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birgco
Joined: 01 May 2007
Posts: 302
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 Posted: Sat Jun 02, 2007 7:05 am Post subject: double wall construction techniques |
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This discussion has caused me to continue researching different sites and while the BSC (Building Science Corp.)site does not specifically address double wall construction details, it is obviously a very valuable resource. (thanks Chris). Rather than assuming that they do not cover double wall construction because it is a bad idea, I believe it is not discussed because it is more costly and is not viewed as part of the mainstream production building process. I am hoping to hear back from them with their comments on double wall construction (pros cons etc. )
The latest thoughts on my wall design after reading up on some fairly complicated vapor research by BSC, is the following:
We are talking about design considerations in zone 6 (ny area)
BSC highly recommends allowing a wall assembly the ability to dry from the interior (as well as[) the exterior. So in their standard single wall
R-19 assembly, they call for no vapor barrier on the interior surface except for vapor permeable latex paint over the sheetrock. On the exterior side there is also vapor permeable 1" foam board and house wrap detail.
Now the question arises that if you construct a wall as follows:
from the interior...........
-5/8 sheetrock (not required but my personal preference)
-2x4 stud wall with R13 unfaced batt insulation (cotton or fiberglass)
- 1" foil faced foam board bonded to the exterior structural wall assembly
(perm rating of .01 or less)
-2x6 stud wall with R-21 unfaced batt insulation (cotton or fiberglass)
-5/8 cdx plywood sheathing (1/2" required for code)
-tyvek or similar house wrap taped at all joints
3/4 t&g cedar siding
Does this design allow for the wall to dry from the interior AND the exterior? Or does effectively stopping the flow of vapor mid wall create a problem?
And therefore, would there be a possibility of condensation occurring on the interior side of 1" foam board foil?
Also as previously stated, additional benefits are as follows:
1- all penetrations are in the interior 2x4 wall (elec. boxes etc.) leaving the 2x6 exterior assembly virtually undisturbed.
2- Air leakage is greatly reduced by the low perm rating of the foam board
3- Combined R-value should be R-40 plus
Now obviously, I am pushing this idea because it is my idea but
I am not looking for people to tell me how smart I think I am, but if you disagree, please give factual data or real life examples of why you think it is not a good idea. This discussion has been really important and I appreciate your input. birgco |
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mx2
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Joined: 18 Jan 2006
Posts: 1968
Location: Miami, Florida
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| Posted: Sat Jun 02, 2007 8:24 am Post subject: |
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Housewrap is not a vapor barrier (exterior). And the foil faced foam board should be attached to the sheetrock, adhered even (interior). The double batt is overkill but does not hurt. But in essence, your "sandwich" of materials has only ONE layer as an air barrier (housewrap) and no layers of vapor barrier (water). Take a look at what I had proposed to see what I was trying to accomplish. Try drawing a series of parallel lines and naming each one (like a sandwich) then draw arrows that depict the flow of air, vapor & heat...and label which materials are designed for which condition...this picture should help you get a better handle on it all...
mx2.5
_________________
*Art of Architecture: The conscious use of skill and creative imagination in the production of an aesthetic building.
*Science of Architecture: The calculated use of technical skill and knowledge in the construction of a functional building. |
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birgco
Joined: 01 May 2007
Posts: 302
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| Posted: Sat Jun 02, 2007 9:23 am Post subject: double wall construction techniques |
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mx, following your points, it's my understanding that the house wrap on the exterior doesn't want to be a vapor barrier but a vapor retarder and air barrier, which allows trapped vapor to pass through it as designed by the manufacturer. This also reflects the thinking of BSC.
My "sandwich" of materials (like the term) has the 1" foil foam board in the middle (or the 35% point from the interior side) which acts as an air barrier and vapor barrier (perm rating .01)
If I put the foil foam board behind the sheetrock, we have to deal with detailing each electrical box and wall penetration.... (through hard experience I don't want to do this which is why I am aggressively pushing for an alternative method, as you already know.....). BSC also mentions that an air barrier behind the sheetrock prevents drying of the wall cavity to the interior side. Your thoughts appreciated.... |
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Sundog
Joined: 03 Jun 2007
Posts: 2
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| Posted: Sun Jun 03, 2007 2:22 pm Post subject: |
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New here and this is a wall that I designed for cold climates. Using wood studs for the interior wall will increase the R-Value, but I used steel for it's straightness. Thoughts please?
....jc
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birgco
Joined: 01 May 2007
Posts: 302
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| Posted: Sun Jun 03, 2007 3:00 pm Post subject: double wall construction techniques |
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| thanks for the thoughtful submission. Where do you build and have you constructed it in the field? I like the addition of the the metal studs for straightness. The metal studs I have used are like a "C" so the insulation could be placed inside the metal stud almost completely. Are you sure it subtracts that much from the R value? Also, is the R-15 fiberglass unfaced behind the sheetrock and what are your thoughts on the air barrier/vapor barrier/vapor retarder issues for this type of wall? |
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Sundog
Joined: 03 Jun 2007
Posts: 2
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| Posted: Sun Jun 03, 2007 3:32 pm Post subject: Re: double wall construction techniques |
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| birgco wrote: | | thanks for the thoughtful submission. Where do you build and have you constructed it in the field? |
I am a SIP dealer in Colorado, and I have yet to use this design. What I don't know is if this design can be built for less than a 10.25" SIP wall.
| Quote: | | I like the addition of the the metal studs for straightness. The metal studs I have used are like a "C" so the insulation could be placed inside the metal stud almost completely. Are you sure it subtracts that much from the R value? |
No doubt. Steel is a thermal conductor, not an insulator, so I gave it an R-Value of Zero. So, if wood 2x4's are used, you get an additional R-3.5 for every stud!
| Quote: | | Also, is the R-15 fiberglass unfaced behind the sheetrock and what are your thoughts on the air barrier/vapor barrier/vapor retarder issues for this type of wall? |
That's your call. Here in Colorado(heating dominated) vapor barriers are pretty much standard on the interior of a wall. I know that Building Sciences designs the barrier for the exterior of the wall and that they want the wall to breathe. But, I'm of the opinion that if you keep the moisture out, no breathing is required. And, I think that a lot of people are looking at 'worst case' scenarios, and assume that there will be a failure.
....jc |
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arch*tect
Joined: 14 May 2007
Posts: 4
Location: United States
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| Posted: Mon Jun 04, 2007 7:31 am Post subject: air as an insulator |
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| csintexas wrote: | Air is not a good insulator (unless it is in small cells), it is only good with radiant barriers. If air is such a good insulator why bother with insulation?
I would not be so sure condensation will not occur inside the wall. How is the airspace insulated at the top and bottom?
The rigid foam board is a good way to prevent thermal bridging between the framing. The airspace in between the two walls is bad design. No radiant barrier is needed or desired there. Foam boards are permeable to different degrees but I think the ones with foil are not. The perms may be listed on the sheet or you could find out that info from the manufacturer.
I don't design for cold climates but I believe the vapor barrier is normally placed under the sheetrock.
So I think the 1" permeable foam between the walls with no air space is the way to go. |
If air as an insulator is not good, then cavity walls would not be around as much as today.
As most would agree that cavity walls are constructed accordingly to the context of a site, for instance environmental conditions.
Would you not try this type of construction in your area? Maybe it worth to experiment it. |
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P.C.
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Joined: 26 May 2004
Posts: 2163
Location: Denmark
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| Posted: Mon Jun 04, 2007 9:31 am Post subject: |
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| Air proberly isn't that bad to insulate if you look at it's properties as such --- anyway most foam material do mainly consist of air -- from what I know, it's rather the fact that air will move and cirkulate, and that way "transport" the energy rather than it's isolating abilities that is the issue here. |
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P.C.
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Joined: 26 May 2004
Posts: 2163
Location: Denmark
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| Posted: Mon Jun 04, 2007 9:34 am Post subject: |
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--- And by being "transported" being warm and moist into a colder inviroment, it also produce the sad side effect, to condensate water.
This water btw. don't nessery show up as something you can see just like that, it also can be just a bit more moisture in the cold arears. |
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P.C.
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Joined: 26 May 2004
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Location: Denmark
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| Posted: Mon Jun 04, 2007 9:35 am Post subject: |
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| Again leading to a poorer insulation due to water's abilities to transport heat. |
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mx2
millennium club
Joined: 18 Jan 2006
Posts: 1968
Location: Miami, Florida
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| Posted: Mon Jun 04, 2007 11:11 am Post subject: |
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The problem with the super wall configuration, sundog, is that the metal studs are the weak points...at every metal stud, heat will look to transfer through them, hence why it is recommeded to adhere foilfaced foam boards to the backside of the gypsum wallboard (sheetrock) which would insulate the studs. Even the wood studs will also let heat through at their points...
birgco, the idea is to keep moisture (via air) out as much as possible, but then allow moisture out when it does enter the wall system. So in essence we do agree, but just using different terms (I proposed a air permeable vapor barrier, which you call vapor retarder, air barrier...a bit confusing don;t you think? haha).
I'm of the thinking that it is best to keep moisture out as much as possible and "dry out" the wall by allowing weepholes and other controlled air venting methods. You don't want a wind tunnel but you do want to let air out for sure. It is indeed tricky business. But like I mention to sundog, the foil faced insulation is best when attached to sheetrock for heat retardant purposes. You should consider using something else behind the studs (perhaps around the studs) to stem the flow of heat thru the outlet penetrations. Builders I've met have always suggested spray foam around the inside of the outlets...
mx2.5
_________________
*Art of Architecture: The conscious use of skill and creative imagination in the production of an aesthetic building.
*Science of Architecture: The calculated use of technical skill and knowledge in the construction of a functional building. |
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birgco
Joined: 01 May 2007
Posts: 302
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| Posted: Mon Jun 04, 2007 11:48 am Post subject: double wall construction techniques |
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Just wanted to mention again that the exchange here is invaluable because it has caused me to research and learn more about vapor barriers, air barriers, pressure differences, etc. than I ever knew before, but it is frustrating at the same time because of the lack of hard answers. I respect the BSC site because they do actual scientific research into the topic, but if you noticed, they do not address very real issues such as electrical boxes, the electric wires drilled through each stud hanging in the middle of the wall cavity, the high probability of very poor insulation installation quality and other on-the-job situations.
I would like to be able to build a wall that addresses the following personal issues for me as a builder of energy effiecient homes:(I realize these are not the same for everyone so I emphasize the word "personal")
1. Use natural materials to insulate such as cotton insulation.
(don't like SIPS and the like because they are made from chemicals and I know they are inert...... just would like something more natural)
2. Have an R rating of at least R-40 (folks tell me it too expensive to build a double wood wall but when you do the math at savings of $300- 500 a month in energy costs in our area, the payback is about 5-7 years)
3. Not have to worry about whether the wall will experience condensation or other vapor related problems as I have seen with some other designs in the past.
4. Improves upon an already fairly successful R-38 /10" wall design, but makes it even better.
Keep those ideas coming....... |
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mx2
millennium club
Joined: 18 Jan 2006
Posts: 1968
Location: Miami, Florida
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| Posted: Mon Jun 04, 2007 2:31 pm Post subject: |
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I thought you might find this interesting;
| Quote: | copied from http://www.naima.org/pages/resources/faq/faq_home.html
Air Infiltration
Thermal insulation, whether fiber glass or cellulose, when installed in side walls has very little to do with air infiltration. The U.S. Department of Energy estimates that up to 40% of a home's heat loss can be from air infiltration. But only 14% of the total loss from air infiltration occurs through side wall cavities, and most of that is around electrical outlets, a problem easily solved with inexpensive, easy-to-install electrical insulated outlet gaskets.
Air infiltration generally occurs in the areas of a home that are not insulated, such as around windows, doors, fireplaces, HVAC ductwork and perimeter joints. It can, and should, be controlled with the use of housewrap, proper caulking, and sealing of band joists, sill plates, header plates, and insulation around doors, windows, electrical outlets and other openings.
Recently there have been claims that some insulation products are better because they reduce air infiltration. Numerous research studies have been conducted to investigate this issue, among them studies by the National Association of Home Builders Research Center, researchers at Penn State University, and by a St. Louis utility. The research consistently demonstrated that if a wall cavity has been properly constructed, using drywall, sheathing and caulking, very little air will flow through the wall cavity regardless of the type of insulation. |
So,...electrical insulated outlet gaskets...anyone? 
mx2.5
_________________
*Art of Architecture: The conscious use of skill and creative imagination in the production of an aesthetic building.
*Science of Architecture: The calculated use of technical skill and knowledge in the construction of a functional building. |
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mx2
millennium club
Joined: 18 Jan 2006
Posts: 1968
Location: Miami, Florida
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| Posted: Mon Jun 04, 2007 2:36 pm Post subject: |
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Found many pictures online...geez. So simple...going to go buy some for my house!! Oh, and those child safety plugs for the outlets...they stop drafts from passing through the openings themselves, and they sell self-closing outlets as well. Simple but very effective details to consider...it's not just about sandwiching more insulation in the walls...all that could be going "out the window"...!!
(pun intended)
mx2.5
_________________
*Art of Architecture: The conscious use of skill and creative imagination in the production of an aesthetic building.
*Science of Architecture: The calculated use of technical skill and knowledge in the construction of a functional building. |
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birgco
Joined: 01 May 2007
Posts: 302
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| Posted: Mon Jun 04, 2007 3:25 pm Post subject: double wall construction techniques |
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mx2, Hey what a great idea, you know I've been building for over 25 years and I've never seen foam electrical gaskets at the hardware store, home depot, lowes, sears, the lumberyard, ................ so I ran over to my neighbors new house and installed them. We couldn't believe it, his heating and cooling bills were reduced from $1000 a month to $100/ month just like that.
Whew, am I glad that problem is solved..... planet earth saved! thanks  |
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