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Dick Russell
Joined: 08 Mar 2006
Posts: 9
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 Posted: Wed Mar 08, 2006 6:06 am Post subject: Insulation: how and when to install in double wall. |
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I have a question concerning installation of wall insulation.
In one double wall design for a superinsulated house, the outer wall bears the roof load, while the inner wall bears the interior ceiling and floor loads. The vapor barrier is wrapped around the inner wall frame before it is raised to a vertical position, so that the vapor barrier is on the cold side of the inner wall, leaving room for installation of plumbing, wiring, and drywall without making holes in the vapor barrier. The two wall frames at any level are tied together above the top plates with 1/2 or 5/16 inch plywood, for support and firestopping purposes. The exterior sheathing is applied to the outside of the outer wall, and covered with housewrap and siding. The cavity between the sheathing and vapor barrier is filled with insulation, which some prefer to be blown cellulose. Before the drywall is applied to the warm side of the inner wall studs, the space between those studs is filled with fiberglass insulation. The finished assembly has the vapor barrier less than 1/3 the way across the total insulation, high total R value, and minimal thermal bridging.
Now the question: how and when is the insulation in the cavity between sheathing and vapor barrier installed? It can't be installed until the exterior wall is closed and the windows installed, to avoid rain damage, so that means installation from the inside. While fastening the vapor barrier to the cold side of the inner wall frame provides a tight seal and excellent protection against migration of interior moisture through the insulation in winter, it means blocking off access to the larger insulation cavity before the insulation is installed. The only thing I can think of, for the case of a two-story house, is to make small slits in the vapor barrier just below the top plate, blow in the insulation, and tape the slits. The same technique would be used to completely fill the spaces below windows. This kind of insulation is blown to a certain density, resulting in slight compression, so as to avoid settling over time. I have to wonder about the pressure of the insulation against the vapor barrier and whether the vapor barrier will give too much. I suppose that could be alleviated by supporting the vapor barrier with some sort of mesh material stretched first over the inner wall frame.
I think the how and when question wouldn't be there for a single-story home. In that case, the plywood firestopping connecting the tops of the inner and outer walls could have predrilled holes. The insulation could be blown in from the top, accessed from the attic space before the attic insulation is installed. The holes in the plywood would be covered after filling the wall cavity. Filling spaces beneath windows might be awkward from the top plate. While the cavity beneath a window in a double wall design would be accessible from the sides, since the framing and headers are only in the wall assemblies, getting the insulation under the window from an attic access strikes me as an iffy proposition.
I'm most interested in comments and suggestions from those well-versed in the application of blown insulation. Envisioning a wall system that gives good performance once completed is one thing, but the practical aspects of building the wall in the first place are something else. |
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gleearch
Joined: 07 Jul 2005
Posts: 185
Location: Oakland, CA , USA
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| Posted: Wed Mar 08, 2006 8:51 am Post subject: |
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You are obviously concerned with condensation happening within your walls, especially with this double wall assembly and the sequencing problem it presents.
You are also mixing two different types of building insulation and trying to find a work around. Maybe what you should be asking is if there is a better way to approach this and forget about the complexities of the proposed wall.
You already have pointed out a number of problems in installing it this way.
Time to ask your design professional for alternatives.
There is more to insulation than just cellulose and fibreglass batts.
If you are in snow country and worried about thermal bridging, there are alternative ways of dealing with it beyond using a super thick wall.
Again this is where your design professional should be able to help you come up with an alternative wall system that addresses your concerns.
_________________
Gerard Lee Architects
http://www.gleearchitects.com
San Francisco bay area, Oakland, CA based sustainable architecture & planning firm specializing in residential, education, commercial, community & civic projects. |
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csintexas
millennium club
Joined: 06 Feb 2006
Posts: 1665
Location: USA
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| Posted: Wed Mar 08, 2006 9:34 am Post subject: |
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Have you seen this system used before or is this just some idea of yours?
I would suggest you research different wall systems and choose one that has been done before instead of trying to reinvent the wheel.
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Chris Stewart
Modern Texas Home Project |
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Dick Russell
Joined: 08 Mar 2006
Posts: 9
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| Posted: Wed Mar 08, 2006 12:01 pm Post subject: |
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Quite a number of various double wall superinsulated homes have been designed and built over the years, using various techniques to address various concerns. The placement of the vapor barrier on the cold side of the inner wall, for the reasons I cited, was used in the Saskatchewan Houses back in the late 70s. However, in that design, the inner wall bore the roof load, and the outer wall was there just to provide a wall cavity for insulation and a place for the siding to keep the rain out. The structural sheathing was on the outside of the inner wall, applied over the vapor barrier while the frame was on the floor. The outer wall was erected over the inner wall, the spacing established, and the whole assembly moved to the vertical position. The document I downloaded, explaining all of this, didn't say how many people were needed to get the wall off the floor and onto the sill. Nor did it say how or when the wall cavity was filled with insulation. They could well have put the insulation in before the outer wrap and siding was applied. If a crane were used to lift the wall into place, the extra weight wouldn't have been a problem.
While the Saskatchewan House design performed well, according to the reports, the design has its own shortcomings. There is no one design that is best for all situations. In Gene Leger's designs in the early 80s, he used multiple designs over the course of a few years. One design placed the vapor barrier on the warm side of the inner wall, but had no plumbing or wiring within the wall. Electrical outlets on exterior wall assemblies were provided as surface-mounted strips. Leger kept evolving his designs according to experience gained on each project and as his thinking changed. Each situation can be addressed with different combinations of techniques, involving different tradeoffs between effectiveness and complexity. The wall assembly I described in my original post addresses some concerns but suffers in other respects, one being the installation of insulation and, in particular, the matter of blown-in cellulose possibly pushing too hard against the vapor barrier.
I have a few years before the house gets built, and I want to understand what is possible and all the ramifications of different ideas before engaging someone to do a detailed design. I certainly don't want to just throw the problem (and a lot of money) at an "architect" and walk away from the problem. I also don't want to just build the house the way it's been done for forty years or more; that just gets me decades old technology, and that is woefully inadequate. I'm hoping for constructive discussion and suggestions here. I know a fair amount about this subject, but not enough. |
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gleearch
Joined: 07 Jul 2005
Posts: 185
Location: Oakland, CA , USA
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| Posted: Wed Mar 08, 2006 12:55 pm Post subject: |
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Ok. Fair enough.
I'll throw a few ideas out. You decide on their merits.
From an ease of construction, using spray in expanding insulation. Open cell and with no CFCs, formaldehyde nor urea. There is a good product out there. It acts as a vapor barrier and therefore negates your need to applying the vapor barrier to the inner stud wall. It's LEED approved and good for the environment as far as other man made insulation products go. With your double wall configuration, no need to worry about any thermal bridging. Your choice to mix two different types of insulation, probably results in higher labor cost. balance that with one sub installing the spray in insulation and see where that gets you. I have given you plenty of keywords to google with.
From an alternative construction standpoint. If you are going with a thicker wall, why not look at straw bale wall construction. Not sure how canadian bldg officials look at it but the technology has evolved over the years and many jurisdictions are now approving them. They perform well as far as insulation goes and because of the way they are installed, thermal bridging is not much of a concern. You do need a vapor barrier and good detailing. There's plenty of information on the web about straw bale beyond the fallacies that people spout without knowing much about it.
There are other methods using conventional rigid insulation and stick framing with a mix of fibreglass batts that can give you a super insulated wall. But I rather not use fibreglass batts and any rigid should be free of CFCs, urea or formaldehdye. So that leaves EPS rigid insulation and spray in insulation. Here it's a matter of where you put each type of insulation and how you choose to frame it without having to do a double wall.
There is cellulose insulation that comes with an acrylic binder to keep it from slumping and settling, this negates your concern about having to compress too much cellulose.
Insulation works by trapping air. When you compress it too much under pressure, you technically reduce it's insulating capabilities.
It all comes down to how you design the wall. It's one thing to talk about materials, it's another to actually work out the assembly. While you may not want to throw this over to your "architect" and spend money on it, the right architect with the right experience and knowledge can help you solve this problem and save you money.
The information I provided above is freely available. I suggest you look at recent snow country construction in some of the newer ski resorts now going up. They are for the most part using energy efficient methods learned from the past with newer sustainable materials and technology.
Good luck on your project.
_________________
Gerard Lee Architects
http://www.gleearchitects.com
San Francisco bay area, Oakland, CA based sustainable architecture & planning firm specializing in residential, education, commercial, community & civic projects. |
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csintexas
millennium club
Joined: 06 Feb 2006
Posts: 1665
Location: USA
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| Posted: Wed Mar 08, 2006 2:43 pm Post subject: |
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Believe it or not there are people currently working on super insulated building systems, double stud walls are not new technology. The reason I say investigate what other people are doing is because your question showed a basic lack of design knowledge. I'm not saying that to insult you I say that to help you. There is a lot more to designing an eficient building than how you insulate the walls. It takes a complete system. Also there may be structural issues that you are not considering. There are also elements which will effect the effeciency more than the wall system.
anyway...
It sounds like you are doing your homework and at this rate you will be very well informed by the time you are ready to build. I don't know of any product that will give you more Rs per inch than rigid insulation. The spray foam that gleearch talked about is a good choice also. Have you looked at SIP's? What R value are you trying to achieve?
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Chris Stewart
Modern Texas Home Project |
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Dick Russell
Joined: 08 Mar 2006
Posts: 9
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| Posted: Thu Mar 09, 2006 7:06 am Post subject: |
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Oh, I know that everything I will need has been done before in some way or another. But I have seen quite a number of combinations of existing techniques described here and there. I'm trying to sort through what I can find to give me something that will satisfy me, in terms of energy efficiency, structural strength, and cost. There will be compromises. I imagine every architect would like all his customers to be well informed and with a clear idea of what type of construction he wants, to minimize the permutations and move quickly to detailed plans.
My initial goal on R value is around 35-40 for the wall and 48-50 for the attic. The attic is easy to do; it's just depth. Super-tight construction, ideally zero air infiltration, and HRV for controlled breathing. The wall R value is what you get with 5.5 inches of foam in a 2x6 wall or cellulose in a double wall with a 10-12" cavity. It doesn't make sense to go with high apparent R value if there is a lot of thermal bridging, because ultimately most of the heat is lost via the wood and extra insulation has a diminishing effect. So double walls, staggered studs, cross-mounted sheathing support studs all are attempts to reduce the thermal bridging.
I did receive some information on the installation of blown cellulose into a closed cavity with just the vapor barrier on one side. There is a product called Par-Pac. basically a fiber-reinforced vapor barrier. After it is applied to the studs, slits are made, the blowing tube inserted, and later the slits are taped. So that particular issue seems to be resolved on that wall design I described. The fellow who sent me that also suggested perhaps a double 2x4 staggered wall system on a 2x6 plate, filled with closed cell foam. I'll study that, too.
One thing I didn't mention in the first post, as it was not part of the cellulose blowing problem, is that I envision using a 2-inch extruded foam board outside the foundation, from footing to sill, protected above grade, of course. This puts the concrete mass inside the insulation. Leger called it a "thermal flywheel." It avoids sweaty walls in the summer, too. However, this means that the top edge of the 2-inch board must be protected. I envision having a 2x12 PT sill extending out past the edge of the concrete. The siding would just cover the foundation insulation. The wall must be born directly over the concrete, with sufficient bearing surface to meet structural requirements (and meet codes, of course). Getting the sheathing out that extra 2 inches with a 2x4 stud outside is problematic. I could do it with a 2x6, perhaps 24" OC to save stud count, but that means thicker drywall inside. If I use an inner wall of 2x4 16"OC, that is a really thick wall for foam insulation, but it does avoid much of the thermal bridging. Then there is the idea of gaining that extra 2" to cover the foundation insulation by using horizontal 2x3, 24"OC, to block the sheathing and siding out that 2".
So, more and more things to think about, and I'd really like to hear about ideas from those who already have thought about this. Thanks for the responses thus far. |
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csintexas
millennium club
Joined: 06 Feb 2006
Posts: 1665
Location: USA
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| Posted: Thu Mar 09, 2006 11:39 am Post subject: |
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Yes those would be my points. Air infiltration, the joints between wall and foundation and ceiling, the windows type and placement, the components that make the rest of the home will all make the most difference. I can't see the savings by using a double wall system will justify the cost. That is why you don't find it used very often. It doesn't make any difference how much insulation is in the wall if the door is left open. So unless the budget is unlimited I think the best use of money and effort would be better used in other places.
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Chris Stewart
Modern Texas Home Project |
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OSG
Joined: 06 Nov 2007
Posts: 3
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| Posted: Tue Nov 06, 2007 7:11 pm Post subject: |
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| Dick Russell wrote: | Oh, I know that everything I will need has been done before in some way or another. But I have seen quite a number of combinations of existing techniques described here and there. I'm trying to sort through what I can find to give me something that will satisfy me, in terms of energy efficiency, structural strength, and cost. There will be compromises. I imagine every architect would like all his customers to be well informed and with a clear idea of what type of construction he wants, to minimize the permutations and move quickly to detailed plans.
My initial goal on R value is around 35-40 for the wall and 48-50 for the attic. The attic is easy to do; it's just depth. Super-tight construction, ideally zero air infiltration, and HRV for controlled breathing. The wall R value is what you get with 5.5 inches of foam in a 2x6 wall or cellulose in a double wall with a 10-12" cavity. It doesn't make sense to go with high apparent R value if there is a lot of thermal bridging, because ultimately most of the heat is lost via the wood and extra insulation has a diminishing effect. So double walls, staggered studs, cross-mounted sheathing support studs all are attempts to reduce the thermal bridging.
I did receive some information on the installation of blown cellulose into a closed cavity with just the vapor barrier on one side. There is a product called Par-Pac. basically a fiber-reinforced vapor barrier. After it is applied to the studs, slits are made, the blowing tube inserted, and later the slits are taped. So that particular issue seems to be resolved on that wall design I described. The fellow who sent me that also suggested perhaps a double 2x4 staggered wall system on a 2x6 plate, filled with closed cell foam. I'll study that, too.
One thing I didn't mention in the first post, as it was not part of the cellulose blowing problem, is that I envision using a 2-inch extruded foam board outside the foundation, from footing to sill, protected above grade, of course. This puts the concrete mass inside the insulation. Leger called it a "thermal flywheel." It avoids sweaty walls in the summer, too. However, this means that the top edge of the 2-inch board must be protected. I envision having a 2x12 PT sill extending out past the edge of the concrete. The siding would just cover the foundation insulation. The wall must be born directly over the concrete, with sufficient bearing surface to meet structural requirements (and meet codes, of course). Getting the sheathing out that extra 2 inches with a 2x4 stud outside is problematic. I could do it with a 2x6, perhaps 24" OC to save stud count, but that means thicker drywall inside. If I use an inner wall of 2x4 16"OC, that is a really thick wall for foam insulation, but it does avoid much of the thermal bridging. Then there is the idea of gaining that extra 2" to cover the foundation insulation by using horizontal 2x3, 24"OC, to block the sheathing and siding out that 2".
So, more and more things to think about, and I'd really like to hear about ideas from those who already have thought about this. Thanks for the responses thus far. |
Hi there. I built this style home. My builders website is www.castlehomesinfo.com My wall 12 inch double walls and are R-40 and attic is R-78. These are his minimums. His standard design is a 16 inch wall that is R-60. He built a 3700 sq foot home that cost 30 dollars/mo to heat or cool (2007 energy cost) in WASHINGTON STATE. He also has a PhD in Engineering (something to due with energy conservation in civil engineering). Good luck and tell me if this helps you.
Last edited by OSG on Tue Nov 06, 2007 7:18 pm; edited 1 time in total |
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OSG
Joined: 06 Nov 2007
Posts: 3
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| Posted: Tue Nov 06, 2007 7:17 pm Post subject: |
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I disagree with some. I have a foot thick wall with R-40, nearly R-80 in the attic, low energy double pane windows, maximum south side windows, minimum north and east/west. I have an overhang engineered to not allow the summer sun from getting through my windows, but the lower angle in the winter comes right through, giving me free solar heat. My energy bills are less than half of my neighbor's house (energy star with R-twenty something). Two walls made with 2X4 use the same amount of wood in weight as a single 2X6 wall, and it has greater strength and insulation value.
An unmentioned bonus is how incredibly quiet the house is. The noise gets lost in those big walls. |
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OSG
Joined: 06 Nov 2007
Posts: 3
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| Posted: Tue Nov 06, 2007 7:24 pm Post subject: |
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| csintexas wrote: | | Yes those would be my points. Air infiltration, the joints between wall and foundation and ceiling, the windows type and placement, the components that make the rest of the home will all make the most difference. I can't see the savings by using a double wall system will justify the cost. That is why you don't find it used very often. It doesn't make any difference how much insulation is in the wall if the door is left open. So unless the budget is unlimited I think the best use of money and effort would be better used in other places. |
A properly engineered dual 2X4 wall (24" OC) cost the same as a standard 2X6 wall of similiar strength. |
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birgco
Joined: 01 May 2007
Posts: 282
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| Posted: Wed Nov 07, 2007 5:18 am Post subject: |
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As oil fast approaches $100 a barrel, my double wall, super-insulated home is looking pretty good. The economics of this type of construction integrated with active and passive alternative energy systems makes more sense each day.
$200 a barrel anyone? |
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csintexas
millennium club
Joined: 06 Feb 2006
Posts: 1665
Location: USA
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| Posted: Wed Nov 07, 2007 6:35 am Post subject: |
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Yes Birgco I think you are right, the return on investment is getting better all the time.
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Chris Stewart
Modern Texas Home Project |
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Francis4344
Joined: 27 Mar 2008
Posts: 1
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| Posted: Thu Mar 27, 2008 2:21 pm Post subject: |
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I am with Dick on that one..
I too am planning a house to be build this summer with the double wall technique. Yes blowing in the insulation is the solution to the problem...
As for your outside foam insulation issue, would you consider having a custom made flashing that would cover it? Thats what I did at my daughter's house (the foam could not be placed inside - the first floor of her house is a horse stable - yes your read right.)
As for insulating your basement, you should consider this very, very good product. We have it in our present home and I will certainly insist on it in the next one. Roughly 15 $ per linear feet for 2 inches.
http://www.styrorail.ca |
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modjohn
Joined: 07 Nov 2007
Posts: 30
Location: Kansas, USA
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| Posted: Thu Mar 27, 2008 3:52 pm Post subject: |
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What about Insulated Concrete Forms. ICFs are very energy efficient, stronger than any other system and very very quiet.
Combine ICFs with some good windows and soy based expanding foam insulation in all other areas and you have a very efficient home. Add in a geothermal heating system for even more efficiency.
You can even do ICFs yourself if you are the DIY type. |
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