double wall construction techniques

[mx2]

[csintexas]

Why have a cavity inside of the wall? What is it accomplishing? It isn't as good of insulation as insulation and it isn't providing the drainage plane. It also isn't as good a thermal barrier as foam.

I don't have anything against airspaces in general but this particular situation is not where one is needed or desired.

If the object is to save the cost of the foam than just use insulation to fill the cavity. If the object is to use less batt insulation make the wall narrower.

We don't use airspaces inside 2x4 or 2x6 walls either. Airspaces have good uses, this is just not one of them.

[mx2]

Sometimes it feels like I'm talking to a brick wall with you...if you don't take the time to read what I post then what's the point? If you don't open your mind and at least consider what the FACTS say about the other opinion that you already established you don't agree with...then what's the point? Either you respond to what I write or at a minimum please stop repeating the same thing...sheesh! Either way, there is no more discussion...

mx2.5

[csintexas]

"But I'm still a big fan of well designed cavity walls with good vapor barrier details...think of it this way, air has a thermal conductivity of 0.025, while the mortar between c.m.u. is 1.73 and wood (yellow pine) is 0.147 (to compare to common materials)...and insulating materials are like fiber insulating board is .048 (twice as conducive) fiberglass and felt insulation is 0.04, or glass woll insulation is 0.04...

...air is still twice as resistive to heat. Agian, the problem is moisture...but then again, it's still a problem in non-cavity walls, so...??? what's the dillie yo? "


Ok, so an air space is twice as resistive to heat as fiberglass insulation? I guess that means we are just wasting money on insulation then.

Where are you getting this information from? Do you seriously expect me to believe we are decreasing the resistance of our our walls by insulating them? Is this insulation thing just some commercial hype from Owens Corning?

[P.C.]

Now it seem the problem is, that warm moister air can simply enter spaces that is colder, and condensation is a strong force --- it will even pass thru from an inhouse warm atmosfere thru materials like wood, and acturly push off the layer of paint , leaving water bobbles on the cold outside.

So it simply have to be made so, that the warm moister full air, can not enter these volumes, the membrame must be on the inside , yet still this is caursed by the whole attitude, by our rigid use of this type of construction and how we over time replaced some of the materials that originaly was used back then, when it was not plywood and poor spruce, cheap universal nail fittings, but special morter ,straw and chicken wire, and when the plywood was honest timbers and good wood planks.

Gee next time someone want to "develob" this plybox fasion, it will be with even cheaper materials ; where did the craftmanship go where did the good materials go.

[birgco]

Hello?? I appreciate the discussion here, but I think this has digressed a bit into an argument over the effectiveness of air spaces and cavity walls. The original topic was introduced as how I might produce a more effective double framed wood wall. I am primarily concerned with gaining insight into the effect of moisture on the following constructed wall system: from the exterior.....
1. 3/4" T&G cedar siding
2. tyvek or similar house wrap taped at all seams
3. 5/8" cdx plywood
4. 2x6 doug fir structural wall with R-21 fiberglass unfaced batts
5. 1" foil foam board taped at all joints (should be virtually 100 %
effective at eliminating moisture passing through to the exterior)
6. 2x4 doug fir non-structural wall with r-13 fiberglass unfaced batts
7. 5/8" fire-rated sheetrock

Remember, all electrical and plumbing roughing is in the interior 2 x 4 wall so there is no need to disturb the 1" foil foam board (vapor barrier)
except for an occassional dryer or bathroom vent which will be very carefully detailed to prevent air leaks.
The primary question is what happens to the interior 2 x4 wall with the
R-13 insulation in it? Will it in any way be adversely affected by vapor trying to get to the outside? And also remember the house will have an air to air heat exchanger to maintain indoor air quality/humidity and also all heating equip, etc. will have outside air supply vents to help minimize negative interior pressure. Now I welcome your comments but please try to stay on the topic. thanks, birgco

[csintexas]

I'm sorry we have had to waste time on this silly airspace issue.

like I said several posts ago:

"I don't design for cold climates but I believe the vapor barrier is normally placed under the sheetrock."

Although now days many professionals say the wall should be allowed to breath unless you are in a very cold climate (more than 9000 degree days heating)

In either case like I said before: the 1" foam board needs to be permeable.

I don't know how I can be more clear on this issue, perhaps you need to do more research on vapor barriers.

here is a good place to start:
http://www.buildingscience.com/bsc/designsthatwork/cold/profiles/minneapolis.htm

[birgco]

Chris, I appreciate your input, but I am looking for information that deals with more than standard building practice. The Minn. site is helpful but it mostly talks about single wall construction for extreme cold climate. As I mentioned before, this project is in the ny metro area, cold but not Minn. cold. Standard practice in most cold climates has been placing the vapor barrier behind the sheetrock. As I previously mentioned, this practice has drawbacks and results in major air infiltration around electrical boxes and other wall penetrations, even with careful energy detailing. I am looking to build a wall that eliminates this "flaw" and results in major energy savings for an active solar house with integrated systems. Hopefully, this can be accomplished by moving the vapor barrier(1 inch foil foam), to the center of the wall. But not being a energy wall design expert, I would like to get as much info as possible before proceding. Also I am not a fan of SIPS or other foam walls for reasons previously stated. If you know of sites that discuss double-wall technology, please let me know. thanks, birgco

[csintexas]

An insulated cavity wall is the same regardless of it's thickness. Using a 2x4 plate and a 2x6 plate side by side is the same as using a 2x10 plate (except that you can use your foam sheeting in between the wood to get a thermal barrier)

If you just want somebody to tell you your idea is good because that is what you want to hear than you are missing the point. There is plenty of information available on this subject. If you think you know more than the professionals who design these wall systems than by all means go for it.
If you want to put a vapor barrier in the center of the wall than do it, it is your house after all. (Unless some building code official prohibits it.)

[birgco]

Chris, whoa, you sound awfully hostile for this site. If you would read my post, I am just looking for people who have experience with double wall construction. I don't claim to be an expert on everything, nor am I looking for someone to tell me what I want to hear. You are giving info that has nothing to do with the topic and then you go on the attack. I have been successfully building houses for 25 years and wish to keep improving techniques that have worked well but what to make them better. Take it easy, read the topics a little more carefully and stick to what you know instead of trying to be an expert in all fields.

thanks again, birgco

[csintexas]

What seems hostile about my last post?

The air space question was a direct result of your first post because the only difference between your new wall design and your old wall design is that you are replacing the airspace with foil foam board. I believe the topic is "Double Wall Construction" and we have stayed on that. We simply haven't been talking about what you want to talk about.

I don't try to be an expert in all fields but I can read and comprehend simple technical data written by people who are experts. If I was trying to be an expert than I wouldn't be advising you to follow the experts advice.

To sum it up

experts say:
put vapor barrier behind the sheetrock or if the heating degree days are less than 9000 don't use a vapor barrier.

you are saying: my wall is a special case because it is thicker than the standard wall so I want to put a vapor barrier in the center of it.

I said: it's your house, if you think you know better than the experts go for it.

I don't see anything hostile about that. That does not say anything as to whether your idea is good or bad and it is not a recommendation of what you should do.

Actually I think there is a fairly good chance that the inside surface of the foil wrapped foam board will likely not be less than the dew point so condensation would probably not be a problem. And the fact that they are now recommending that no vapor barrier is needed in anything but cold cold climates tells me that condensation within the wall is not an issue. I simply don't see any reason to disregard expert advice and add a vapor barrier to the center of the wall.

What is this foil foam board supposed to do? If it doesn't have an airspace in front of it than it won't act as a radiant barrier. Other than that foil is pretty conductive and I don't see that as a benefit.

[mx2]

I can't believe it!!! I just wrote a rather lengthy response and accidentally closed before posting....aaaargh!!

Long story very, very short; 3 methods of heat transfer =

Radiation
Conduction
Convection

Two other problems with wall insulation = air & vapor flow.

Foil can be any material but aluminum is a good heat resistant material of radiation, as opposed to air. Air is a great resistor to heat transfer by conduction. That said, back to birgco's request, I would conclude with a proposal like this, starting from exterior:

wood siding
housewrap (vapor permeable air barrier)
foam insulating sheathing (instead of plywood)
studs w/batt in between
air space (of course I prefer 2", but...you know)
air permeable vapor barrier membrane
studs w/batt in between
foil faced foam board insulation (sealed w/metallic tape and bonded to gyp. bd.)
gypsum wallboard

Keep in mind, the foil faced insulation is a radiant heat retardant, not a vapor barrier, so the penetration around the outlets need to be sealed but it's mostly for heat control, whereby the house remains warm during winter and cool during summer. The vapor barrier behind the studs is the only place I could think of, using the 1/3:2/3 rule of thumb...1/3 towards warm side, 2/3 towards cold side.

In this assembly, you cover all bases; air & vapor flow and heat transfer resistance (insulation). In your proposal, putting the foil between the studs is too far to be effective as a radiant barrier where the heat has already entered the "cavity" of the wall and your worry of air flow is valid, but air will flow regardless, hence why I opted to place it somewhere in the middle. Hope this helps and please keep us informed on new info you come across and (curiosity killed the cat) what you finally choose. Best of luck.

mx2.5

[mx2]

Oops...correction:

air will penetrate anyway, hence why I placed the vapor barrier at 1/3. Once air penetrates the two extremes of the skin (exterior shel and interior shell) you want the air to flow (not in extreme flow, but find it's way out), however you do want to stem the flow of vapor, which is also why I opt for air. That said, the penetration thru the outlets is a valid point and I would add that a goode detail is to spray foam inside the cut out when installing the outlet/junction boxes. The closed cell foam acts like an air barrier and will help stem the flow of moisture at these weak points. Also by adding the foil faced foam boards behind gyp. bd. it acts as both vapor/air barrier but also insulates the studs, which is commonly the weakest link is heat transfer.

mx2.5

[birgco]

mx2, I like the fact that you are thinking outside the box (no pun intended), but I believe you are correct that the electrical boxes and other penetrations are the weak link in any insulated wall. I remember reading about super insulated houses and the danger of even the smallest hole in the vapor barrier. These breaks in the vapor barrier acted as accelerating funnels for moist air to enter the wall and could cause water damage because of very localized high levels of condensation. Now that being said, in the early days of trying to prevent vapor from moving into the wall, polyethelyne plastic sheets were used. I remember a few years back, ripping out one these walls and finding the heavy plastic "vapor barrier" badly deteriorated and wet insulation. So my concern (obsession?) for eliminating the potential for leaks into the wall and hopefully a better type of wall system. It was also my understanding that a good aluminum facing on foam board is, as you say, a radiant barrier but it is also 99.9% effective in eliminating the flow of vapor through a wall. The wrench in the current system, as previously stated, are the wall penetrations. thanks for the feedback.

[mx2]

The greatest pleasure is thinking outside the box and working towards a new solution...that said, it's a great exercise and learning experience for me as well.

The weakest link is air intrusion in the walls, even if air will penetrate regardless what you do. But the air pressure will seek and find the largest opening and create greater flow which invotes condensation at that point. The openings in the drywall are common areas of "slop" and prone to air infiltration, but I have seen and heard that sprayed in foam behind the junction boxes helps greatly alleviate this problem. At any rate, you're welcome, and thanks! Keep us informed...

mx2.5