double wall construction techniques

[Antisthenes]

see: Strata (90% air) the worlds first, i have worked on, able to stand up to hurricanes and and with the exoskeleton principles of a insect
or : Pumice-Crete, witch i have been using for a long while here i can get that variable mass to insulation ratios depending on the mix anywhere from 1:25 to 1:10, know as one of the first building materials to man.

no insect or moisture or fire problems there...

i like Radiant Barriers. i made these things called Pop-Ins they are basically 1 side covered in foil foam inserts then wrapped in white on one side black on the other cloth for my windows that repel sun heat and keep my home like a cave/thermos. saves me a bundle on AC thats for sure. while my neighbors pay around 250 bills i am paying under 50. soon i will have my PV hooked up too, i really want to be in a Net Zero residential dwelling. If i have to pay my power co. to be my batteries and to buy green energy credits to spur there investment that direction i will.

[csintexas]

Yes I agree that your system would probably perform a little better than calculated because of the reduced obstructions. I don't think it would be correct to compare your wall to a "typical" poorly done 2x6 wall but instead compare it to a well built 2x6 wall. (for example the type that building sciences would recommend) Because I am sure you would build either system as good as possible.

I raised the issue of building codes early on (it may be that they require it on the inside).

I think Antisthenes is referring to thermal mass. This brings up another idea that may help the performance of your wall system. Lately I have read that an 1" layer of sheet rock can provide enough thermal mass to function in a 12 hour cycle. Although this would be most effective as part of a passive heating and cooling system.

[Antisthenes]

yes the same exact home was built framed with 2x6 as well as the foam one and the foam one has 4x the performance (100R roof) and it is built in one the highest end communities so i am certain the craftsmanship on the wood one was the best it could be where as the foam one was the worlds 1st, so it will only gets better as wood and steel prices go up i predict and new methods are developed to work with the system.

not here to slam wood, as there is allot of reasons to, but to say a single wall of different material can be more than comparable to a double wall and a wall that combines insulation and mass is very important, a different kind of dual wall.

a house i am building now i am using OmniBlock where you have a little different that standard CMU block that has offsets, to prevent dirrect thermal transfer, and combines EPS inserts to retard that heat transfer again, the same goes for heat loss as well for you cold climate guys and gals.

today it is 111 and humidity 30%, poor people with only evap. coolers and leaky homes!

[tfurry]

Well, I'm getting lots of good ideas. I still think $300 is too high for summer AC bills, my dad's house is maybe 500 sq ft less but he cools his with a 2-ton AC for about $90 for the same time period. I'm still thinking about what to do...I'm a dyed-in-the-wool DIYer, so the labor isn't a problem...I just finished hand-digging about 300' of trench for our front yard sprinklers (too many roots for a machine). There are a lot of things I can tighten up on the house first, though, before I get nuts with the walls...increasing attic insulation, upgrading the 1984 AC unit (probably a SEER 6 or , foundation insulation on the added family room, etc. I'll keep checking back for more information and ideas. I'd love to build a house from scratch but don't see it happening for 20 years or so.

[csintexas]

yes I agree 300 is to much, I just said it was not unusual for a house of that age.

"There are a lot of things I can tighten up on the house first, though, before I get nuts with the walls...increasing attic insulation, upgrading the 1984 AC unit (probably a SEER 6 or Cool, foundation insulation on the added family room, etc."

I think you will find money spent on these other upgrades will help a lot.

[peake]

Gentlemen/Ladies

Excuse me for butting in, I have just stumbled accross this site, which I find very interesting, as a time served and now retired plumber, can I add a few notes which I made about cavity walls, for the non trade person

Cavity Wall's and Damp Proof Courses

The first cavity wall buildings date from about 1875 in England, and were first used to prevent dampness in the building, caused by rain water penetrating the wall, rainwater penetrating the outer skin would not be able to pass through the second wall, but would be either evaporated by the air current, or trickle down the outer skin and escape through 'dry perpend's' (vertical joints without mortar in the lowest course of bricks) at ground level

Normaly the east wall of a building would be the wall which is the most sheltered from driveing rain, (the rain & wind is mostly from the west), so could be built as a solid brick wall, the other wall's would be built as a ventilated cavity wall, 2 No 41/2" brick skin walls with wought iron ties, the ends of the tie were 'ragged' (split and the ends opened out), or the ends were cut on allternate edges to form a type of fishhook edge the idea was to help the tie to resist the walls spreading apart, the ties had a 'U' formed in the middle of the tie, this was built with the bottom of the 'U' pointing down, to stop water bridgeing the cavity) to tie the 2 cavities together. The outer skin built out of first quality bricks, the inner skin built with reject bricks, both walls were built in a sand lime mortar, air brick's or cast iron grills were built in to the outer wall, a void was formed in the inner skin, below the D.P.C. and about 6" above ground level, the idea of this was twofold; one to provide a through air current below the suspended timber ground floor, second to allow a passage of air up the cavity wall, and keep the inner wall dry; the cavity was not closed at the top, but kept open to allow the moisture escape

Windows and doors were built in the cavity, behind a 'nib' of brick, of the outer brick skin, the inner skin opening had a timber lineing, tounged into the door or window frame this finished the internal opening

The timber floor to the ground floor was out of T&G floor boards, on timber joists, layed on timber wall plates, on honeycombe sleeper walls (bricks layed with a 41/2 inch space between the bricks in the same course

Damp proof course's were normanly either a pitch sand mix, layed hot, or a double layer course of slates, layed with stagered joints between the courses, both are supseptable to fracture, with the movement of the building, but the slates being ridgid these are more supseptable to fracture, later on and in the more office or higher class establishment, the damp course would built with two courses of engineering bricks, beded in a strong sand cement mix, 3 to 1 mix, sometimes the upper course was a 'squint' (a bevel was formed on the top half of the brick) or a 'bull nose' brick, the 2 courses were sometimes layed on top of a 131/2" wall, this formed a plinth to the bottom of the wall, sheet lead and copper have been used, but these need to be layed by plumbers because of the jointing the sheets with lapped and welted joints

Other forms of wall tie were a 'Z' shaped brick, layed has a 'header' brick, with the lower portion of the 'Z' built into the outer wall, then came twisted stamped steel ties, and wire 'butterfly' type of tie, the wire was bent to form 2 loops which were built into the two brick walls the ends of the tie were twisted together in the middle of the tie, and when built in should point downwards, this is to allow any water to drop off the tie, in the middle of the cavity, it goes without saying that the cavity had to be kept clean of mortar droppings and "snots" (protruding mortar), a cavity board was used to assist in this, a board just wide enough to fit in the cavity, was layed on the lower wall ties and this was pulled up the cavity has work progressed and layed on the next higher set of wall ties

Later on towards the 1960's the cavity was built as a closed cavity, the top of the cavity had a brick layed over the top of the cavity to close it off, the idea of this was to stop air circulation and escapeing, so helping to assist in thermal insulation of the building, even later, mid '70's somebody came up with the 'bright'? idea that if the cavity was filled with an insuating substance, rockwool, fibreglass, or a liquid foam which set on contact with air, this would be even more benificial in insulating the building.
There are a few problems with this form of inulation; a) the insulation can induce 'wicking' of moisture, and transmit the moisture to the inner skin, b) if foam is used, it can also induce moisture to the inner wall, and also has it sets it can give off formaldehide gas which is toxic, c) if the insulation is 'blown' into the cavity 'cores' have to be cut into the wall to allow the nozzel of the machine access to the cavity, which have to be made good, d) if ridiged fibre glass or rock wool 'bats' were built in the cavity has the work proceds, the work needs to be kept dry and rain water needs to be kept out of the work in progress, otherwise the insulation bats will become saturated with water and will take for ages to dry out, in the UK some of the cavity walls are haveing ridgid polystyrene slabs incorperated in the cavity wall instead of fiberglass or rockwool 'bats' to avoid the problems of rainwater whilst the walls are being built

The Canadians also have a cavity wall with a 'styrofoam' insulation fixed to the inner wall, built out of blocks, and with dry perpends to the bottom course of brick work on the outer skin of the wall, to allow any dampness from driveing rain to evaporate, thus haveing the best of both worlds

[Riversong]

I, too, just Googled onto this site and am amazed by the amount of misinformation being promulgated.

I am an instructor in building science, specializing in sustainable building, and have been building super-insulated houses for 25 years, including a number of double-stud wall systems.

Let me first address some of the misinformation.

"Cavity" or what is now called "rain screen" construction is, as the last poster pointed out, merely a technique for equalizing vapor pressure on both sides of the outer skin of the building envelope and for shedding any rainwater that penetrates the outer skin. It contributes nothing to the insulative value of the envelope.

Vapor barriers, or retarders as they're now called, though often still required by code, are largely unnecessary except in very high humidity cooling-dominated environments such as Florida, where they must be located on the warm - or outside - surface. In mixed heating-cooling climates, no vapor barrier is recommended. In primarily heating climates, an inside (warm side) vapor barrier is recommended.

Building science, however, has determined that only 1% of moisture in walls is caused by vapor diffusion through building materials and that 99% is caused by moisture-laden air transport into walls and ceilings. Stop the air movement and you stop moisture problems (obviously excluding rain infiltration).

There are three options for stopping air exfiltration into wall and ceiling assemblies. An inside air barrier (like the air-tight drywall system), a cavity air barrier (like dense-pack cellulose or foam insulation), or an outside air barrier (like Tyvek or Typar). Using all three is a belts and suspenders approach.

However, regardless of which air-sealing approach is used, it is wise to build an envelope which can "breath" or absorb and release moisture with daily and seasonal cycles. Any envelope which contains an impermeable layer cannot breath. For this reason I advocate against vapor barriers in heating climates, and against any plastic layer in the envelope including foam insulations.

The air-tight drywall system uses acoustic caulk or EPDM gaskets at each framing assembly joint (1st deck to sill, bottom plate to deck, top plate to band joist, etc) and drywall caulked or gasketed to bottom and top plates to complete the "membrane". As for electric outlets, they are easily addressed with polypans - plastic outer boxes with large flanges to seal to the drywall. Obviously all other penetrations must be caulked or foamed. It is particularly important in any house system to seal penetrations in the upper ceiling, as the stack effect drives most moist air upward. And it is vital to remove moisture at the source, with thorough slab or crawl space vapor barrier, bath fans, kitchen hoods, and vented dryers.

Placing a vapor impermeable layer inside a double-stud wall can cause condensation problems. Though the 1/3-2/3 rule of thumb (placing the vapor barrier no more than 1/3 of the R-value from the inside) may work depending on the climate and inside relative humidity, it may also cause problems if moisture and air exfiltration isn't carefully controlled.

It is easy to calculate the temperature at any surface inside an evelope section. Assuming an indoor temperature of 65°F and a worst-case outdoor temperature of -10°F, the double wall system that birgco describes will have a temperature of 40° at the inside surface of the interior foam board where there is an R19 batt beyond it and a temperature of 29° where it rests on a 2x6 stud. If the indoor RH is greater than 38%, there could be condensation on the foam where it covers the batt, and as little as 24% RH would create condensation where the foam covers a stud.

As far is being "impossible" to thoroughly insulate a wall cavity that contains mechanicals, you're thinking only about batt insulation which in may ways is problematic, particularly fiberglass (the worst insulation ever invented!). Blown cellulose (100% recycled, low-embodied energy, completely non-toxic and non-allergenic) will fill any cavity completely, will prevent air movement, is a firestop, prevents insect or rodent infestation, is an excellent sound attenuator, and is easy to remove if repairs are required. It's R-value, unlike fiberglass, increases as the temperature either increases or decreases from room temperature. It can absorb 30% of its weight in moisture reversibly (in other words, can breath) and is so hydrophilic that it removes moisture from wood framing members thus protecting them (foam will concentrate moisture in the wood frame).

While the double-stud wall system in any of its varieties can be an effective super-insulated building envelope, the Larsen Truss or my version - the modified Larsen Truss - is less expensive, more energy-efficient and requires fewer forest resources. The modified Larsen Truss system also creates only a single insulation cavity that can be filled with dense-pack cellulose, thus creating a true R40 wall with a 12" thickness.

The wall system that birgco describes would have a theoretical R-value of approximately 42 and an as-built R-value of 39.6 if the inside and outside studs are staggered and the wall plates are not continuous between the two walls (this ignores the problem areas of band joists, etc). A Larsen Truss wall, because it is built outside of the structural wall and surrounds the floor and ceiling assemblies, eliminates thermal bridging and offers nearly 100% of its theoretical R-value. It is also one of the most cost-effective methods of super-insulation.

In terms of payback periods: if the house is mortgaged, then the incremental additional cost of monthly payments is more than offset by the incremental savings in utility bills over an energy-code standard house and the payback is instant.

- Robert

[csintexas]

Hi Robert,

I like your system and think your reasoning is sound. I don't know if I would go so far as to suggest instant payback in all cases because I think there are a lot of factors involved that have to be considered. Also I think it is not quite fair to compare any new wall system to a typical minimum wall system.

Why is the truss added to the outside of the wall and not the structure of the wall itself?

[Riversong]

[birgco]

Robert, thank you for your helpful response. I have not started construction yet but have since been in contact with a few knowledgeable building science professionals like yourself. I like your idea for the modified Larsen truss wall system but the planning, drawing, construction phase it too far along to switch gears at this point, but let me fill you in on further progress for the current super insulated wall system.
At this point, the double stud wall will consist of a structural 2 x 6 exterior wall (24 in. o.c.) and a 2 x 4 non-structural interior wall (staggered and 24 in. o.c.). Each wall has a seperate plate system so there is little/no thermal bridging and the walls will be 2 inches apart for a total wall depth of 11 inches. The idea of batt insulation and a vapor barrier has been abandoned in lieu of a cellulose filled wall cavity (totally agree this is a better way to go than batt insulation.) I was initially concerned about cellulose compaction issues but would be interested in your thoughts on the subject. The remainder of the wall system will remain the same.... 5/8" fire-rated sheet rock, double staggered stud walls, 5/8"cdx plywood, Tyvek house wrap, 3/4 inch cedar siding. The 14" attic ceiling/floor truss will also be completely filled with cellulose insulation and capped with a 5/8 inch plywood attic sub-floor. An air tight drywall system will also be utilized. Exterior electrical wall outlets will be installed with insulated boxes. Also band and ceiling joist areas will be blocked and filled with either cellulose or expanding foam insulation. I don't believe the few extra interior 2 x 4's (24" o.c.) is much of an additional material use for the added energy saving benefits.
This insulated wall/attic system should perform well in hot and cold climate conditions, prevent potential condensation problems and move the project a bit closer to the goal of an energy efficient home.
Looking forward to your comments and thanks again for your time and interest.

[csintexas]

I should have explained further about the payback period. You may be specifically talking about birgco's case but I would not want all people who may be reading this to think it applies to them.

Also the 5% can only be a very rough estimate dependent on factors like -whether you except the wall overhanging the foundation or not, increase in roof area, labor costs, the proportion of exterior wall to interior volume and finally in calculating it's energy efficiency you have to consider window and door area which can take up a variable amount of that area. Not only that but unfortunately it may raise your property tax.

My point about the comparison is that it excludes other systems which may or may not perform better and makes it sound like our only choice is between the current standard wall system and this wall system. Someone who is considering a super insulated wall is interested in more than just the standard.

I like the modified system.

[Riversong]

[Riversong]

[csintexas]

I read it carefully but there was no way for me to determine that you where talking about energy efficiency in general and not the larson truss wall system or even other super insulated double wall systems which is the topic of this thread.

Since that is the case I think the discussion would be better placed in the current thread:

Conventional Wisdom About Energy Efficient Construction
http://www.designcommunity.com/forums/viewtopic.php?t=17417

I think it a great topic.

[birgco]

Robert, wanted to mention that I was initially concerned about insulating the band joist areas because of the reasons you had previously outlined. A reasonably good solution to this potential problem is to order the 14" tall, 2 x 4 web trusses with a verticle 2 x 4 block about 12 inches back from the exterior wall plates. A one inch piece of foam board can be friction fitted and caulked into the area between this verticle 2 x 4, plywood attic subfloor and the interior 2x4 top plate. After the sheetrock is installed, the cellulose can then be blown in from the second floor (in this case the attic floor) to complete a continuous R 40+ wall/band joist area of insulation .
I agree that the mesh netting some installers use could create drywall installation problems.
On a side note, an interesting window detail is also possible with the 11 5/8" thick double wall. The left and right sides of the window jambs can be flared out at a 10 - 12 degree angle to allow more light to enter a room and create an interesting visual effect. Thanks again for your comments/input.