I am in the final planning stages for my new home.  Budget is a BIG concern.  I had originally planned on using an open cell spray foam for the entire envelope of the house and use standard OSB sheathing with Tyvek house wrap.  The foam is going to cost a lot more than expected, so I am switching gears and need input.  I am going to concentrate more on sealing the house.  I will still foam the roof decking, but will go with either a blown cellulose or fiberglass.  Any opinions?  As far as sealing the house, I need to do it as inexpensive as possible.  Would regular OSB, priming and taping the seams, and caulking the top and bottom be my cheapest way to go?  Budget is tight.  I appreciate any input.  I am building in East Texas, zone 3 with 2x4 framing.  The house is single story 2153 sq ft main level with a 350 sq ft bonus room above the garage.

Jason

I'm sure the real experts will be here shortly. However I recommend reading some of the papers available online at buildingscience.com. Search "air sealing" and there are several pertaining to the airtight drywall approach, sealing penetrations, sealing windows, etc.

Good start:
http://www.buildingscience.com/documents/digests/bsd-104-understanding-air-barriers
Airtight Drywall
http://www.buildingscience.com/documents/information-sheets/4-air-barriers/air-barriers-airtight-drywall-approach
Sealing penetrations
http://www.buildingscience.com/documents/information-sheets/4-air-barriers/sealing-air-barrier-penetrations
Sealing windows
http://www.buildingscience.com/documents/information-sheets/4-air-barriers/air-sealing-windows

My understanding is that cellulose insulation will help in providing an air barrier, although at only 3.5" you are looking pretty thin on the insulation no matter what you use.

Buck3647 please read the original post. jacker had planned on using foam but must go a different route due to cost considerations.

From a moisture transport point of view cellulose, while much more air-retardent than fiberglass batts or low-density blown fiberglass, it is still not an air barrier. But from a thermal loss & infiltration point of view it's close- even better when dense-packed to 3lbs+ densities. But for best R/$ you're looking at wet-sprayed cellulose, which would give you a center cavity-R of ~ R13, and a clear wall value of ~ R12 (due to the thermal short circuting by the studs.) It's pretty-tight, but you'll still want to use air-tight methods on both the sheathing and the interior walls.

Gluing/caulking the OSB to the studs as it goes up is quicker & cheaper than caulking after the fact. Don't forget to lay a bead under the plates as well. To seal the seams of the sheathing it's best tape it with housewrap tape, but that requires first painting the sides of the seam with a quality acrylic-latex primer to get reliable long-term adhesion with the tape. On the wallboard, same story, then foam-seal around all electrical & plumbing penetrations.

I don't know what you're planning to use for siding, but putting at least an inch of Type-I or Type-II EPS on the outside of the sheathing will give a significant boost in thermal performance, raising the clear-wall R value from something like R12-R13 to R16-ish (~R17 in winter, ~R15 in summer, due the tempco of the EPS.) If you set it up with at least 3/8" of gap between the foam and the siding, it's worth using foil-faced EPS with the facer on the exterior to reduce summertime cooling loads. If you use unfaced EPS the housewrap or 15# felt goes on the exterior, but if you use foil-faced goods with a gap FSK taping the seams forms an acceptable drain-plane & air barrier. (Some use housewrap between the foam & sheathing as both a slip-surface and secondary air barrier, but it's probably not necessary in most applications.)

Play around with this a bit to see what an inch of EPS does for the stackup: http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/Calc.htm

1.5" of 2.5lb foam over the exterior can air-seal it well, but you'd only be at ~ R11-R12 for a clear-wall R, at (most likely) a higher installed cost. But where time is money, it can go up a lot quicker. If buck3647 is willing to give you a cash guarantee on that triple back returns, get it in writing, eh? ;-) Closed cell polyurethane is a premium product at a premium price, but no way will 1.5" of 2.5lb foam outperform a moderately air sealed cellulose & EPS stackup @ ~R16. It would likely outperform an R13 fiberglass batt cavity fill approach though, since it doesn't lose as much R with higher delta-Ts as highly air-permeable low density fiberglass, and it's partially translucent to infra-red radiation the way fiberglass is.

In the attic, low density cellulose is also a better value than fiberglass batts, and you could install 3x the R in cellulose up there as you could with closed cell foam. In hurricane zones with a cathedralized ceiling /insulated roof deck approach it can still be worth the money to do part of the R with cc foam though, since it glues the deck to the rafters & top plates, while adding substantial rigidity to the overall structure. Splitting the R between the roof deck & attic floor is legit, but you'll still have to insulate any air handlers or ductwork installed there.

Vented attics make little sense in east Texas- it's a mis-application of a practice more useful in heating-dominated climates. Near the gulf coast venting the attic to the outdoors brings more moisture into the attic than it purges, particularly in air-conditioned buildings where there's cool joist-edges or ductwork to condense on. Sealing the attic puts the attic inside the water-vapor & air-pressure boundary of the structure, and even though it's hotter up there than conditioned space, it's dewpoint will track the conditioned-space air fairly closely.

Thanks for all the input, some really good info.

Dana1, if I understand the concepts correctly, I could do the eps on top of the osb sheathing (taping the seams on both) and this would not only increase my r-value, but also do away with an exterior wrap such as Tyvek.  According to this website, I am in the zone that should not have any vapor retarder, http://www.energysavers.gov/your_ho...opic=11810.

Posted By jacker on 14 Dec 2010 11:31 PM
Thanks for all the input, some really good info.

Dana1, if I understand the concepts correctly, I could do the eps on top of the osb sheathing (taping the seams on both) and this would not only increase my r-value, but also do away with an exterior wrap such as Tyvek.  According to this website, I am in the zone that should not have any vapor retarder, http://www.energysavers.gov/your_ho...opic=11810.

Tyvek isn't a vapor retarder- it's a drain-plane and air-barrier, but it's highly vapor-permeable.  Caulked/taped seams on multiple layers of sheet good CAN make excellent air-barriers, but you still need some sort of drain-plane between the siding and the OSB, and I'm not sure unfaced EPS would be adequate in some stackups.  It would be for most if there's at least a 3/8" gap (aka "rainscreen") between the siding and the EPS, and it might be OK behind vinyl siding, which is inherently naturally back-ventilated.  Good old #15 felt is more than adequate, and cheaper than most housewraps, and it's highly permeable when damp, semi-permeable when dry.

Closed cell foam IS a vapor retarder, but if it's more than half your R value it won't matter what side of the assembly it's on anywhere in TX.

In east TX it's a good idea to use borate-loaded EPS to limit the ant & termite tunneling potential. (Some ants seem genetically hard-wired to crave the stuff- a real problem with EPS core SIPS of the 1980s.)  Alternatively, copper flashing both top & bottom of the EPS (particularly at the foundation sill) could mitigate this too.