I am an owner/builder who will be subcontracting out to build a custom home in texas at climate zone 2. I plan on making this extremely effiencent and wanted an opinion. My thoughts are to build 2x6 walls with open cell spray foam filling the cavity. I want to wrap the entire house in 1/2in plywood and also 1/2in foil backed rigid foam board to reduce thermal bridging within wall studs. Is this a bad idea. Do I need a house wrap between the two layers of sheathing and if so, what kind. I know this is probably overkill but I want the lowest energy bills possible. If I'm not mistaken, open cell foam will allow inward drying in the event of moisture making it to the plywood. I plan on having a 1 in airgap between outside of foam and a brick exterior. Any suggestions would be appreciated.

As for the roof, I was told to use at least 6 in of open cell spray foam under roof decking making sure to cover rafters entirely for thermal bridging prevention and 5/8 radiant barrier plywood decking with 30# felt and shingles

Posted By bgrygar on 11 Feb 2014 01:04 AM
... Any suggestions would be appreciated.

Use spray applied cellulose in the wall cavities and use the money you saved over spray foam (~$2/sq. ft. of wall area) and apply that to more/thicker exterior rigid foam and air sealing.

Posted By bgrygar on 11 Feb 2014 01:18 AM
As for the roof, I was told to use at least 6 in of open cell spray foam under roof decking making sure to cover rafters entirely for thermal bridging prevention and 5/8 radiant barrier plywood decking with 30# felt and shingles

6" of open cell spray foam is only ~R21.  That doesn't meet building codes anywhere. 

Radiant barrier decking is useless if it isn't exposed to an airspace.  Spraying foam against the foil face of the roof decking completely eliminates any benefit provided by the foil radiant barrier.

Where will your HVAC unit and duct work be located?

I was not aware of the airspace required for the radiant barrier. I keep hearing that spray foam though more expensive is superior to fiberglass batting and cellulose because of its high R value and air sealing properties. They also say that if the entire attic is within the envelope, a smaller a HVAC unit is generally used because of lighter load on the unit. I will have to make sure that my bid includes at least 9in of depth to roof decking insulation to meet R30 requirement of Climate zone 2. My HVAC unit will be in a closet centrally located in the house with ductwork in the attic. I am looking at an electric heat pump system but have heard about geothermal but will have to compare cost/benifts of that more expensive system. There is no natural gas at my lot without me providing my own tank so gas is not really ideal. I have heard that insulated metal ductwork though more expensive is more efficient because of its smooth flow design.

Yeah, radiant barriers only work with high differences in surface temperatures, and only in air gaps. Radiation travels through open voids, not solid materials. If solid materials touch, then you have conduction, not radiation (at least in a practical sense). So radiant roof sheathing should only be used if you are including a ventilation channel. The foil facing on your exterior wall insulation may help with radiant heat transfer between you siding and the foam, But that is only if you are using the foil face as the WRB (weather/water resistive barrier), plan on having a rain screen gap, and not adding exterior tyvek or similar.

Spray foam is higher R/inch and better at helping air seal (though it can get brittle and separate from the stud bays as the walls move over the years). But cellulose is so much less expensive that it allows you to use the extra $ more effectively elsewhere to save energy, and is still good at air sealing. No matter what, you will want to air seal the exterior walls using other methods, as the majority of air leakage spots are NOT in the stud bays themselves, but rather at intersections between the walls and the floor, ceiling, windows, and other exterior walls (corners). So spray foam is not really quite the magic bullet that people say it is. Use the $saved by going to cellulose and use it for proper air sealing materials/methods. You'll save more energy with the same/less $.

I have been reading more material about using open cell foam on the roof decking and the possibility of roof sheathing rot due to humidity in the attic space air increasing moisture content of the plywood decking. I'm now not so sure what to do, vented attic with r30 ceiling insulation with the best ductwork sealing I can do or unvented attic with ductwork in space and possibility of sheathing durability problems. Though the application of radiant barrier plywood would be incorrect for its intended purpose, would it in effect seal the underside of the sheathing from high moisture contact from humidity in the attic with the open cell foam applied underneath. I now realize there is no "this is the end all be all best way to do it" due to the area of the country you live in and the numerous variables that exist with home construction and area of the country you live in... Would you still keep the 2x6 walls and fill cavity with spray applied cellulous. Is there a best all around application for the whole house system in the central texas area that falls in climate zone 2?

2x6 stud walls @ 16" oc with blown cellulose, 7/16" OSB sheathing, and 1" foil faced rigid polyiso with edges and seams sealed/taped along with a 1" air gap and then brick would give you a wall with a whole R value of around 23-24. If you went with 2x4 walls instead of 2x6, the whole wall R value would be 19-20. This is considering the R value provided by the radiant barrier and 1" air gap between the insulation and brick. Either would be appropriate for your area. Probably not much difference in cost either way. You just lose 2" interior wall space around the perimeter with 2x6.

In the attic, I would go vented with at least R38 blown cellulose (R49 doesn't cost but just a little bit more). I would also probably opt for the foil faced roof decking for your area to help keep the summer heat down. Also, insure the ducts are well sealed with at least R6 foil faced insulation.

In Zone 2 TX climate you can do just fine with cellulose directly applied to the underside of an unvented roof deck as long as you don't have a tile roof. See Table 3 in this document:

http://www.buildingscience.com/docu...of-systems

Building the upper floor studs a foot or so taller to accomodate a horizontal chase for running the electrical & ductwork, with and blowing R50 cellulose above that is going to be cheaper than any unvented approach insulated at the roof deck though.  Air-sealing between the top side of that space and the attic is critical to overall performance though.

Cellulose has the advantage of deterring wood boring insects, and is preferable to open cell foam, as long as you define a primary air barrier on all 6 sides of the cube, that's fully continuous at all of the seams, from the slab's poly vapor barrier to the capillary break under the sill plate to the exterior sheathing to the supporting OSB or gypsum of the ceiling (or roof sheathing, if unvented) etc., and air-seal any plumbing/electrical/other penetrations at the primary air barrier with the appropriate durable materials.

I'll second Arkie on the 2x6 cellulose + at least 1" of foil faced iso as the best bang/buck on the wall assemblies.

Dealing with the thermal bridging at the top of the foundation and brick siding is always awkward.  With other siding types you can build the foundation wall or slab-on-grade stemwall with exteior foam at the same plane as the wall-foam for a continuous thermal break but brick needs the support of the foundation. You can get some amount of thermal break with aerated autoclaved concrete (AAC) block between the top of the concrete and foundation sill, but it's less than ideal. It's often a real weak point in the thermal performance of brick clad homes in colder climates, but is probably somewhat tolerable in zone 2.

So why is there so much propaganda about how superior spray foam is? They have all the local HVAC companies I have talked to set on how superior it is. Does spray applied cellulose ever compress or sag in the wall cavities like it does in ceiling. Some of the sites I have read talks about that it could do that in the wall cavities leaving uninsulated gaps that you never know exist unlike in an attic that you can see. I am concerned with the long term research on foam as this a newer insulation type and I don't know that enough studies have been conducted on these buildings that have considerable age since they don't exist yet. Dana1, what exactly is the "Building the upper floor studs a foot or so taller to accomodate a horizontal chase for running the electrical & ductwork" look like. Is there a website that can give me a visual of what this looks like? If I were to use cellulose on ceiling instead, how important is it to have all the ceilings the same height? If there is height differences, how should I insulate those areas sufficiently. Thank you again for everyones advice. Before this posting, I was all in with the spray foam but am having some serious second thoughts about that now.

Also, with the insulation combination you both recommend, Is it still cheaper overall then foam? Will it also be more energy effiecient overall? Im building this house on my ideal land and want to live there till they put me in the ground 40 or 50 years from now hopefully so I want to make my electric bills as low as possible.

Posted By bgrygar on 12 Feb 2014 11:39 PM
Also, with the insulation combination you both recommend, Is it still cheaper overall then foam? Will it also be more energy effiecient overall? Im building this house on my ideal land and want to live there till they put me in the ground 40 or 50 years from now hopefully so I want to make my electric bills as low as possible.

Call or e-mail me if you are open to alternative building technologies.  I do not sell any products or service.  Since retiring from teaching in the School of Architecture at Auburn University, I donate my free time to interesting and challenging residential projects.  Tell me about what you want to build and let us see if I may be of assistance in advising you about alternative building technologies. 

Posted By bgrygar on 12 Feb 2014 11:39 PM
Also, with the insulation combination you both recommend, Is it still cheaper overall then foam? Will it also be more energy effiecient overall? Im building this house on my ideal land and want to live there till they put me in the ground 40 or 50 years from now hopefully so I want to make my electric bills as low as possible.

Open cell spray foam runs about $0.50 per inch per sq ft.  Assuming a 2x6 cavity, the cost for spray foam would be about $0.50/in-sq ft x 5.5 in = $2.75 per sq ft of wall area.

You should be able to get a 2x6 wall sprayed with cellulose for $0.75 to $1.00 per sq ft or roughly one third the cost of spray foam.  Plus, the cellulose generally has a slightly higher R value per inch (R3.7 vs. R3.5) compared to 0.5# open cell spray foam and it adds more mass to the wall.

1" thick rigid foil faced polyiso insulation should run about $0.50 per sq ft.  Add roughly another $0.10 per sq ft for cap nails, tape, and sealing caulk.

With 2x6 walls filled with open cell spray foam and no exterior rigid foam, you are looking at a whole wall R value of ~R15 at a cost of ~$2.75/sq ft for insulation (including labor).

With 2x6 walls filled with cellulose and 1" rigid polyiso on the exterior, you are looking at a whole wall R value of ~R23 at a cost of ~$1.50/sq ft for insulation (labor included with the cellulose but not with installation of the rigid exterior foam).

The cellulose filled 2x6 wall with rigid exterior foam should be more energy efficient and cost less.

Posted By bgrygar on 12 Feb 2014 11:35 PM
So why is there so much propaganda about how superior spray foam is? They have all the local HVAC companies I have talked to set on how superior it is. Does spray applied cellulose ever compress or sag in the wall cavities like it does in ceiling. Some of the sites I have read talks about that it could do that in the wall cavities leaving uninsulated gaps that you never know exist unlike in an attic that you can see. I am concerned with the long term research on foam as this a newer insulation type and I don't know that enough studies have been conducted on these buildings that have considerable age since they don't exist yet. Dana1, what exactly is the "Building the upper floor studs a foot or so taller to accomodate a horizontal chase for running the electrical & ductwork" look like. Is there a website that can give me a visual of what this looks like? If I were to use cellulose on ceiling instead, how important is it to have all the ceilings the same height? If there is height differences, how should I insulate those areas sufficiently. Thank you again for everyones advice. Before this posting, I was all in with the spray foam but am having some serious second thoughts about that now.

A complex set of questions...

Spray foam, particularly open cell spray foam does an excellent job of air-sealing the framed cavities, and fills the wall cavities to near perfection. In markets where air-sealing isn't taken very seriously, and super-air-permeable low density fiberglass jammed imperfectly into un-sealed stud bays, that's an instant benefit to a foam approach.

But even 2.5lb low density cellulose (damp sprayed or dry blown) is FAR more air-retardent than the fiberglass standard, fills as completely as any foam application.  From a thermal performance point of view 2.5lbs or higher cellulose is as good or better than half-pound polyurethane, as long as the cavities are reasonably air tight.  It's R-value is more stable across temperature than half-pound foam, and it can buffer seasonal moisture drives, protecting the structural wood.  Caulkihg the studs to the sheathing, and the stud plates to each other, and the bottom plate to the sub-floor, and even foam-sealing the lateral electrical penetrations in the studs and back foaming the electrical boxes seems like a lot of detailing, but it goes pretty quickly. Even a spray-foamed wall cavity still needs that detailing at the top & bottom plates, since the foam can only seal the cavities.

Settling of cellulose is a function of it's installed density and it's seasonal moisture cycling, and there is a well defined density for any given amount of seasonal moisture cycling at which it won't settle.  The amount moisture cycling is a function of both climate, and construction- in cold climates where the outermost layers of cellulose are below the dew point of the interior air for many weeks/months every winter the moisture accumulated over a winter is much higher than in a dry temperate climate.  Typical 2-hole method dry blown cellulose comes in at ~2.2-2.5lbs/cubic foot, but even that is usually dense enough for wall cavites in southern New England, as long as the siding is back-ventilated (vinyl siding, or clapboards on furring), or with R5 of rigid foam on the exterior of the sheathing.  (I've gutted simple clapboarded walls 30 years after the fact that showed no signs of settling, but I've also seen installations of sub-2lb density that settled by more than a foot.)  Damp sprayed cellulose has a dry density in the mid-2s, and has the additional benefit of a water-activated adhesive to keep it from settling.

Open blown attics are usually installed at 1.2-1.5lbs density, and in a vented attic in areas with summertime dew points north of 65F (like the gulf coast, or most of the eastern seaboard) it's the summertime moisture drives that causes it to settle, but that settling stops  when it reaches the density at which the mechanical creepage of taking on and releasing that moisture has found it's elastic limits.  In general,  shooting for a 10-15% deeper initial installed depth is sufficient for long term performance.  As in walls, using damp-sprayed is useful in vented attics, which will limit (but not eliminate) settling of low-density cellulose, but it also keeps wind in the soffits from moving the stuff around on you.

The horizontal service chase approach is  becoming common in high performance homes, as outlined in this bloggery bit or this one or here. This one's a bit narrow for running ducts, but it doesn't take rocket-science framing to make them a foot or more deep if need be to handle ducts & air handlrers, etc, since the mini-joists are only holding up the  finish ceiling and lighting fixtures. The  continuous OSB air barrier mounted to the under side of the structural attic joists (or bottom chords of trusses) handles the insulation load, and is more easily & reliably air sealed, with an order of magnitude fewer electrical/plumbing/flue/duct penetrations.

The idea of a horizontal chase for interior ductwork seams very appealing. The different heights of the ceiling throughout the house may make design difficult but I will have to look at plans to get a better idea. Taking interior ducts out of the equation for now, How does different height ceilings affect the efficiency of a insulated ceiling as opposed to insulated roof decking? For instance, how is the portion of the wall that extends from a 9ft bedroom into a 10ft living room insulated to maintain a proper R value thickness? Is this detrimental to the ability of the insulated ceiling to maintain high efficiency

This has nothing to do directly with insulation, but does anyone have experience with geothermal heat pump HVAC stsyems? These systems are considerably higher then conventional systems but to my understanding have some tax credits available. I just don't know if the cost associated with this system is better overall then compared to a high efficiency electric heat pump system. Any opinions would be appreciated.

minisplit air to air heat pumps may have rebates available from your utility. The tax credits you'd get from a geothermal system still leave you with a more expensive and more complicated system than with minisplits.

It's also not always the case that ground source heat pumps would be as efficient as ductless mini-splits in a TX climate. A well design GSHP would beat most ducted air-source solutions, but variable-speed variable refrigerant volumes can see truly huge efficiency for ductless under part load conditions.

The primary shortcoming of ductless is the distribution, but whether you need much in the way of distributing that heating/cooling is a function of the building envelope design. A doored off bedroom on the west side with a "sunset view" would be a real problem, but more reasonable designs can get by with a minimal number of ductless heads. If by careful envelope design you can get the total load down under 2 tons (cooling OR heating) it takes a heluva lot of subsidy for GSHP to make it competitive with ductless solutions at any efficiency level.

And where there's an efficiency difference, given the price trends in rooftop solar pricing in TX, it can sometimes be cheaper to make up any difference in annual power use with PV.

http://www.greentechmedia.com/articles/read/SunEdison-And-The-Texas-Solar-Non-Miracle

http://www.greentechmedia.com/articles/read/can-u.s.-solar-pv-costs-keep-falling

Thinking longer term, in any new construction it pays to orient a major roof surface for reasonable solar gain- even if you don't install PV as part of the original build, it's guaranteed to be cost-effective to do so before 2025 (maybe even before 2020), with or without subsidy. By 2030 PV, barring serious breakthroughs in other technologies, PV will be hands-down the cheapest grid power available, whether on your roof or on some utility or other party's grid-scale solar-farm. The fact that it's scalable and cost-effective even at the single home rooftop level makes it a technology very disruptive to many utilities' business models, but TX is a real hotbed of power utility innovation regarding both PV and wind. The net metering deals will change as more PV gets built out, but it'll still have real payback unless some very conservative stakeholders out-lobby the regulators to limit distributed generation. But they'll still lose in the end- it's getting to be just TOO cheap, now that the lifecycle cost per kwh is below fixed residential retail rates for half the country.

Is there a specific caulk recommended for sealing the plates and studs?

Posted By joallen001 on 24 Feb 2014 10:01 PM
Is there a specific caulk recommended for sealing the plates and studs?

Tremco Acoustic Sealant has a long and successful track record on air sealing, and stays flexible pretty much forever.  Most other acoustic sealants would work too.

Most construction adhesives will work too, but in some areas would be disallowed for sealing studs to sheathing (which would make the wall assembly too rigid & inflexible for seismic safety during big earthquakes, causing the wall to walk off the foundation during the shake rattle & roll.)