I've been looking for 1" foil faced polyiso at the usual suspects (Home Despot and Lowes) and do not seem to find it. I do find 1" foil faced styrofoam and thinner polyiso without the foil. Perhaps these are temporary shortages???
It seems that polyiso has about a 40% greater R value, but are there any other properties of styrofoam that would make it unsuitable for a solar collector? (probably air)
Google is is unrevealing as to polyiso vs styrofoam comparisons. I've seen some annectdotal comments that make me think polysterene (styrofoam) is more flamable and more water resistant. Resources?
Be careful about the '40% greater R value' claim. Polyiso, *with a foil face* can have a higher R value than polystyrene, but it's the foil face that makes the difference. I bought some polyiso at Lowes, 1" thick with single foil face. Reading the printed information carefully, I found that it is R-value 5.0 if installed between two other layers, and R-7.6 if installed such that the foil faced a dead air gap. Unless you're building with a dead air gap between it and another layer of material, you don't really get the full R-value of 7.6.
Unfaced polystyrene is a fire hazard and doesn't meet building codes in many areas. It's okay if you're putting a layer of drywall or sheathing over it or something, but it can't be left exposed (much like kraft-faced fibreglass batting). Of course, a collector may not be subject to building codes, but there you are.
The polyiso I bought didn't mention if it met fire codes on the un-foiled side, so I don't know about it.
Polystrene probably wouldn't be the best choice for a collector if you expect to get high temperatures from it, it tends to degrade/soften if heated much past 150F.
I've heard most people find the foil's radiant R-value so confusing to estimate (depending on mean and temp diff, emissivity, orientation, direction of heat flow, and air space dimensions) that the FTC will not allow manufacturers to advertise or mention or count it, even tho it can be estimated with Table 2 on page 22.2 of the 1993 in the ASHRAE HOF (which contains over 800 numbers :-) OTOH, the Atlas Energy Shield folk say the foils increase the aged R-value by keeping the gas in the board over time, compared to EPS or Styrofoam.
I'm surprised they were allowed to mention that.
With 2 foils and no air gaps, you might get R6.5 from the Atlas product. Nick
Article 101464 of alt.energy.renewable: From: snipped-for-privacy@ece.villanova.edu Subject: Re: Heater for outdoor "cat house" Date: 13 Nov 2005 07:03:48 -0500 Organizati>I "built" a house out of a sturdy cardboard box lying about
This sounds good, especially if the top of the door opening is a few inches below the floor inside (like an igloo) so cat-warmed air won't leak out. An ASHRAE-standard 6.61 pound cat with a basal heat generation of 27.21 Btu/h could keep a 1'x2'x1' tall house with 6 ft^2 of exterior walls and ceiling
70 F on a 30 F day if 27.21 = (70-30)6/Rv, with Rv = 8.8 walls, eg 1" "R6.5" double-foil polyiso board with aluminum foil-taped seams.
The ASHRAE HOF says a wall surface with a 50 F mean temp and 30 F temp diff and a 3.5" airspace and e = 0.05 has R2.55. A similar ceiling surface with upward heatflow has R2.01, for R2.55+6.5+2.55 = R11.6 walls and an R10.52 ceiling, so G = 4/11.6+2/10.52 = 0.535 Btu/h-F, and the house could be 70 F on a 70-27.21/G = 19 F day. A normally-active or shivering vs basal ASHRAE cat might keep it 70 F on a 70-68.02/G = -57 F day
We might add an entrance tunnel and a few tiny clerestory windows, eg
2"x4" holes with 0.020" clear flat polycarbonate taped over each side.
Written write on the unfoiled side of the board. Plain as day.
If there's 'no air gap' then the emissivity of the foil becomes pretty much irrelevant. Direct conduction is much higher. In that case I suspect only the R 5.0 for the polyiso is the only relavance.
Table 2 on page 22.2 of the 1993 ASHRAE HOF covers air gaps down to 0.5", with a footnote a: ... Thermal resistance R = 1/C, where C = Hc + EeffHr, Hc is the conduction- convective coefficient, EeffHr is the radiation coefficient ~ 0.00686Eeff[(Tm+460)/100]^3, and Tm is the mean temp of the air space... For extrapolation from Table 2 to air spaces less than
0.5 inches (as in insulating window glass), assume Hc = 0.159(L+0.0016Tm)/L, where L is the air space thickness in inches and Hc is heat transfer through the air space only.
So, the surface conductance is the sum of its radiation conductance EeffHr and Hc, which becomes a lot larger than EffHr as L decreases. For instance, with Eeff = 0.05 (1 foil) at 50 F, EffHr = 0.0455 (R22 :-), but Hc = 0.159(L+0.08)/L, ie 0.17 (R5.8) for L = 1", 0.29 (R3.5) for 0.1",
1.43 (R0.7) for 0.01", and 12.9 (R0.08) for L = 0.001".
Each foil can count, on double-foil foamboard, but 2 facing foils with an air gap only reduce the combined emissivity from 0.05 to 0.03 (1/Eff = 1/E1+1/E2-1) OTOH, 2 foils may retain inert gas longer than 1 foil.
Notes b and c say
Values apply for ideal conditions, ie air spaces of uniform thickness bounded by plane, smooth, parallel surfaces with no air leakage from the space... Thermal resistance values of multiple air spaces must be based on careful estimates of mean temp differences for each space.
A single resistance value cannot account for multiple air spaces; each space requires a separate resitance calculation that applies only for the established boundary conditions. Resistance of horizontal spaces with heat flow downward are sustantially independent of temp diff [and large, eg R8.17 for e = 0.05 with 3.5" and a 50 F mean and 30 F temp diff.]
I understand there is an associated R value associated with air gaps. However, I would have assumed that the R value of of conventional insulation, say fiber glass, would have been better than that of an air gap. At least for gaps that were greater than the separation of the fibers.
What I understood was that the foil on foil backed insulation had a different function. It was to "Reflect" infra red radiation. This reflector can be behind sheet rock which is able to transmit the infra red back to the source.
Fibregalss is not a great insulation. However air is a good insulaltion when you stop it from transmitting heat by convection. Fibreglass woll makes "trapped air" a good insulation.
The foil people state an air gap is needed to reflect the radiant heat. If in contact it is still a metal to conduct the heat very well.
Add the air gap's R-value to the insulation's R-value.
It could do that, with an air gap bgetween the foil and the sheet rock, but the foil won't do much if it touches the sheet rock, except to act as a vapor barrier.
As long as we're talking about insulation, I've been trying to figure out the radiation from the insulation and am not getting far.
From a EffHr 0.0455 (for that .05 foil Eff) and:
Thermal resistance R = 1/C, where C = Hc + EeffHr.
It appears to me that the max effective R value would be 22, and that for an infinitely thick blanket with a foil outer barrier. It would seem that the radiation from the insulation would mean these high R blankets would have diminishing returns.
It also seems that a foil barrier on the ambient side of the insulation would be very valuable.
But things are not done this way, what have I misunderstood?
Which you add to the convection conductance, which typically brings the combined R-value of the foil down to something between 1 and 10.
For the foil radiation alone. But you have to add the convective conductance Hc for the foil to its radiation conductance EeffHr. Then take the reciprocal, then add the insulation's R-value.
No. Add the insulation's R-value to the foil's R-value...
It's valuable on either side, but people don't like foil walls, and the foil would weather badly outdoors, and wind would raise its convection loss.
You've just reinvented the "linearized radiation conductance" :-) G = 4x0.1714x10^-8Tm^3 Btu/h-F-ft^2, where Tm is the mean Rankine temp. But air spaces also transfer heat by convection and conduction...
That also depends on convection and conduction, which depend on the temp diff and the direction of heatflow.
Our local college keeps liquid helium for their electron microscope's superconducting magnet in a Dewar vacuum flask surrounded by liquid nitrogen, with insulation around that. H2 boils at 4.2 K. N2 boils at 77.3 K. So 2 mirrors with e = 0.03 would lose
5.67x10^-8x0.03x40.75^3 = 0.000115 W/m^2C by radiation, ie 0.00002027 Btu/h-F-ft^2, ie US R49335, vs an R20 house wall.
Well, the law of diminishing returns certainly applies to any insulation project. But you can certainly insulate with higher effective R values than
Don't see what you're getting at there. One foot thick fibreglass can give you about R-38, irrespective of the coatings or any 'facing' such as drywall or OSB over it.
Remember, a higher R-value *behind* the foil surface makes the foil surface temperature closer to the ambient temperature. And that reduces radiant losses as well.
As far as a foil barrier on the ambient side, it isn't really all that valuable in most circumstances. Yes, it would certainly reduce the heat gain from direct sunlight, and reduce radiant heat loss. But if you look at how much heat is lost to the environment due to simple convection, you will realize that even if radiant heat losses were cut to zero, it wouldn't reduce the total heat losses by a significant percentage in most cases. You won't get a lot of improvement for the 'buck'. And a lot of folks don't like the idea of living in an aluminum foil sided house.
Radiant losses can be a big issue if the temperature difference is large and/or you've already taken steps to reduce the other forms of heat loss (conduction/convection). Or if your goal is to reduce absorption from the sun.
Sounds rather permanent. The Zomeworks Beadwall system moved small styrofoam beads into and out of a window cavity with a vacuum cleaner. It worked well, but the beads required lots of storage space and they wouldn't flow well through fittings, so each window cavity required a separate store and vacuum cleaner. And the multiple vacs required an electrical sequencer to avoid blowing fuses.
"Replacement foam insulation" (filling the space between two glazings with soap bubble foam at night) seems more practical. It's being applied to greenhouses now. In one system, a shop vac pushes air through a 100'x2" pipe with some holes in a 10% detergent solution near the ground, making bubbles rise to the top of a 100' long quonset-shaped greenhouse. When the bubbles reach the top, the vac automatically turns off until they recede, then starts again for a few seconds every hour or so to replenish them during the night. The bubble system turns off at dawn and a small blower inflates the space between the 2 plastic glazings with air.
As I recall, I've heard reports that there were other issues with the beadwall system. For instance, the foam beads would break down over time.
This might be fine for a greenhouse but I question it's usefullness in a house. For instance, how clear and streak free are the windows when the foam goes away? How do you insure that the window cavities are sealed well enough that they don't ever leak in some hard to detect fashion and cause damage to the structure? With a bubble foam system, how do you design the windows so that they can open?
How about this for a possible solution. There are double pane windows being sold now that have window shades or blinds inbetween the panes. Mostly, this means that they never get dusty and you won't find the cat has hung himself from them. Air is a pretty good insulator except when there is some kind of circulation going on. A cellular shade could be produced using thin mylar or paper such that it folds up into a small space at the top or bottom of the window cavity and yet can unfold to fill the entire space with small air-filled pockets. One or more layers of aluminum coatings could be added as well to help cut down on radiant loss.
Not exactly. They tended to clump if never cycled. IIRC, cycling once a month would fix that.
Moreso than my $500 500 ft^2 cloudy plastic film sunspace :-)
I'd probably make the "windows" with 2 layers of 0.020" clear polycarbonate from a 48" roll, over plastic 2x4s, with lots of silicone caulk.
You don't. A few plain windows might do that.
Even tiny circulations.
It could be...
Good idea. Scheme 18.7 on page 168 of Bill Shurcliff's 1980 Brick House book "Thermal Shutters and Shades" describes 5 sheets of metallized Mylar with springy spacers that unfold when it's rolled down. Scheme 18.8 on page 170 describes an interesting self-inflating Mylar shade. Alas, these are no longer being made. Perhaps they can be recreated with an iron or a $118 -RS1 hot roller for plastic film seam-sealing from Hillas at (800) 952 7274.
Symphony "energy track" shades with tracks on each side to reduce air leaks are fairly expensive and low performing. They (877) 966-3689 say their room darkening shade has a R-value of 3.2, when used with an R1.8 window :-) This increases to R4.8 with side tracks. A 3'x6' shade costs $170 with the tracks.
Tiny cold soap bubbles can have the same R-value as fiberglass. A 6" window might transmit 80% of the sun during the day and become an R20 wall at night. Nick
I was considering this as a seasonal plan. Fill the space between a double sash window and storm window on the north wall with packing peanuts in the fall and remove in spring. I could then throw them away or keep them for next season. There is also the stationary half of my patio door, (hinged french door), that has a cavity of 35" x 78" x 4".
Cost: $0.00, if using used packing peanuts that currently go to landfill. Convincing wife to agree, priceless.
1/2" is just typical spacing between glazing on double-paned windows. It doesn't factor into radiant transfer if the two surfaces are flat planes whose area is >> spacing between them. Yes, the emissivity of many materials are higher than 0.5. But the example (half of which has been snipped here) compared the radiant heat loss with/without a low-e coating, versus the conduction of having two films in direct contact. (remember Duane had questioned why I said that emissivity is 'pretty much irrelavent' if there is no air-gap).
The absolute temperature of most building materials is >> the delta temperature between said materials.
The affects of convection and conduction were already discussed. Those affects far outweigh the effects of radiant heat transfer in most building applications. Only after reducing conduction with conventional insulation, and controlling convection by properly designed air-gaps, does the use of low-e coatings become a significant factor.
Interestingly, the spacing of glazing in double-paned windows is chosen to minimize both convection currents and conduction. From a conduction standpoint, a wider gap is better, but a wider gap allows stable currents to set themselves up between the panes, bad from a convection standpoint. Knowing the properties and pressure of the fill gas, one can choose a gap that has the falling gas film on the cold pane interfere with the rising gas film on the warm pane, and inhibit long-path (full height of pane) circulation.
I'm sure you mean the 'He' boils at 4.2K ;-)
I think there might also be fire issues with packing peanuts. Not like they are going to spontaneously combust but if they ever did catch a spark they would burn intensely and produce lots of toxic smoke.
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