I design many wood boiler systems with back up propane or electric boiler which automatically come on when water temperatures go too low (when you are away for the weekend. It is a piping and controls thing. You have to find out the cost of each to decide.

ICF homes must have heat recovery ventilators.

First question is heat load and floor coverings.

Find a certified designer first and beware the fellows who tell you that you don't need this and you don't need that (especially if they are not selling it) wood boiler people come to mind).

And please no blankets or bubble foil under the slab.

So I would need a:
Wood boiler
Propane boiler
propane water heater, could this be done by the wood and propane boiler?
Heat recovery ventilator, would I need this if the boilers were outside or in the attached storage building?
A/C, is there a system that would work well with the rest of this?

Why " no blankets or bubble foil under the slab"? I use 2-4" of XPS under slabs.

the boilers could do domestic.

ALL ICF buildings need ventilation no matter what you do with anything else. They are supertight and you will have moisture and/or health issues if you do not address indoor air quality.

right now there are no commonly available or commonly designed cooling systems utilizing radiant, for residential systems. even if you found someone who could do that kind of design, you would have to have dehumidification handled in an air system. More typically you would have a separate cooling system, which might tie in your HRV, if cooling is necessary in your climate.

Foil is for solar gain in hot climates and never, never properly used under slab.

Blankets may provide an effective vapor barrier (better done by 6 mil plastic less the "insulation".
The R-value of the "blankets" is listed low and most likely diminished further when compressed by the concrete. I believe their success comes from naive DIYers and the fact that typical soil has a considerable resistance to heat flux, so once the blanket is in place and the slab and ground below heated up no one is the wiser.

Having run the numbers I would rather have 4' of 2" XPS at the perimeter than a double "blanket" under the entire slab here in Minnesota.

I will have a perimeter frost wall 40"-48" of ICFs (5" of EPS) and 2-4" of tapped XPS with poly under the slab.

Badgerboilermn,

Could you elaborate on the last sentence in your post?


With a full basement ICF foundation (48" below grade min) does under slab insulation become less effective or serve no purpose?

 

Posted By GuyB on 19 Mar 2010 11:08 AM
Badgerboilermn,

Could you elaborate on the last sentence in your post?


With a full basement ICF foundation (48" below grade min) does under slab insulation become less effective or serve no purpose?

 

Slab insulation has plenty of purpose even 48" below, but foil insulation has none.  For radiant barrier to have any function it needs a significant temperature difference ( delta-T) between the radiating element (the slab) and the radiated-upon element (the dirt, or upper layer of XPS),  and both need to be fairly infra-red emissive (which polyethylene, concrete, and soil all are) but also needs sufficient air or other infra-red translucent insulation between them to ensure that radiated heat transfer is a significant (or even primary) portion of the total heat flux.  When the elements are in  direct contact, with no air separating them heat conducts through the radiant barrier from the slab to the dirt (or insulation) the delta-T is nil, and there is NO radiant heat transfer happening within the stackup, with or without aluminized surfaces.

With typical 1/8" thick bubble pack there is a miniscule amount of radiant heat transfer from one side to the other across the bubble, but the convective heat transfer within the bubble is at least two orders of magnitude greater in this instance.  There is but a very small delta-T, so the amount of radiated heat transfer is also tiny. While the aluminizing ends up blocking ~97% of the radiated heat transfer from one side of the bubble to the other it's 97% of an immeasurably small fraction of the total heat flux through the bubble pack.   (And that's before the bubbles eventually collapse under the weight of the slab over time...) 

You need at least 1/2" of air between the two surfaces before the radiated heat transfer can become a significant fraction even with a 50F delta-T.  If the slab is never going to be hotter than ~75F you'd need to be in the land of permafrost-subsoil for the foil to have ANY value, even if the bubbles were 1/2" thick(!), and it would be barely matching the R2.5 performance 1/2" XPS filling the same half-inch gap.  If your subsoil is above freezing, the effectiveness of the foil insulation is dramatically LESS than R2.5 XPS in the gap.

Compare that to using doubly aluminized bubble pack on the rafters in an attic, with a 150F roof deck and a 100F attic floor:  Heat radiating off the roof deck bypasses the 100-120F attic air, heating the attic floor directly, and accounts for over half the heat transfer from roof-deck to attic floor (the rest being conducted & convective heat transfers of roof to air, air to floor.   Placing a reflective layer a few inches away from the roof deck re-radiates 97% of the radiated back at the roof deck, raising it's temperature, but it also heats up the reflective material which can still radiate some heat downward.  The low emissivity of the aluminum limits this heat transfer.  In a doubly aluminized bubble pack the 3% of heat that got absorbed on the first surface is mostly-reflected back up by the second surface, and even with signficant conducted & convected heat transfer within the bubble, a real delta-T develops across the bubble pack, so instead of rejecting 97% of the radiated heat transfer to the attic floor it's rejecting more like 99% (I'm not sure that's a difference worth very much in dollar terms though.) 

The bigger the delta-T the greater the fraction of the total heat transfer from roof deck to floor is radiated, and the larger the "effective-R-value" the reflective layer becomes. Conversely, the lower the delta-T, the lower it's effective-R is.  Applications where the delta-T is small, and there are conductive or convective thermal short circuits across it (like direct contact, or miniscule air gaps) the value of reflective insulation falls rapidly to zero.  Putting ANY other insulation below the concrete & foil insulation without significant air gap renders it's value zero.  In the tortured argument of the Reflective Insulation Manufacturer's association, with an isothermal plane of 125F above a 2" slab, with 5" of well-drained gravel (for some air-gap) between the slab and 55F dirt (a 70F delta across the assembly), inserting a foil between the gravel and the slab yields an effective R1.1.  Since the gravel is about R0.75, and the concrete is about R0.1 for a total of R0.85, the effective-R1.1 by adding the foil results in a 56% reduction in heat loss. See this. 

Wow, 56% in heat savings!  Sounds pretty good right? 

But at lower delta-Ts and higher beginning R-values from other materials you'll be lucky to get an effective R0.01 out of the foil, and 0.0001% reduction in heat loss with the inclusion of the foil.  Your slab will never see 125F (unless your house is on fire, burning to the ground), and relying on a few inches or a few feet of gravel for an insulation levels of under R5 is ridiculous.  Typically R10  (2") of XPS is cost-effective in a radiant slab anywhere the subsoil is 60F or lower, significantly more is cost effective in areas where the subsoils are under 45F.

> For radiant barrier to have any function it needs a significant temperature difference ( delta-T) between the radiating element (the slab) and the radiated-upon element

Radiant barriers work well at low delta-Ts - when delta-T is zero, a radiant barrier stops all heat transfer :-).    The problem is that unless you can put a vacuum in the required gap (like a thermos), convection and conduction limit the value. 

In other words, if the surrounding air is a different temperature than your surface, then a radiant barrier can help you.

To answer your question. You have addressed the greatest potential heat loss in radiant floor applications by generously insulating the perimeter where the greatest temperature differential will occur e.g. the frost line.

Insulation below the slab, though less effective, will increase response time and lower the heating load in most northern climates.

Foil under a slab has little to do with Delta T and much to do with conduction. If the slab is touching the foil conduction and not radiation is the driving factor. In other words foil doesn't add to the U value and is U-sless when place in direct contact with the slab.

Most of the studies you find will be for attics in warm climates where they have marginal success dependent on construction and climate.

Unless your house is on fire. hehehehee, I'm going to use that! ehehehee

Dana. I love you man. Seriously. I want to have a stack of pamphlets printed up with the post you just made and hand them out at airports.

How much benefit is there to do a deeper frost wall, and therefor a deeper insulation at the perimeter?

Little benefit as the temeratures below frost reach "deap earth" temperatures pretty quickly. Put it under the slab.

Careful Rob. I think dana is a wo-man.

No offense.

Posted By BadgerBoilerMN on 21 Mar 2010 10:54 PM
Little benefit as the temeratures below frost reach "deap earth" temperatures pretty quickly. Put it under the slab.

Careful Rob. I think dana is a wo-man.

No offense.

Last time I looked (which was admittedly over an hour ago) I was still a member of the masculine persuasion, but who knows?

Posted By jonr on 19 Mar 2010 03:52 PM
> For radiant barrier to have any function it needs a significant temperature difference ( delta-T) between the radiating element (the slab) and the radiated-upon element

Radiant barriers work well at low delta-Ts - when delta-T is zero, a radiant barrier stops all heat transfer :-).    The problem is that unless you can put a vacuum in the required gap (like a thermos), convection and conduction limit the value. 

In other words, if the surrounding air is a different temperature than your surface, then a radiant barrier can help you.

But with radiated heat the temperature of the surrounding air is completely irrelevant.  With equally emissive surfaces on opposite sides of the air gap (air of any temperature), heat is being absorbed and radiated equally by both surfaces, at the same rate, each radiating heat toward the other.  In this scenarior any heat transfer would be convected, and a radiant barrier would have no more effect than any other air barrier.

Put another way:  Two co-planer panels of the same material, same temperature, with an intervening gap (air or vacuum) each send and receive an incident radiation level of X- X being the emission of that material at that temp.  Put a mirror between them so that each reflects their own heat back to them, they're still both seeing a radiation level of X- it just happens to be the X that they were radiating instead of the X from the material on the other side of the gap, but neither are absorbing more radiated heat than they're radiating away.

For radiated heat transfer to take place one side has to radiate less (the cooler side), cooling the warm side, warming the cool side until they're balanced.  When emissivity and absorptive properties of the materials on each side are different that balance will occur with some residual temperature difference, but concrete and dirt or stone are pretty similar in absorption & emissivity.  That heat transfer process can be interrupted with reflective materials such as aluminum, which is highly reflective of both infra-red and visible light, with a correspondingly low emissivity.

The very low emissivity of aluminum & chrome also cause them to retain heat- they warm up.  In sun-drenched locations on a calm day it's common to have chrome car door handles see mid-day temperatures many 10s of degrees above the more absorptive, (but also more emissive) painted car door, and approaching a hundred degrees above the air-ambient.  This is why  both solar absorption and IR emissivity are specified for "cool roof" materials- both are necessary, neither are sufficient.  A bright aluminum roof on a flat roof deck (no induced convection), still results in high temps a the roof deck despite reflecting well over 90% of the incident energy.

> But with radiated heat the temperature of the surrounding air is completely irrelevant.

Unless you care about total heat transfer (by any method). Then, with air near the surface temperature, you can't interrupt much heat transfer no matter what you do radiant wise. Radiant barriers have great benefit in a thermos (at any delta-T) and not much in a small air gap. But overall we agree.

Sorry Dana,

I thought I saw a reference to lipstick...but I could be putting my foot further in the bucket...

Posted By jonr on 22 Mar 2010 05:14 PM
> But with radiated heat the temperature of the surrounding air is completely irrelevant.

Unless you care about total heat transfer (by any method). Then, with air near the surface temperature, you can't interrupt much heat transfer no matter what you do radiant wise. Radiant barriers have great benefit in a thermos (at any delta-T) and not much in a small air gap. But overall we agree.

But you were saying that radiant barriers were useful anywhere the air temperature is different from the surface temps, which is simply not the case.

If both surfaces are the same temp, and the air between them is higher or lower there is a conducted heat transfer between the air and each surface, but none surface-to-surface.  The surface temps must be different from one another independent of the air temp between them for a radiant barrier to have any effect.  If the air is near the surface temp of one surface and far from the other, radiated heat transfer can still be a very large fraction of the total when delta-Ts are large, since air is fairly insulative.  A radiant barrier can interrupt that quite well.  But it's the difference in surface temps, not the difference between air & and either surface that makes a radiant barrier useful.

In a thermos the space between the glass is near-vacuum, not air, and nearly ALL of the heat transfer is radiated, which is the ideal situation for using low emissivity highly reflective materials as insulation.

It's a rare case that one is concerned about heat transfer between two surfaces at the same temperature, but agreed, that was assumed but not stated.