We are building a 3200 sq ft, 2 story home. We love the radiant floor idea and were convinced to go with a single story install, as our home is pretty air tight. We are building an ICF/stick home, with the 1st story being 6" core ARXX and 2x6 with spray foam on the 2nd. Open beam ceiling on the 1st floor to help heat transfer. Engineer has calculated a 15% heat loss and was baffled why we'd want to do 2 floors when so little heat was needed to heat the home. Thoughts? I understand that radiant keeps things warm towards the floor, but the efficiency leads us to believe it wouldn't have a problem heating the upstairs. PS: We also plan to install small wall heaters, just in case. Thanks!

15% heat loss?

what does that mean?

radiant doesn't stratify well. some homes can get away without 2nd floor heat but only if it's cooler upstairs or wide open to below and very few of those work out comfortably.

With the "small wall heaters" and a high-R house with decent windows, and not too much window area, comfort wouldn't suffer.

Location matters though- 2x6 with no exterior continuous insulation isn't very high-R for places with heating design temps below 25F. An ICF with 2.5" of EPS on each side comes in at about R22 in a cold-weather climate, whereas 2x6 with open cell foam is only ~R13 ish, and 2x6 with 5" of closed cell foam is still a bit shy of R15 at a 25% framing fraction. That's a pretty big difference in wall losses between the first floor and the second. Ideally you'd want to put at least 1.5-2" of rigid foam outside the sheathing on the 2x6 to make it at least SOMEWHAT consistent with the loss rates of the first floor walls. But you also won't be getting the mass-effect benefits upstairs, and there will be a time lag between peak heating loads on the first floor relative to the second floor- they'll be a bit out of phase. But as-is it's not even close to the same steady state U-factor on walls even if you discount the mass effect.

Use open cell foam or cellulose in the wall cavities, spend the real foam budget on 2" of exterior rigid iso and you'd be at about the same U-factor, in which case stratification MIGHT get you there, (assuming good windows and an R50 roof.)

The 15% was the number given after calculation of the R ratings in the walls, etc. Thought it would be a figure some would know. Thanks for the info. To all the people that wish to tech dump on me, sorry, I don't have my contractor's badge and am only giving the info I have... as a consumer.

Posted By NRT.Rob on 02 Nov 2012 12:04 PM
15% heat loss?

what does that mean?

radiant doesn't stratify well. some homes can get away without 2nd floor heat but only if it's cooler upstairs or wide open to below and very few of those work out comfortably.

I've never heard of heat loss stated as a percentage. (A percentage of WHAT?)

In N. America heat loss is usually stated in BTUs per hour, or (rarely) in kilowatts. In Europe it's usually stated in kilowatts. (15 kw would be ~51,000 BTU/hr which could be a plausible heat loss for a house like that in a cold/very-cold location, or if there is a lot of window area.)

Make sure you have an idea of how the air is going to move in the home. If you assume that the HRV will SUPPLY to the bedrooms, and EXHAUST from bathrooms you've got a start. Every home will have a natural airflow, you just want to make sure you know what it is and can control it. You also might consider adding a water to air coil module to temper the incoming air from the HRV.

Consider what Dana1 says about exterior iso. As much as I love my closed cell sprayfoam, I MIGHT have gone with the exterior sheets IF I could have found a subcontractor that knew what he was doing with that.

I would never favor in a two story home just heating the lower level.The comfort level deminishes the further one is from the radiant source. Over heating the lower level to obtain comfort on the second floor is never a good idea.For the cost of a proper design vs.the value of your finished home, it just does not make any sense to skimp on the heating system. Does this home not need any cooling capabilities in the summer months.

I'm heating a 6000 sf (unfinished) home at 49N right now with a minisplit running one head in the basement and another on the main floor. Nothing on the 2nd floor. Everyone who sets foot in it remarks immediately on how comfortable it is.

How about a proper heat load and HVAC design by a seasoned professional (the vast majority of "builders" and architects need not apply)?

Radiant floors work well in nearly ever application but will not heat rooms to which they are not attached.

There is rarely an application in new construction where a panel wall radiator is needed to supplement a radiant slab panel. We use a lot of panel radiators in older home here in Minneapolis to supplement sub-floor radiant systems that can't keep up with un-insulated walls and original windows.

Posted By ICFHybrid on 03 Nov 2012 02:16 PM
I'm heating a 6000 sf (unfinished) home at 49N right now with a minisplit running one head in the basement and another on the main floor. Nothing on the 2nd floor. Everyone who sets foot in it remarks immediately on how comfortable it is.

49N represent!  I apologize for not knowing what a 'minisplit', but sounds similar to what I have going if you negate the basement.  So you have no heating on the 2nd floor and it is comfy?

Again, apologies to those that can't make heads or tails of a 15% heat loss... as that is what I was told by the engineer.  My house is insanely stingy on letting heat out.  Let's put it that way. 

Thank you for all the thoughts and questions.  I'm trying to answer what i can.

One is forced air and one is radiant heat, they do not function the same. With the mini-split the floors are cold, with a properly designed radiant floor heating system the floors are heat source. They are not cold.

Posted By uw91 on 04 Nov 2012 11:25 AM

Posted By ICFHybrid on 03 Nov 2012 02:16 PM
I'm heating a 6000 sf (unfinished) home at 49N right now with a minisplit running one head in the basement and another on the main floor. Nothing on the 2nd floor. Everyone who sets foot in it remarks immediately on how comfortable it is.

49N represent!  I apologize for not knowing what a 'minisplit', but sounds similar to what I have going if you negate the basement.  So you have no heating on the 2nd floor and it is comfy?

Again, apologies to those that can't make heads or tails of a 15% heat loss... as that is what I was told by the engineer.  My house is insanely stingy on letting heat out.  Let's put it that way. 

Thank you for all the thoughts and questions.  I'm trying to answer what i can.

How about sharing your location? (Which would give some sense of the outside design temperatures.)

The total amount window area on the first floor and on the second floor and the window's U-factors would also be significant.

Short of separate kilowatt or BTU/hour load numbers for the first and second floors it's hard to say, but at any given outdoor temp, but of the window area is comparable between floors,  as-constructed you would have more heat loss out of the upper floor than the first floor due to the roof losses and the differences in wall-R.

you also have to account for the ability of air to migrate between the areas. wide open cathedral with a couple bedrooms off a loft vs a full 2nd story with just a stairwell is very different.

also with forced air the upper story might be warmer than the lower in some cases. with radiant that will almost never be true, you will have to have a temperature differential to drive heat transfer so the upper floor will always be cooler.

for radiant based systems I penciled out a likely range that stays in the 60's upstairs IF and ONLY IF the upper floor is very well insulated indeed with little glass and you have a single plank floor between lower and upper floor such as in post and beam construction. Joist cavities and drywall won't do it in most cases.

any other method of doing this relies on air movement in some way.

What are the effects of an HRV? Don't they move that air around?

Posted By ICFHybrid on 05 Nov 2012 11:16 PM
What are the effects of an HRV? Don't they move that air around?

While HRV systems can be designed to facilitate moving air from one space to another, they aren't necessarily set up that way. 

It's a reasonable way to assist in evening out room temperatures in high-R homes with point-source heating, but even in houses heated with mini-splits it's usually done with at least one head per story, with the HRV supply in the room with the cassette, and HRV exhaust everywhere else.  This isn't likely to be very effective when the heat source is in the higher-R/lower-loss part of the house, and moving it between stories. 

With large open convective spaces spanning floors and similar loss characteristics for each floor it can be sort of OK (it works at my uncle's place in western WA), but that's probably only true for fewer than 5% of homes.

air coming out of the HRV is, by necessity, below the average weighted room temp (weighted by the CFM draw of your exhaust points). it pulled in room temp air and only got part of that heat back in the exchange process. You can't heat with an HRV if you want truly consistent temperatures.

It can pull some room temp air from an adjacent room into an exhausting room, and it can even out *some* differentials. for example if you had a bedroom that was going to be 50 degrees without an HRV and your HRV is dumping 60 degree air into it because the rest of the house is at 70, it can help narrow the differential. but it will never make that bedroom the same as other rooms, and MOST HRVs/ERVs are not super high efficiency units and will actively cool, at least a little, any space they supply air to in the winter as their discharge temp will often be below any room temp you'd find acceptable.

so it can help keep the upper bedrooms, say, in the low 60's for some people. it's not going to help if you actually want 65-70 degree bedrooms. unless you're keeping the rest of your house at 80 or something

You can't heat with HRV, but you can balance, if your losses are low enough. Pushing the HRV supply into the fully conditioned room and only exhausting from the remote rooms pulls fully conditioned air into those remote rooms, not air at the HRV supply temps. In a high-R house (R35+ for whole-wall R) with only modest amounts of
Most single-zoned homes even with distributed heat see differentials of ~5F between the warmest & coolest room even at outside temps well above the 99% condition.

But most homes have much larger wall & window U-factors, and will see much larger differences. I don't see that approach working AT ALL in this house, unless the 99% design temps are truly modest.

sure. the price you pay there is multiple though. you do have a temperature differential, still. secondarily, this means that you most likely are going to have to ditch bathroom exhaust via your HRV/ERV which basically cedes 20-40% of your total ventilation energy recovery.

not saying there aren't times that this can work for some people... just not for most and even for people it "works" for it's not exactly perfection.

Agreed,temperature balancing via HRV is not going to be viable for most houses (and certainly not for THIS house), and nothing is perfect, not even milli-micro zoning.

Here is the info;

Home is located in NW Washington. Lows probably get to 30's with dips to the high 20's, but usually in the 40s during the cold season.
2nd floor will be 2x8 walls with spray foam. There will be no exterior insulation. ARXX ICF on the 1st floor. 1st floor has only 1 wall with most of the windows. Of 1600 sq ft of floor space downstairs, only about 200 sq ft of window opening.
Slab on grade.