Any reason to use foil-faced board insul
Jeff_Clarke
| Posted in Energy, Heating & Insulation on
Application is insulation board on the interior of solid masonry exterior (extruded polystyrene board)
With this application (interior between furring strips) is there any reason to use foil-faced board?
Jeff
Replies
just my usual caveat......
no foam unless it's treated ....
too many bad experiences with carpenter ants, termites and other vermin
With this application (interior between furring strips) is there any reason to use foil-faced board?
There are two advantages to foil facing. On polyisocyanurate, it dramatically slows outgassing of foaming agents and prolongs the original high R-value.
With XPS, the foil facing offers no R-value benefit unless it is used as a radiant barrier which requires an air space between foil and next layer (DW). A 3/4" vertical air space in a wall assembly with foil one side and paper (DW) other side will offer a "free" R-2.8 and raise the surface temperature of the drywall to increase the mean radiant temperature of the conditioned space.
Because human skin has one of the highest E-values of any material (0.87) the mean radiant temperature of a living space has about the same impact on comfort as the air temperature. That's why everyone loves radiant heat, whether it's the sun coming in a window, or a woodstove or radiator or a radiant floor or wall panel. Radiant ceilings, however are the least effective, since human comfort requires a higher temperature at the feet than at the head.
Solar & Super-Insulated Healthy Homes
Yes - thanks and in the 1" I have I can get R 6.5 vs. R 5.0 with foil/iso
Jeff
the 1" I have I can get R 6.5 vs. R 5.0 with foil/iso
You've got it backwards. 1" XPS is R-5. 1" foil/iso has a stablized R-6.5.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Yes - you are right. Meant 5.0 with extruded polystyrene.
Jeff
<< A 3/4" vertical air space in a wall assembly with foil one side and paper (DW) other side will offer a "free" R-2.8 >>
I'm curious how you figured that...not that I disagree, just wondering how a planned vertical airspace is good, and an unplanned vertical airspace is bad. Seems like an area prone to convective loops, no? Though i suppose the convective loops wouldn't matter if there was no fibrous insulation being affected....I may have just answered my own question, ha. If you just give me a few minutes that sometimes happens!
Justin Fink - FHB Editorial
Edited 1/12/2009 2:29 pm ET by JFink
I'm curious how you figured that...not that I disagree, just wondering how a planned vertical airspace is good, and an unplanned vertical airspace is bad. Seems like an area prone to convective loops, no?
I didn't figure that. It's from the ASHRAE Book of Fundamentals, the "bible" of the HVAC industry.
Yes, there will be internal convection in any vertical air space of more than 1/2" - 3/4", which is why the R-value benefit of a vertical radiant space will diminish beyond 3/4" thickness.
A horizontal radiant air space (floor or ceiling), however, will increase in R-value with thickness and will insulate better if heat flow is downward than if it is upward. A slanted air space (roof) is in between the two.
Riversong HouseWright
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Edited 1/12/2009 4:30 pm ET by Riversong
I always thought that when people said foil insulation would "reflect" heat back outside/inside. Since photons are not going to get thru the sideing, nothing to reflect. Heat transfer thru any material will vary but nothing is bounced back. Your point on stopping outgassing is interesting. I've never heard of that before.You get out of life what you put into it......minus taxes.
Marv
I always thought that when people said foil insulation would "reflect" heat back outside/inside. Since photons are not going to get thru the sideing, nothing to reflect.
Radiant barriers don't reflect photons. They work by not emitting infrared wave lengths (heat). Their value as radiant barriers is determined by their emissivity rating (E), which is why we use windows with a lowE coating - they don't emit radiant energy, which is the primary heat loss mechanism between glass panes. Foil has extremely low emissivity (E-0.05, compared to E-1 for perfect emissivity).
Your point on stopping outgassing is interesting. I've never heard of that before.
Polyisocyanurate foams are rated with a Long Term Thermal Resistance (LTTR) R-value representing a 15 year weighted R-value. This is in response to issues of thermal drift of the polyisocyanurate products. Thermal drift occurs due to the gasses produced during the forming of the foam. These gasses slowly diffuse out of the product over time and are replaced by air. Since these gasses also have more thermal resistance than air, the R-value of polyisocyanurate diminishes over time as the gasses diffuse out of the product. Facings on the insulation board, such as aluminum foil, will slow this process down as the diffusion can only occur out the edges of the product and not through the front and back faces. Most polyisocyanurate products have an LTTR R-value of R-6.5 per inch.<!----><!----><!---->
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I thought XPS was preferred for insulation of basement walls?Building sciences specs 2" of XPS against interior basement concrete walls, than a small gap (1/2" I think?) then your standard 2x4 stud wall and DW.Would polyiso be a better solution than XPS?I'm looking into finishing out a section of my basement soon (ordering materials this weekend).
I thought XPS was preferred for insulation of basement walls?
Would polyiso be a better solution than XPS?
No. Foil-face foam board cannot breathe. While XPS has a perm of 1.1, so is a vapor retarder, it will still allow some drying to the inside.
Using a near-perfect vapor barrier, like foil or 6 mil poly, is almost always a recipe for problems. Wall assemblies ideally should be able to dry in both directions.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Thanks.XPS it is.
Riversong,I've always a hard time following this reflective barrier business.Your saying reflective barriers don't reflect photons??? How about a mirror? Sure seems to reflect photons.You said reflective coatings don't emit I/R waves?? I would think they do, and I would think the wave length they emit is a function of the temperature of the foil.What am I missing here?Harry
Edited 1/12/2009 8:11 pm ET by inperfectionist
I've always a hard time following this reflective barrier business.
Because it's not a reflective barrier - it's a radiant barrier.
Your saying reflective barriers don't reflect photons??? How about a mirror? Sure seems to reflect photons.
I'm saying that's not the function of a radiant barrier. Why put a mirror inside your wall? The function of a radiant barrier, say in a roof assembly, is to prevent the sun's intense radiant energy from penetrating into the attic and into the living space. It does this by not re-radiating IR because it has a very low emissivity.
You said reflective coatings don't emit I/R waves?? I would think they do, and I would think the wave length they emit is a function of the temperature of the foil.
What am I missing here?
Radiance is a function of the temperature difference between two surfaces (or two bulk fluids, like air and water masses) multiplied by the emissivity of each surface.
So a perfectly matte black surface (E-1) at a certain delta-T will emit twice as much IR heat energy as a a medium blue tile (E-0.51) at the same delta-T.
Foil (E-0.05) will emit virtually no IR wavelengths no matter how hot it gets.
Emissivity is roughly the same as absorptivity, so foil also will not absorb IR.
In a double-glazed lowE window designed for a heating climate, the low emissivity coating is on the outside of the inner pane, so that when that pane gets warmed from the inside air it will not emit IR wavelengths to the outer, colder pane regardless of the temperature difference.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 1/12/2009 9:47 pm ET by Riversong
Riversong,Maybe we have our wires crossed terminology wise.I/R radiation is EMR w a little longer wavelegth than (red) light,,, as in photons,,, right?Radiant energy is photons. The foil reflects photons. ( I don't know the limit but I don't think it would do much to stop gamma rays). Lets say it reflects visible light and anything less energetic.Also, I feel the foil does indeed emit radiation as a function of it's temperature. Think for a momement, if I were to heat up the foil to a bright cherry red. Don't you feel it would be "radiating" a lot of heat.How, excactly, does this wall assembly with the 3/4" air space and the foil faced foam end up with an additional 2.8 R. I don't see it. I'll give it some more thought.Thanks, Harry
I/R radiation is EMR w a little longer wavelegth than (red) light,,, as in photons,,, right?
Forget the photons - it's only confusing you. Photons, waves, wavicles - let the quantum physicists debate what light really is.
What is important, from a building thermal engineering perspective, is that the sun's energy arrives in a broad spectum of wavelengths. That energy gets absorbed by building materials and is re-emitted as infra-red (heat). The amount of heat emitted is proportional to the fourth power of the delta-T between that object and any other object in a direct unobstructed sight line, and directly proportional to the emissivity of the warmer surface and the absorptivity of the cooler surface.
Also, I feel the foil does indeed emit radiation as a function of it's temperature. Think for a momement, if I were to heat up the foil to a bright cherry red. Don't you feel it would be "radiating" a lot of heat.
I never said it didn't. But it will emit IR (heat) in direct proportion to its emissivity, which is extremely low compared to the black box standard of E-1. In other words, hardly any for any given temperature difference (it's not the material temperature, but the delta-T which counts - red hot foil will not radiate any net heat back to the sun or to the torch which heated it).
How, excactly, does this wall assembly with the 3/4" air space and the foil faced foam end up with an additional 2.8 R.
In the same way that thermal mass offers a fictional "effective R-value" without changing the actual conductivity of the wall. The effective R-value of a lowE air space is significant because most of the heat transfered between non-continuous solid materials is by radiation, not conduction.
As I said earlier, most of the heat transfered between the two panes of a double-glazed window is by radiation from the warmer glass to the colder glass. Thus a lowE metalic film (even though metal is highly conductive) on the outside of the inner pane will stop most of the radiant heat transfer. That's how a lowE window can have at least 50% greater R-value than a clear glass window which is identical in every other way.
It's also why a lowE window feels warmer when standing next to it at night than a clear-glass window. Since the inner glass can't radiate it's heat out to the colder glass, it remains at a siginificantly higher temperature and so there is a smaller delta-T between it and our skin and hence less radiant heat transfer from our skin to the glass (glass and skin have nearly identical E-values).
The human skin has an emissivity of 0.87, almost as high as black epoxy paint, so we are very sensitive to radiant heat gain or loss. And that is the reason that the mean radiant temperature of a space is just as important for comfort as the air temperature. It's also the reason that MOM said to put on a hat on a cold winter's day.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 1/12/2009 11:58 pm ET by Riversong
Edited 1/12/2009 11:59 pm ET by Riversong
You said reflective coatings don't emit I/R waves?? I would think they do, and I would think the wave length they emit is a function of the temperature of the foil.
What am I missing here?
Ah, that magic thing called thermal mass.
Get a cookie sheet out. Put aluminum foil on hte cookie sheet. Make sure the corners are turned up. Put tostada chips on the foil, then put grated cheese and picante sauce on the chips. Put in a 350º oven for 16-18 minutes.
Now, you will need an oven mitt or pot holder or the like to get the cookie sheet out of the oven. The foil, on the othe hand, you can grab with your fingers to slide on to a plate to serve your nice hot nachos.
The foil has low mass to go with its low emissivity. The cookie sheet is low emissivity, but has more mass per area.Occupational hazard of my occupation not being around (sorry Bubba)
Capn,Now you have me wanting nachos.Thanks, Harry
Now you have me wanting nachos.
Well, I did include my take on Ignacio's recipe in case of need for that reason.
if you pick the serving plate first, you can rough mash the foil into it, and you know how big to pile stuff. Using the oven (convective heat) instead of the broiler (radiant heat) helps the nachos staw warm longer. The foil lets you transfer things neatly to a plate, restaurant style, too.Occupational hazard of my occupation not being around (sorry Bubba)
In response to your post concerning the out gassing of polyisocyanurate and it's eventual loss of R value. Is this true of the spray foams also? or singular to that one
material.
In response to your post concerning the out gassing of polyisocyanurate and it's eventual loss of R value.
Is this true of the spray foams also? or singular to that one material.
The only foam insulations which maintain their R-value over time are the thermoplastic foams: EPS and XPS.
The closed-cell thermoset foams, which include all the urethane and polyisocyanurates will lose R-value as they lose their foaming agents.
Pentane-expanded foam will decrease from R-6.8 to R-5.5, though even their initial R-value will depend on the mixing proportions, density and temperature at installation.
CFC/HCFC-expanded foam will decrease from R-7 or R-8 to R-6.5 over 5 to 10 years.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Riversong commented:
Radiant ceilings, however are the least effective, since human comfort requires a higher temperature at the feet than at the head.While radiant heat in the ceiling is not effective, isn't high insulation of the ceiling important for minimizing radiant cooling from a large surface near people's heads? Similar to how radiant heat brings comfort, cold surfaces cause discomfort from radiant cooling.---mike...
Madison Renovations
Cambridge, Mass.
While radiant heat in the ceiling is not effective, isn't high insulation of the ceiling important for minimizing radiant cooling from a large surface near people's heads? Similar to how radiant heat brings comfort, cold surfaces cause discomfort from radiant cooling.
Not for radiant cooling or radiant comfort (we prefer our heads cooler than the rest of our bodies).
The primary reason we traditionally put lots of insulation in the ceiling (or only in the ceiling) was because it was an unused space that was accessible and had enough room for lots of loose-fill fiber.
Also, in leaky old houses, there were large vertical temperature gradients, so the upstairs ceiling had the warmest air and the greatest heat loss. For retrofits, the most bang for the buck was in dumping lots of insulation in the ceiling.
But in a new, tight house the ceiling is no more important to insulate than any other part of the thermal envelope.
Heat doesn't rise. Only hot fluids (air) rises. A tight well-insulated house has little vertical temperature gradient and no significantly greater heat loss through the ceiling.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
When I upgraded our furnace and could actually get 160 deg vs. 120 deg out of our radiators, I could finally feel the radiant (IR) heat from across the room. Putting something between the radiator and me would remove that warming feeling, even with the same room temperature.Similarly, a cold wall or window will make you feel cooler, even if the room temperature is the same. Cathedral (vaulted) ceilings add cold surface to enhance the effect.The following link probably won't work, but it explains some of this effect and the difference in how baseboards vs. radiant floor transfer heat.http://www.achrnews.com/Articles/Technical/5a1b00d0db67a010VgnVCM100000f932a8c0____or try googling: operative bald spot achrnews.comEven if the ceiling is tight, you will sense a cold surface and feel cold even if the room temp at your body is the same.---mike...
When I upgraded our furnace and could actually get 160 deg vs. 120 deg out of our radiators, I could finally feel the radiant (IR) heat from across the room. Putting something between the radiator and me would remove that warming feeling, even with the same room temperature.
I'm not at all sure what you're trying to say in this post, but a baseboard "radiator" is primarily a convective heating appliance. Any warm surface, however, will radiate some heat.
Similarly, a cold wall or window will make you feel cooler, even if the room temperature is the same. Cathedral (vaulted) ceilings add cold surface to enhance the effect.
Actually, what our body feels (in addition to the air temperature) is the mean radiant temperature of all room surfaces. The mean radiant temperature you feel is dependent upon your distance and angle to the various surfaces, in addition to their radiant temperatures and emissivity.
Even if the ceiling is tight, you will sense a cold surface and feel cold even if the room temp at your body is the same.
Unless the ceiling is markedly colder than other surfaces (i.e. not insulated), it will not decrease comfort. In fact, while we can tolerate a very warm wall or window, we prefer that a ceiling be no more than 9° warmer than other surfaces and the air temperature at our heads to be no more than 5° warmer than at our ankles. We like cooler heads.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
M> When I upgraded our furnace and could actually get 160 deg vs. 120 deg out of our radiators, I could finally feel the radiant (IR) heat from across the room. Putting something between the radiator and me would remove that warming feeling, even with the same room temperature.R>I'm not at all sure what you're trying to say in this post, but a baseboard "radiator" is primarily a convective heating appliance. Any warm surface, however, will radiate some heat.Not baseboard; I have FHW radiators that look like the old steam ones.M> Similarly, a cold wall or window will make you feel cooler, even if the room temperature is the same. Cathedral (vaulted) ceilings add cold surface to enhance the effect.R> Actually, what our body feels (in addition to the air temperature) is the mean radiant temperature of all room surfaces. The mean radiant temperature you feel is dependent upon your distance and angle to the various surfaces, in addition to their radiant temperatures and emissivity.You can identify a particularly cool surface not just the average of all surfaces. For example, if one wall is particularly cooler than others, that direction will feel cooler.M>Even if the ceiling is tight, you will sense a cold surface and feel cold even if the room temp at your body is the same.R>Unless the ceiling is markedly colder than other surfaces (i.e. not insulated), it will not decrease comfort. In fact, while we can tolerate a very warm wall or window, we prefer that a ceiling be no more than 9° warmer than other surfaces and the air temperature at our heads to be no more than 5° warmer than at our ankles. We like cooler heads.What I don't know is how much cooler makes a difference in comfort. I agree with "warm feet, cooler head," and I was not recommending heating the ceiling. But I was just pointing out that you still need a reasonable amount of insulation R-value even if you stop air movement (e.g. a thin layer of spray foam would not be enough).
If you can rework the placement of the foamboards they can be taped to produce a vapor barrier.
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Like Riversong stated ... for the foil to work, you need the airspace. And then it's benefit is marginal as if it gets dirty/dusty during const. you lose the ideal benefit of a shiny surface (IMO).
The extruded polystyrene is about R5.0 per inch and the polyiso foil faced is about R6.5-7.0 per inch. You'll have to do the economics. Both should be OK for the application (as far as e.g. moisture goes). Foil faced (i.e. polyiso or Thermax) is more expensive, but you generally get what you pay for ... more R-value.
I'm surprised no one mentioned the other inherant factor: vapor barrier.
Scott.
I'm surprised no one mentioned the other inherant factor: vapor barrier.
Posts #5 and #15 did address that issue. Perhaps you should have read them first.
A near-perfect vapor barrier, such as foil or poly almost always creates more moisture problems than it prevents.
The thermal envelope should ideally be able to dry out in both directions. Vapor retarders are one thing, but a vapor barrier is almost never a good idea. An air barrier is 100x more important than a diffusion barrier, and a house needs to breathe.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 1/13/2009 12:38 am ET by Riversong
The thermal envelope should ideally be able to dry out in both directions. Vapor retarders are one thing, but a vapor barrier is almost never a good idea. An air barrier is 100x more important than a diffusion barrier, and a house needs to breathe.
You're preacin' to the choir. I've been jumped on several times for suggesting something similar.You get out of life what you put into it......minus taxes.
Marv
I'm afraid I don't understand your shorthand description of this wall.
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