Replies

1. 
   Tim_Dianics | Jul 16, 1999 09:13pm | #2

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   The foil backing on drywall is an infrared reflective barrier, normally used with an airspace.

   Tim

2. 
   Guest_ | Jul 16, 1999 09:36pm | #3

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   Hi Tim,

   You wouldn't happen to know the original intent of this foil backing was by any chance?

   Also, I've seen it at the suppliers, but have never seen it on any of the projects that I have been involved with, so do you know where it has been used?

   Gabe

1. 
   Guest_ | Jul 17, 1999 06:45pm | #4

   *USG promotes the foil-backed panels as an integral vapor retarder system for regions outside the South. The panels are also firecode-rated which may account for some of the price difference.You could use regular panels and skip the poly, too. See discussions here ad nauseum. However, I'm a convert.

1. 
   Guest_ | Jul 17, 1999 08:35pm | #5

   *Gabe: Thanks for the reply. The one case I have heard of it being specified for a job involved a warehouse build-out. One room was to be finished with the foil-backed. There were vague references to radars and testing mentioned. An interesting use that might appeal to some people.One benefit I can see to using it, when carefully installed, is that it would allow glueing and screwing to the studs.Francis

1. 
   Guest_ | Jul 17, 1999 08:48pm | #6

   *Andrew: Thanks for the references. I have been following the discussions. Unfortunately my head exploded at some point.Francis

1. 
   Guest_ | Jul 17, 1999 11:23pm | #7

   *Hi Andrew,Right, you were never a believer to start with so it's academic. You have to first understand what it is that your converting from and that is where your argument is weak.I can inspect a poly installation and you can't inspect a foil-backed panel installation for defects.Gabe

2. 
   Guest_ | Jul 18, 1999 05:32am | #8

   *Gabe: Right,English: I have to admit, you’re right.Gabe: you were never a believer to start with...English: if you are counting on poly to do anything, it helps to pray a lot...Gabe: You have to first understand what it is that your converting from...English: You have to first understand that polybattventism is a religion...Gabe: ...that is where your argument is weak.English: ...there’s no argument that poly is just too weak to be a critical building component.Gabe: I can inspect a poly installation and...English: I can inspect a poly installation and if the pipe fitters and rockers trash it later, it's not my problem.Gabe: ...you can’t inspect a foil-backed panel installation for defects.English: ...you don’t need to inspect a foil-backed panel installation for defects and that puts the product way ahead. As long as there is drywall coverage, a complete diffusion barrier is guaranteed.Translationally, Fred

3. 
   Guest_ | Jul 18, 1999 05:38am | #9

   *I see the idiot is out defending those that can't speak for themselves.Gabe

4. 
   Guest_ | Jul 18, 1999 06:03am | #10

   * Gabe,

   Idiots? You give them more credit then they deserve.

   Joseph Fusco View Image

5. 
   Guest_ | Jul 18, 1999 06:56am | #11

   *Hi Joe,Well you know me, always willing to give the benefit of a doubt.Things are up about 27% over last year so my play time is being limited. Right in the middle of a major electrical storm right now, temperature was over the century mark all day and miserable.Chat laterGabe

3. 
   Tim_Dianics | Jul 18, 1999 08:13am | #12

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   Gabe,

   I believe that the original intent is for use as an infrared (IR, radiant heat) barrier, with an air space.

   Tim

1. 
   Trent_ | Jul 18, 1999 06:59pm | #13

   *While on the subject of foil could a radiant barrier be installed under the felt on a roof shingle job or would it damage the shingles?

4. 
   Guest_ | Jul 18, 1999 09:37pm | #14

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   Hi Tim,

   I don't know. The publicity is aimed at using this foil backed sheating OVER strapping nailed across the wall studs and WITH insulation board between, so that doesn't allow for any air space.

   It has a technical spec. indicating the foil's resistance to vapour but nothing at the seams or joints.

   Also, it isn't recommended for ceilings, so that eliminates the wall to ceiling continuety for vapour barriers.

   Makes for a useless product as far ar residential applications go.

   Gabe

5. 
   Tim_Dianics | Jul 19, 1999 01:04am | #15

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   Gabe,

   Stories change with time and circumstance. If you go back to the 1970's in the south western deserts a typical residence (new construction) had an insulated ceiling (R19) furred out walls, over slump block, and foil backed DW. No vapor barrier, no wall insulation. The DW mfg's claimed and additional 1-R plus from the foil backing. Good old DW had become an energy efficient product.

   The DW mfg's are literal in their touting of foil's vapor resistent qualities. The are expecting us to ignore many of the necessary attributes of a vapor barrier.

   Tim

6. 
   Guest_ | Jul 19, 1999 01:16am | #16

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   Hi Tim,

   I'm not familiar with house construction in that neck of the woods, what is slump block?

   I know where your coming from with this. The claims made by some manufacturers, would have us believe that one has to simply lay a piece of insulation on the ground and we can feel the heat coming from it.

   You're absolutely right though, by linking qualities of a product to our expectations, they are not directly misleading, simply not informing enough.

   Gabe

7. 
   Tim_Dianics | Jul 19, 1999 08:46am | #17

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   Gabe,

   Slump block: A unit manufactured to have the appearence of an adobe block. The face has an irregular slumped appearence on the exposed vertical surfaces.

   Tim

8. 
   Guest_ | Jul 19, 1999 05:31pm | #18

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   Thanks Tim,

   We had the same idea, except they were called boyd blocks and resembled cut stone.

   Gabe

1. 
   Guest_ | Jul 21, 1999 10:08am | #19

   *Francis, you are one of the lucky ones! And you probably are still ahead...It is educational and healthy to question conventions. many are driven more by what manufacturers want to sell than by what we need to buy. Others are just plain superstition.

1. 
   Guest_ | Jul 22, 1999 03:10am | #20

   *Some questions on vapor barriers.1. If you inspect the poly before the dry wall is placed, now do you know there are no additional holes, tears, etc. placed after the dry wall is in place? Do you inspect from the outside?2. If you paint the outside of the house, the paint is supposed to protect the wood from moisture. Now assuming it rains in your part of the country, wouldn't that imply that paint on the inside of a house would serve as a moisture barrier of sorts that would be more easily inspected than poly behind drywall?3. What happens to poly as it ages? The stuff I have seen in remodels looks like hell. 4. Lastly, why the heck doesn't someone build a simple mock-up and test these crazy ideas?5. Please note, I don't have the faintest idea of what is best or what works, or what doesn't; but I have a passing knowledge of thermodynamics, and some of the discussions here are weak in theory to be polite. Dennis

1. 
   Guest_ | Jul 22, 1999 06:50am | #21

   *Not to sound like a broken record, but these questions have been addressed in the archives and, I think both or all sides well represented. There have been a variety of mock-up studies, but none conclusive ... so builder's indivdual anecdotal experiences still prevail.One of Gene Leger's crucial arguments -- the one that was key to my new understanding -- is that the amount of moisture moved by escaping air (under considerable pressure from the stack effect and ill-designed forced air systems) dwarfs that excreted via diffusion; so all this concern about permeability is really misplaced. It is the airtightness of the assembly that counts, and to this poly, kraft paper, or fiberglass contribute nothing important or lasting. Permeability is still important, but focusing on it at the expense of plain old leaks is a forest-for-the-trees error.Paint does limit permeability, especially oil-base and glossy. I refer you to the archives for further elucidation -- where they don't turn personal, they are very interesting (and some of the personal stuff is amusing too).

1. 
   Guest_ | Jul 22, 1999 07:56am | #22

   *Well said Andrew.Dear Dennis,Your instincts are sound. Poly in framing is a "just kidding" moisture protection measure that can never stand up to those questions and other concerns. So why be polite about it? This is a serious problem.Pressure variations, and the resulting air movement, in low density walls is a very complicated thing. It varies with house design, construction defects, and weather. We do know for sure that a pressure boundary on the inside of walls is good. This keeps moist air from flowing into the framing and condensing on cold sheathing. That boundary is formed primarily by the drywall, regardess of whether poly is used or not.One thing that didn't come out in earlier discussions, is that fiberglass insulation must have some type of diffusion barrier between it and moist air. In the North that's inside and in the South it's outside. The poor folks living in mixed climates have to tear it out and reinstall it oppositely from season to season, I guess.Politely, Fred

2. 
   Guest_ | Jul 25, 1999 02:00am | #23

   *Sounds like it might have been spec'd as Faraday shielding to some extent. If foil-backed drywall is used with metal studs and grounded correctly, it might form a fairly good method of shielding radio waves. Worked on drawings for a high power TV transmitter building once that was to be placed high in the Sierras and have living space. The living areas were going to have copper screen shielding in areas to keeping the people from slowly being slowly microwaved. Microwaves, radar, and TV transmissions are similar in frequency and power.If used with wood studs, the panels would have to be electrically connected to each other to provide good shielding from electromagnetic fields, I think.I think the foil was originally intended to be used as a heat barrier, but someone found another use.

3. 
   Guest_ | Jul 25, 1999 07:58am | #24

   *Would this help prevent the CIA from monitoring my thoughts and sending me instructions?

4. 
   Guest_ | Jul 26, 1999 07:18am | #26

   *Andrew, Yes, definately, but I've found lining your baseball cap with tinfoil works almost as well.RM

9. 
   Guest_ | Jul 26, 1999 07:18am | #25

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   Hello. Does anyone have any thoughts on using foil-backed drywall as a moisture barrier/retarder instead of conventional stapled-up poly? It costs about $5.00 more per 4x12 Type x sheet than the non-foil Type x, which would seem to make it an decent alternative to the poly route. Thanks.

1. 
   Guest_ | Jul 16, 1999 01:12am | #1

   *Dear Francis,No it is not a decent alternative to poly. It's called a sales gimmick.A proper poly installation can be inspected and approved prior to the installation of drywall to ensure quality compliance.Foil backed drywall cannot. Not only is it more expensive to do, but also more labour intensive after the fact. You still have to seal everything afterwards.But I'm sure some who are too lazy to do a job right will swear by it.Gabe

10. 
    Critster | Dec 05, 2020 11:36pm | #27

    Faraday cage. Vapor barrier. Those are the reasons it is used. Depending on the skill and commitment of the installer it can work well.

11. 
    Steven_Baker | Dec 02, 2021 02:26pm | #28

    I taught and did research at the University of Oregon from 1973 to 1982, then worked at the Oregon Department of Energy for many years. My areas of expertise were solar energy in energy conservation in buildings.

    Foil-back gypsum became wide used in Canada during the early conservation years (1970s) as a vapor barrier. Special tape would be applied where there were to be joints between sheets so that the result would be a continuous vapor barrier. A high quality vapor barrier is very important when the outside temperatures are very low and consequently any moisture that comes out in the wall can turn to ice. I worked on Native Alaskan and Aleut village projects in Alaska during the early 1970s at MIT and we even specified that all cabinets on exterior walls be installed with spacers off the wall so ice would not form inside the cabinets ( a common occurrence at the time).

    During the same time in the US, poly sheets were widely adopted as a vapor barrier. I suspect that this was because of availability of the foil-backed gypsum in Canada. US suppliers hadn't started making this product yet. Plastic was also big in the US (see the song by Paul Simon "Ms Robinson" from the movie).

    I haven't followed building research in Canada and other northern climates for years, but I suspect that foil-back gypsum is still widely used. Gold Bond makes foil-backed gypsum so there must be a market. One of the justifications was that the foil-backed vapor barrier was installed as part of the sheetrock process and not another building step, which saves labor. Yes, foil with an air space can give you a slight R-value boost if there is an air space. At the time in the US in Canada, most fiberglass batt insulation came with a foil face. So foil-back gypsum wasn't deployed for the foil face.

    Poly vapor barriers require a separate building step for installation. The poly is easily torn or punctured after it is applied before and during sheetrock installation. Joints between sheets are usually overlapped, not completely sealed. It is difficult with poly to provide a continuous vapor barrier between say a first floor and a second floor.

    If foil is touching the insulation (no air space), there is no increase in R-value since the heat transmission is by conduction. For those interested in details, any version of the ASHRAE Handbook of Fundamentals (going back to the 1960s) has tables that show the R value of an air space of various dimensions with foil on one or two sides. The R-value depends on orientation, width of the air space, and the direction of heat flow. So foil with say a 1/2" air space has a greater R value for heat flow downward (in winter climates for a floor over a crawl space where there is no convection) than for heat flow upward (roof where convection upward becomes the dominant heat flow mechanism).

    The Florida Solar Energy Center conducted a lot of tests, deployed and promoted multi-layer aluminum foil assemblies as a excellent form of attic roof insulation in Florida. The multiple foil layers created many foil-backed air spaces that could reduce the heat flow downward with a minimal amount of material. In this case for heat flow downward, only conduction (determined by the thickness of the air space) and radiation are the mechanisms of heat flow. I haven't followed this work to see if Florida builders are now using this technique. This would be dependent on building materials manufacturers making a multiple-layer foil product and builders using it.

    Heat transmission in deep space is solely by radiation since there is no gas or material to support conduction and convection.

    Some energy-conserving architects and builders in Oregon now use a vapor barrier paint after sheetrock is applied rather than poly. Another finish layer of paint is applied over the barrier paint. When local architects were asked, they had never heard of foil-back gypsum.