bare feet than the cool concrete.

Each house and owner is different so I hope you find the best solution for your situation.

neonweb, I cannot thank you enough for this information! Definitely noted the point that you do not get "toasty toes", that led me to readjust my expectations! :-) I think I'd be ok as long as my toes do not freeze like they do now.

If you do not mind answering just a few more questions: you mention that your upstairs is a carpet with padding underneath. So do you have any heat there at all? If so, what type?

You mention a 2 hour lag. Do you notice a similar lag during cooling down also?

Also, do you have heated floors underneath bathroom floor on the main level too? (just wanted to confirm!) - in our case, main floor bath will be used by us (it's guest bath cum powder), but hey, it's ~40 sqft, and might as well!

Then, about your concrete floors: I wanted to find out your experience with those, because I've heard that they crack over time and need ongoing maintenance. My husband and I are intrigued by those but we're not willing to take the leap (we are still designing but ...) due to the maintenance stories we heard.

I definitely found your experience very useful. Thanks for sharing.

Our upstairs is wood joists with the pex between them tacked to the underside of the wood subfloor with metal deflector plates. They are on their own zones and turned down low since they get warmed a lot from the downstairs rising heat - even with the floor being insulated.

The lag time for cooling seems longer, but I open the windows to cool before I turn on ac, so it depends on the outdoor conditions quite a bit.

The entire house is heated with radiant - upstairs and downstairs. The ac is a wall unit centally located and cools the entire house. Before we turn the ac on we also have a whole house fan for quick evening cooling. I was unwilling to pay for two types of systems so I had to choose between radiant and forced air. So far the only downside I have found to not having a furnace is having a freestanding humidifier rather than a furnace mounted one

Concrete will always crack, that's why cut joints are so important. The concrete will crack straight in the joint and look nice. Our HVAC guy also warned that having the floor temp (there's a sensor in the concrete) too high can cause the concrete to crack. I think ours is set to a max of 80 degrees. Not really sure about what maintenance there would be. Think about concrete driveways...they take more of a beating than inside a house would and still last a lifetime. If they do crack outside of the joint there's not much you can do about it and about impossible to hide, but that shouldn't happen.

Hi, homey-bird. We put in hydronic floors in our (insulated) slab, for several reasons, but it is crucial to note that with 21st century insulation and windows, they do not feel toasty underfoot. Not like the electrical under-tile installation in our old and drafty house. We did it because we didn't like any of the other options. We are in the Pacific NW and don't use A/C, so had no ducting. We have an open floor plan and locating anything on the walls would be awkward.

I would say our lag time is probably more than two hours, but haven't measured it yet. We tend to arrive at night, turn the thermostat up from 62 to 68, and wake up to a toasty house. Once we arrived on a cold morning, and it seemed like a lot longer than 2 hours before I was actually comfortable.

Some people say the concrete is hard on their legs and feet. I don't like walking on it all day so I have slippers with cushy soles that I wear in the house. My husband wears socks. This is in anything other than the hottest weather. We didn't stain our concrete, but we did have some pigment added to warm the color up some from "sidewalk gray."

In your situation, I would probably make different decisions. I wouldn't do it if I couldn't insulate the slab and if I had to buy and install ducting any way. Some people think it is over kill and "not worth it" in our climate too, but we decided it was worth it to us.

But when we are living there full-time, we won't adjust the temp, but will just leave it at 68 unless we are away. Our bedroom is upstairs and is relatively small, so we just put in a couple of toe-kick heaters "just in case." Last weekend, it was around freezing outside, and when the downstairs was 68, the upstairs was 65, so about perfect sleeping temperature. (Note that 68 in a house with no air moving is a lot more comfortable than a forced air version of 68 in a drafty house.)

Hope you find something useful here. This was not a decision we took lightly-- the downstairs is only 1100 sf and I think the system was estimated at 10K, but of course it is also our finished floor, though I'm sure we paid some extra for the polishing and sealing at the end.