double wall construction techniques

[birgco]

I read with interest the topic of double wall construction in march 06 or thereabouts. I built a double insulated wall system for a my current home about 7 years ago and was quite happy with the results (approx. 50 % reduction in heating / cooling costs from standard 2 x 6 new construction. Some one commented about the cost of such a system and if it was worth it. I am planning to build another home this fall, and believe the previous method I used, ( 5/8 cdx ply with house wrap, 2 x 6 exterior wall with r-19 fiberglass, 1 inch air space, 2 x 4 inner wall with r-11 / foiled faced vapor barrier ) worked well, but I believe this system could be improved by installing a 1 inch foil foam board in place of the 1 inch air space and then installing unfaced r-13 in the 2x4 non bearing stud wall. Btw, I expect to use cotton insulation in place of the fiberglass with the hope of greatly improving the quality of the installation because of the non-toxic nature of the cotton insulation. Would be interested in your comments. thanks.

[JWmHarmon]

Have you considered using Structural Insulated Panels (SIPs) instead of conventional stick framing with 2X6 and 2X4 lumber? SIPs have a core of foam insulation between a sandwich of osb (oriented strand board).

You might want to compare the cost of this type of construction with what you did before or are planning now. You would have to include the labor factor since the labor to construct and insulate with fiberglass batt insulation may be more than the extra cost of the SIPs.

I also vaguely recall when double-wall construction was first being considered back in the 20th century "energy crisis" days, that some houses were built with trusses placed vertically to increase the depth of the wall to allow for more insulation. I don't know if a product like Trus-Joists could be adapted for this purpose, but it might be worth investigating.

After viewing tornado damage over the years, I wonder if using a trussed wall system integrally connected to the roof trusses might stand up to tornadic winds? Any engineers out there looking for a challenge?

I note that sports cars are designed to withstand 200 mile per hour winds when they are driven. Big-rig trucks can withstand 100 mile per hour winds without damage. Modular homes transported over the highways typically can withstand hurricane force winds without damage. Why can we not design houses to withstand hurricanes or tornados?

[JWmHarmon]

A little googling produced the name Larsen truss wall

http://www.greenfret.com/house/larsen.html

Also, a complete truss-built house components site gives some usefull photos.

http://www.swelite.com/

I am not affiliated with these products in any way.

Another idea from way back when... why not build with a double wall with enough space to walk through? This space would act as a buffer between the outside and the inside and could be ventilated or heated as needed, keeping the inside comfortable. This is the same basic principle as putting continuous closets around the outside walls of the house. Continuos closets on the outside walls provide and additional layer to buffer the temperature extremes.[/url]

[birgco]

Thanks for the feedback, I had considered SIPs at one time but I didn't know if the OSB was a good idea because of potential outgassing. My concerns may have been overstated. Also, do SIP's lend themselves to custom-type construction techniques. I will talk to a manufacturer to address these questions. Because I frame the walls myself, I wasn't as concerned with labor type costs and the additional 2x4's and insulation costs were not substantial. My preference for the double-wall system revolves around its ability to almost total isolate the electrical and plumbing roughing in the inner, non-structural, (2x4) wall, leaving a totally insulated and sealed 2 x 6 (exterior structural) cavity. My primary interest was the vapor barrier location and if someone had used the 1 inch foil foam board in-between the two walls. Also, I wanted to know if anyone had any experience with the new cotton roll insulation. Thanks again for your interest.

[harrysoe]

If you are looking for a 'green type" product for insulation, you may want to have a look at a NOP ( Natural Oil Product ) based polyol that will give you a rigid type polyurethane ( PU ) at this website:http://www.maskini.com.my[url]
The company seems to have experience in producing various products/ system using the technology. I am myself considering their product / system for my own house as a retrofit

[mx2]

[birgco]

thank you for your comments. It's not that I am totally go-hung on the 1 inch foil board between the 2 x 6 (exterior) and 2 x 4 (interior wall). It's more to the point that I am looking for an optimum energy efficient wall system that pays for itself over a certain number of years (10 let's say), is a model of good construction, insulation, vapor barrier technique, isolates systems roughing from air penetration in the exterior wall, and also results in a house that easily lends itself to the integration of active solar systems because it requires so little energy to heat and cool it. I believe that current code requirements of a single 2 x 6 wall with R-21 batt insulation is inadequate if only for the reason that almost no one doing production type housing takes the lengthy amount of time required to properly insulate a wall with electrical wires and plumbing lines present. Insulating with foam is on the right track but I would not want to be the mechanic who needs to trace a leak or a faulty wire in a wall that has been foamed. I like your idea about adding an additional 1 inch of air space to the 1 inch of foam board but I am concerned that the wall is approaching a thickness that starts to compromise valuable living space. A larger question looming in my mind, is that are we quickly approaching a point in energy costs where a additional 10 K spent to super insulate a house, will almost become a necessity.
In any event, the discussion is extremely helpful, thanks again.

[mx2]

typically the way plumbers and electricians repair faulty lines is to isolate the fault and cut it at two points and bypass it with new line. The point being, to repair the lines, whether it's in air space or buried in material (often embedded in concrete for example), it's easier and faster to do the bypass rather then actually fixing the one faulty point. They essentially end up re-building and re-finishing the walls anyway...

That said, the code minimums are just that....minimum requirements. If left to their own devices, builders would opt out of any extra costs and not put any insulation if they didn't have to put it in. But there is law against improvements and higher effeciencies. You make a valid point about valuable real estate...you would indeed be encroaching into occupiable space and that's a trade-off that has to be factored in. That's why it's typically a single cavity wall system with an exterior insulated & moisture resistant wall and an interior insulated wall. The foil backed insulation is best towards the interior because it "reflects" the heat/coolness back into the space that needs it...and like I mentioned, if you want greater effeciency, look into improving the air space. The greatest hindrance to this is the lack of proper venting. Hot air will radiate heat, but if it were vented enough to get fresh air then it's a very effecient insulating "layer". It's not common to detail this, but there are many ways to increase venting...another interesting idea I've read about was using the grey water from your plumbing system to run through the walls and either heat or cool the walls...but again, there are many alternative concepts and methods and it requires further investigation. All and all, when pushing the envelope, so to speak, it's good to experiment and take a chance, but you do so with a lot of research to back it up...in this specific issue, I would tend to agree the 3rd layer of insulation "sandwiched" between the walls without any air space is not worth it. I wuld look to using higher insulating materials for your walls, including the finish (ie, better than gyp. borad & paint??).

mx2.5

[csintexas]

It's pointless to have an air space in the middle of a wall. What is it supposed to be doing? The whole purpose of a double wall is to create a larger space for insulation and to prevent thermal bridging through the studs.

Airspaces are typically only used with masonry. You need to be more concerned on how the walls breath. In other words is any moisture able to get into the wall system and if it does how will it be removed?

What climate is this being designed for?

[arch*tect]

Air space between walls are the cheapest form of insulated wall construction.

If one wants more efficient insulation for walls, then one should go for beyond airspace wall construction, for instance foils, foam board...etc. But one should never deny air, because it is one of the best insulation material you find and it is useful.

It is highly recommended that airspace wall construction requires proper ventilation to avoids moist contents and to prevent damp on interior wall.

arch*tect

[birgco]

Several good points are made. I believe the most critical (and the one I am struggling with the most) is what happens to moisture entering a double wall with a 1 inch foam board in-between. It should not condense because the foam board will be pretty warm with R-21 on the outside of it and virtually no penetrations or obstructions in the 2x6 exterior wall to adversely effect the full R factor. Btw, the house will have an air to air heat exchanger to keep fresh air circulatiing thru-out the house. In my previously constructed double wall, there was a 1" air space. Now if that space was totally void of air movement, it would probably be a good insulator and thermal bridge. I doubt this is the case, so this is why I am looking hard at the 1 inch foam board instead. If the foam board is properly sealed (tyvek or similar tape at the seams) this should result in a virtually leak free wall. The problem of sealing electrical boxes and pipes is eliminated. The wall should have an R value in the neighborhood of 40. Trying to build a double-wall with the vapor barrier behind the sheetrock is not easy because of the before-mentioned electrical boxes, etc. I have seen houses built with a 2 inch foam board on the exterior wall but this concerns me more with condensation potential and also difficulty in attaching exterior finish, etc. Interested in comments about the placement of the 1" foam in the middle of the wall and thoughts about moisture, condensation, etc. thanks, birgco

[birgco]

Btw, house to be built in metro-new york area. 0-45 degree winters on average. birgco

[csintexas]

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.

[mx2]

I understand your fear of air chris, however, you underestimate the value of a well detailed cavity wall, since air is a great energy resistant "material" and it's free...

The problem lies in moisture control and the design and installation has to be perfectly executed or else...trouble. But compared to triple layers of insulation, that technically conduct heat from one material to the next, I would be surprised if the total R-values matched. I'll have to try and calculate it some other time... I found this online which I'll paste the most imporant passage below. Hope this helps...

mx2.5

[csintexas]

If this was a masonry wall system than a cavity or drainage plane inside the wall would be good.