polyiso vs styrofoam

Question:

   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?    Cheers, Jeff

Response:

>   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?

I just found this: <URL: http://www.pima.org/technical_bulletins/tb201.pdf > which indicates vastly inferior performance (of styrofoam) at temps over 165 degrees as well as some other nasties.    Jeff – Hide quoted text — Show quoted text ->   Cheers, > Jeff

Response:

>    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

Perhaps you should try an actual lumberyard. Mine has Celotex in any thickness you’d like, and I’ve been noticing some interesting things as I watch prices there and at HD – for instance, HD has a roll of 9" wide flashing (Vycor) for about the same price that the lumberyard does. But HD’s is 33 feet long, and the lumberyard’s is 75 feet long… — Cats, coffee, chocolate…vices to live by

Response:

>   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)

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. daestrom

Response:

I use Johns Manville Ap foil faced poly foam,,good to temp 250 f . www.jm.com

– Hide quoted text — Show quoted text ->   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) > 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. > daestrom

Response:

>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’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 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.

I’m surprised they were allowed to mention that. >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.

With 2 foils and no air gaps, you might get R6.5 from the Atlas product. Nick Article 101464 of alt.energy.renewable: Organization: Villanova University >I "built" a house out of a sturdy cardboard box lying about >and glued 2" foamboard insulation to it. >Added an extra 2" on the bottom. >"Door" opening just big enough to allow cat to enter. >An old blanket on the floor inside for comfort.

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. Nick

Response:

– Hide quoted text — Show quoted text ->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’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 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. > I’m surprised they were allowed to mention that.

Written write on the unfoiled side of the board.  Plain as day. >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. > With 2 foils and no air gaps, you might get R6.5 from the Atlas product.

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. daestrom

Response:

I wonder what the net result is for "trapped air" insulation such a spun wool. Would the thermal conduction apply and the radiant reflection still be defeated, as per the manufacturer’s warnings? I would think they have done thorough testing of different structures.

– Hide quoted text — Show quoted text -> 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. > daestrom

Response:

Hi Daestrom;

 > If there’s ‘no air gap’ then the emissivity of the  > foil becomes pretty much irrelevant. Would you mind giving a citation for this? I have seen this stated a number of times and no where can I find any studies that support this idea. I suspect this is a "wives tail". Nick, can you give us citations pro or con?  > daestrom Duane —      Home of the $35 Solar Tracker      Receiver     http://www.redrok.com/led3xassm.htm      [*]    Powered by                          //|   Thermonuclear   Solar Energy from the Sun / | Energy (the SUN)                  /  /  | Red Rock Energy                   /   /   | Duane C. Johnson   Designer      /   /    | 1825 Florence St  Heliostat,Control,& Mounts | White Bear Lake, Minnesota    ===   /    | USA      55110-3364                ===     | (651)426-4766        use Courier New Font  | http://www.redrok.com  (Web site)          ===

Response:

> Hi Daestrom; >  > If there’s ‘no air gap’ then the emissivity of the >  > foil becomes pretty much irrelevant.

I was struck by this also. This seems to have gotten muddled. If we were talking about emissivity then I believe this "radiated loss" is proportional to the temperature difference. The temperature difference in sandwiched material with the foil in the middle, would be small. On the other hand, if this was the last "barrier" before ambient, the tempearture difference would be at it’s greatest and this would be the only place where a low e foil barrier would be usefull. My understanding of this is shaky… There may be more going on as reflection may not equal emissivity.    Cheers, Jeff – Hide quoted text — Show quoted text -> Would you mind giving a citation for this? > I have seen this stated a number of times and no where > can I find any studies that support this idea. > I suspect this is a "wives tail". > Nick, can you give us citations pro or con? >  > daestrom > Duane

Response:

> > If there’s ‘no air gap’ then the emissivity of the > foil becomes pretty much irrelevant.

I agree. >Nick, can you give us citations pro or con?

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". L         Hc    EffHr    U = Hc+EffHr   R = 1/U 1"        0.17  0.0455   0.2155         4.6 0.1"      0.29  0.0455   0.3317         3.0 (surprisingly large) 0.01"     1.43  0.0455   1.4755         0.7 0.001"   12.90  0.0455  12.9455         0.1 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.] Nick

Response:

> Hi Daestrom; > If there’s ‘no air gap’ then the emissivity of the > foil becomes pretty much irrelevant. > Would you mind giving a citation for this? > I have seen this stated a number of times and no where > can I find any studies that support this idea. > I suspect this is a "wives tail".

Well then, "I suspect" you haven’t studied heat transfer and fluid flow for 30 years like I have The heat transfer by radiation between two metal films is orders of magnitude lower than conduction through direction contact between the films. The only reason radiant heat transfer even becomes an issue is because it is on a comparable level with heat transfer through a stagnant air gap (in which internal convection currents are minimal). Two surfaces spaced a modest 1/2 inch apart with emissivity/abortivity of about 0.5, with temperatures of 460 R and 461 R will have a net radiant flux of q’ = emissivity*Stefan-Boltzmann * (461^4 – 460^4) = 0.34 BTU/hr-ft^2 (R-value of 3.0).  Even without the metal foil, which would up the emissivity/absorbtance to almost 1.0 (ideal black-body), you only get a radiant heat flux of 0.67 BTU/hr-ft^2 (R-value of 1.5). Compare that with the conductive heat transfer for 2 layers of Al foil for a total of 0.039 inch (1 mm, or 0.0033 ft).  Al has a conductance of 136 BTU/hr-ft-R, so the heat flux through the two films when in contact with 1 degree R difference would be q’ = (461-460)*136 / 0.0033 = 41200 BTU/hr-ft^2 (R-value of 2.4e-5) (even worse if you think the Al film might be thinner). So you see the foil layer in direct contact with another surface provides basically no ‘R-value’ at all. Interestingly, if the foil is facing into a room with no other material ’shielding’ it, you have performance in between.  In this case, the metalized film does reduce radiant heat transfer slightly over other surface emissivities, but you now have convection heat transfer.  And how much heat is transfered by convection can depend on a lot of factors.  A second material properly spaced from the metal film, forming an enclosed ‘air-gap’ can minimize convection currents, leaving radiant heat transfer as the only significant factor.  But if the metalized film is just exposed to an outside wind of 15 mph, the convection heat losses through the air layer will far exceed the radiant heat losses, regardless of using a low-e metal coating or not. This is the ’state of the art’ with regard to double-pane windows. Conduction and convection heat transfer between the panes has been reduced to the point where radiant heat transfer between the panes is a significant factor.  Hence low-e (low emissivity) coatings are applied to the face *between* the panes to reduce the radiant heat-transfer component. "Principles of Heat Transfer 3rd Edition" Frank Kreith is a good source.  Or search google for radiant heat transfer.  One of the major window manufacturers has a good writeup about how low-e coatings work in their product, but I don’t remember which (Andersen maybe?  or the national-fenestration web site (www.nfrc.org)??) daestrom

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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. Duane —      Home of the $35 Solar Tracker      Receiver     http://www.redrok.com/led3xassm.htm      [*]    Powered by                          //|   Thermonuclear   Solar Energy from the Sun / | Energy (the SUN)                  /  /  | Red Rock Energy                   /   /   | Duane C. Johnson   Designer      /   /    | 1825 Florence St  Heliostat,Control,& Mounts | White Bear Lake, Minnesota    ===   /    | USA      55110-3364                ===     | (651)426-4766        use Courier New Font  | http://www.redrok.com  (Web site)          ===

Response:

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. – Hide quoted text — Show quoted text -> 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. > Duane > — >      Home of the $35 Solar Tracker      Receiver >     http://www.redrok.com/led3xassm.htm      [*] >    Powered by                          //| >   Thermonuclear   Solar Energy from the Sun / | > Energy (the SUN)                  /  /  | > Red Rock Energy                   /   /   | > Duane C. Johnson   Designer      /   /    | > 1825 Florence St  Heliostat,Control,& Mounts | > White Bear Lake, Minnesota    ===   /    | > USA      55110-3364                ===     | > (651)426-4766        use Courier New Font  | > http://www.redrok.com  (Web site)          ===

Response:

>I understand there is an associated R value associated with air gaps.

Sure… >However, I would have assumed that the R value of conventional >insulation, say fiber glass, would have been better than that >of an air gap.

Add the air gap’s R-value to the insulation’s R-value. >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.

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. Nick

Response:

- Hide quoted text — Show quoted text ->I understand there is an associated R value associated with air gaps. > Sure… >However, I would have assumed that the R value of conventional >insulation, say fiber glass, would have been better than that >of an air gap. > Add the air gap’s R-value to the insulation’s R-value.

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?    Cheers, Jeff – Hide quoted text — Show quoted text ->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. > 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. > Nick

Response:

Daestrom> Two surfaces spaced a modest 1/2 inch apart with Daestrom> emissivity/abortivity of about 0.5, with temperatures Daestrom> of 460 R and 461 R will have a net radiant flux of Daestrom> q’ = emissivity*Stefan-Boltzmann * (461^4 – 460^4) Daestrom>    = 0.34 BTU/hr-ft^2  (R-value of 3.0). Seems like the air-gapped foil-backed insulation’s R-value is dependent on the foil temp. i^4 – (i-d)^4 = -4i^3d +6i^2d^2 -4id^3 +d^4 ~= -4i^3d So the air gap heat flow is just about linear with delta-T, but goes up as the cube of ambient temp.  So while you got an R-value of 3.0 for the interior of a cold refrigerator, the same foil against the interior of a house would see an R-value of 2.0. Seems like the R-value would be a lot higher if the foil faces the cold side.  If it’s -30 F outside, R-value of that gap is 3.75.

Response:

>> Add the air gap’s R-value to the insulation’s R-value. >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)

That’s a radiation conductance. >Thermal resistance R = 1/C, where C = Hc + EeffHr.

Which you add to the convection conductance, which typically brings the combined R-value of the foil down to something between 1 and 10. >   It appears to me that the max effective R value would be 22…

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. >It would seem that the radiation from the insulation would mean >these high R blankets would have diminishing returns.

No. Add the insulation’s R-value to the foil’s R-value… >It also  seems that a foil barrier on the ambient side of the insulation >would be very valuable.

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. Nick

Response:

– Hide quoted text — Show quoted text ->>I understand there is an associated R value associated with air gaps. > Sure… >>However, I would have assumed that the R value of conventional >>insulation, say fiber glass, would have been better than that >>of an air gap. > Add the air gap’s R-value to the insulation’s R-value. > 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.

Well, the law of diminishing returns certainly applies to any insulation project.  But you can certainly insulate with higher effective R values than 22.  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. > 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?

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. daestrom

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>>I’m not on usenet and don’t know how to post there, but I read your = >posts frequently… >If you can read usenet posts, you can probably post yourself… >Might be worth learning how.

OK. Looks like I got it figured out. Now as far as window insulation goes, what about packing peanuts between the panes? Dennis

Response:

>Daestrom> Two surfaces spaced a modest 1/2 inch apart with >Daestrom> emissivity/abortivity of about 0.5, with temperatures >Daestrom> of 460 R and 461 R will have a net radiant flux of >Daestrom> q’ = emissivity*Stefan-Boltzmann * (461^4 – 460^4) >Daestrom>    = 0.34 BTU/hr-ft^2  (R-value of 3.0).

What 1/2"? Why 0.5? Most materials are closer to 1. >Seems like the air-gapped foil-backed insulation’s R-value is >dependent on the foil temp. > i^4 -(i-d)^4

= i^4 -(i^2-2d+d^2)(i^2-2d+d^2) = i^4 -(i^4-2i^2d+i^2d^2-2i^2d+4d^2-2d^3+i^2d^2-2d^3+d^4) =           2i^2d-i^2d^2+2i^2d-4d^2+2d^3-i^2d^2+2d^3-d^4 =           4i^2d-2i^2d^2     -4d^2+4d^3            -d^4 >~= -4i^3d

Yes, with a + vs -, if i>>d. >So the air gap heat flow is just about linear with delta-T, but goes up >as the cube of ambient temp.

You’ve just reinvented the "linearized radiation conductance" G = 4×0.1714×10^-8Tm^3 Btu/h-F-ft^2, where Tm is the mean Rankine temp. But air spaces also transfer heat by convection and conduction… >So while you got an R-value of 3.0 for the interior of a cold refrigerator, >the same foil against the interior of a house would see an R-value of 2.0.

That also depends on convection and conduction, which depend on the temp diff and the direction of heatflow. >Seems like the R-value would be a lot higher if the foil faces >the cold side.  If it’s -30 F outside, R-value of that gap is 3.75.

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 5.67×10^-8×0.03×40.75^3 = 0.000115 W/m^2C by radiation, ie 0.00002027 Btu/h-F-ft^2, ie US R49335, vs an R20 house wall. Nick

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>>If you can read usenet posts, you can probably post yourself… >Might be worth learning how. >OK. Looks like I got it figured out.

Congratulations! >Now as far as window insulation goes, what about packing peanuts >between the panes?

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. Nick

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>>Now as far as window insulation goes, what about packing peanuts >between the panes? > Sounds rather permanent. The Zomeworks Beadwall system moved small > styrofoam beads into and out of a window cavity with a vacuum cleaner.

… 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. > "Replacement foam insulation" (filling the space between two glazings > with soap bubble foam at night) seems more practical.

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. Anthony

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>… I’ve heard reports that there were other issues with >the beadwall system. For instance, the foam beads would >break down over time.

Not exactly. They tended to clump if never cycled. IIRC, cycling once a month would fix that. > "Replacement foam insulation" (filling the space between two glazings > with soap bubble foam at night) seems more practical. >… I question it’s usefullness in a house. For instance, how clear and >streak free are the windows when the foam goes away?

Moreso than my $500 500 ft^2 cloudy plastic film sunspace >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?

I’d probably make the "windows" with 2 layers of 0.020" clear polycarbonate from a 48" roll, over plastic 2×4s, with lots of silicone caulk. >… how do you design the windows so that they can open?

You don’t. A few plain windows might do that. >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.

Even tiny circulations. >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.

It could be… >One or more layers of aluminum coatings could be added as well to >help cut down on radiant loss.

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

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– Hide quoted text — Show quoted text -> Daestrom> Two surfaces spaced a modest 1/2 inch apart with > Daestrom> emissivity/abortivity of about 0.5, with temperatures > Daestrom> of 460 R and 461 R will have a net radiant flux of > Daestrom> q’ = emissivity*Stefan-Boltzmann * (461^4 – 460^4) > Daestrom>    = 0.34 BTU/hr-ft^2  (R-value of 3.0). > Seems like the air-gapped foil-backed insulation’s R-value is > dependent on the foil temp. > i^4 – (i-d)^4 > = -4i^3d +6i^2d^2 -4id^3 +d^4 > ~= -4i^3d > So the air gap heat flow is just about linear with delta-T, but > goes up as the cube of ambient temp.  So while you got an > R-value of 3.0 for the interior of a cold refrigerator, the same > foil against the interior of a house would see an R-value of 2.0. > Seems like the R-value would be a lot higher if the foil faces > the cold side.  If it’s -30 F outside, R-value of that gap is 3.75.

Yes, you have an interesting point.  The absolute temperature involved does make a difference.  But contriving a system where you can get the foil to be coldest isn’t easy.  You can’t just put it on the outside surface, convection losses would over-shadow any radiant issues.  If the air gap is embedded in the wall, then it will be at some temperature between inside and outside, depending on exactly where the air gap is in relation to other materials.   I suppose ideally we would put the air gap as close to the lower temperature side of the construction as possible.  That sounds like an outer surface with a low emissivity foil, then protected from excessive convection losses by some thin film to form the air gap.  Not sure how practical such a construction would be in a high wind situation though. daestrom

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