How much is enough ? Will 1/2 double foil polyiso be enough for some benefit if there is enough batting to take care of the general R factor in the walls ? Im just looking for it to break the thermal bridging issue....any R value added to the wall from it is a bonus. Thanks,Mark

To know if it is enough we need to know what are you trying to accomplish energy usage wise?
Just for estimating purposes, if you are starting with a 2*4 wall with fiberglass batts, you can expect a R-10 to R-11. Adding 2 layers of 1/2" polyiso will add R-6 (whole R-6, not a partial like the fiberglass) yielding about an R-16 to R-17. Starting with a 2*6 wall, you will start with about an R-15 to R-16, adding R-6 will take you to a R-21 to R-22. This estimation will get you fairly close.

To truly determine the R values, you would have to convert the R values of the stud and polyiso area as well as the fiberglass and polyiso area to U values, and take the weighted average of the U values with the weights being the approximate percentage of wall area covered by each area, and then take the inverse of that averaged U value to find the final average whole wall R value.

Are you going to have to put furring strips before you put the siding on? If you have to do that, I would try to get some thicker 2” polyiso and do one layer of that and attach the strips with 3.5” screws. You can get recycled 2” cheaper than you can buy 2 layers of new 1/2” anyway.

Also, what kind of insulation is going to be above and below? Are you using trusses or rafters? Cathedral or flat ceiling? Your attic is the easiest place to get R value, so you need good insulation there too. If your attic insulation is crap or you ignore the floor insulation, doing all this to the walls won’t end up mattering that much. I have seen too many houses with 2*6 walls, but don't have anything in the crawl space and don't use raised heel trusses, so their attic insualtion depth at the edges is 2" if they are lucky. Funny thing is, all these homeowners think their houses are superinsulated just because of the 2*6 walls, and apparently ignore the $xxx.xx a month heating/cooling bills (in tennessee).

lol ... Aardvarcus, you are like a mad scientist when it comes to explaining R-Values.

The point you are making is very valid. You need to consider all the components involved when calulating the R-value of a given cross section. Further more the entire structure must be evaluated not just one section. You can have an R60 wall, but if your attic is not insulated it won't help retain energy.

The bottom line is that the heat you are trying to contain in your home wants to escape and neutrilize with the outside conditions, and will do so through the weakest point in the building envelope. When constructing an energy efficient building you need to consider all the components that make up the envelope.

Yeah, but it's sorta like asking "How much money do I need to save to deal with the retirement issue? Will $100K be enough, provided I live in a country where health care is a given? I'm just looking to have some partying and recreational expense money- any thing beyond that will be a bonus. ". :-)

The answer will always start with "It depends...." and will be dramatically different for different people, depending on a lot of factors.

But anywhere in EastMark's ME half-inch iso would be very much on the skimpy side in standard stick framing, and if it's going to be foil-faced stuff, needs to be placed on the warm in winter INTERIOR side of the studs, (not the cold-in-winter exterior), or you're very likely to end up with condensation inside the insulation. I you went with fiber-faced iso or XPS it's OK on the exterior though (up to only about 2" in the XPS case, much much more for fiber-faced iso.)

You can safely put foil faced iso on the exterior in ME if it's ~65-70% or more of the total R value. You'd need ~3" (R20 iso) if it's a 2x4 16" o.c. studwall with fiberglass batts, double that if it's 2x6" 24" o.c.

I had no idea it was so complex. Basicly heres what I planned on doing....
Timberframe. 2 X 6 -24 OC w/R21 fiberglass insulation surrounding the exterior of the frame.
What I was considering was if there was any benefit to 1/2" polyiso (double foil if applicable) on the outside of the sheating and before the wood siding not really for increasing R factor but to break the bridging ?
I see the info everywhere on thermal bridging...this is the only reason I am considering using this...to try to cut that down some. Will it help any ?
Thanks guys ! Lots of knowledge here...wow. Keep it simple for me...Im pretty simple. ;-)
Mark

Yes there is a benefit from a thermal bridging point of view, but don't use foil or poly facers, use vapor permeable/ semi-permeable stuff like fiber reinforced iso or extruded polystyrene (XPS- blue or pink board) if it's going on the outside.  The foil will trap moisture inside the wall structure, which needs to dry toward the outside to keep mold conditions at bay.

The effect on clear-wall (no window & door framing) & whole-wall (other framing features included) R values is significant, since the R value of the 1.5" edge of the 2x6 is about R5, compared to the R21 of the cavity fill.  Even though the framing is only 10% of the area, it's a much larger fraction of the heat loss.  An inch of XPS cuts that framing heat loss roughly in half, while cutting the center-cavity heat loss by about 20%  

Play around with this for a bit:

http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/SimCalc.htm

Doing a little interpolating (the tool doesn't have R21 batting or R5 per inch foam as options)- adding R5 sheathing makes the difference between an R20 and R25 clear-wall values, but more importantly, the difference between R15 and R20 whole-wall values- (a full 25% reduction in heat loss from walls.)

R5 (1" XPS or 3/4" no-foil iso) would be the minimum for anywhere in ME, but more may very well be cost-effective depending on your exact climate and fuel.  Half inch iso is ~R3- enough to be useful, but more is definitely better.

Great info and thanks.
I was thinking the 1/2 polyiso/double foil/exterior...thinking I could side right over it without having to do strapping (and getting some help with bridging which I see it will help some )but I now see the vapor issue would be a problem in this cold climate.
How thick can you would be able to sheath safely without strapping do you think ? Probably going to log side it.
Mark

Log siding is kinda hefty, eh?

I dunno... 1" for sure, but maybe 1.5", long-screwing the siding through the XPS into the sheathing?

Depending on your overhangs and local wind/weather averages it might be worth long-screwing vertical furring at every stud to form a rainscreen-cavity between the siding & sheathing.  That way any rain/snow wetting events don't end up trapping moisture on the backside of the wood siding. (This is done all the time in commercial construction everywhere, as well as for stucco, but it really should be the standard for residential in rain-prone climates.)  Back-trapped rain moisture is a cause of wood siding cupping/splitting/rotting even on old-skool felt-underlayments.  But if it's a 1-story structure with 2' eave overhangs in a low-wind area your wetting events will be minimal, and mostly on the lowest courses of siding from backsplash, probably not a problem.

Caulk/tape every seam of the XPS and it becomes an effective air-barrier (or housewrap over it.  Housewraps are highly permeable.)  You can go as thick as 2" with XPS before it becomes too vapor-retardent, but taped/caulked it's an effective air barrier even at 1/4"  EPS/beadboard starts getting air-leaky if you go too thin, but it's not bad at the thicknesses we're talking.

Dana is correct about the furring strips to form a rain screen. If you are going to invest in quality wood siding of any type, you have to worry about moisture. The rain screen allows the moisture to dry quickly, thereby reducing the chances of deterioration. You should also treat the wood with a stain/preservative on all sides (including end cuts) before installing.

Saw some limited info on the "Dow SIS Structual Insulated Sheathing" in another thread. 1" is R5.5 and solves the OSB,Foam for bridging,and vapor barrier in one shot. Ive got to look further into this product but Dow is generally pretty sharp. Its been out almost a year. This along with 2 X 6 with R21 would make a pretty nice wall.
I saw it estimated at $32/sht by someone for the 1" which is steep but it does save quite a bit of labor it seems. Anyone use it or know someone who did ?
Mark

Dow SIS is NOT a vapor barrier (nor should it be, in most US climate zones).  It's a class-II vapor retarder though, and needs to be treated as such when designing a wall stackup.

It's not the only sheathing system with both structure R-value out there- some have higher permeance than SIS, which may be a good idea in ME.  SIS is rated at <0.3 perms at either thickness.  (Class-I vapor retardency is <0.1 perm, Class-II is between 0.1-1.0 perms. )  In cold climate with exterior side permeance that low if you need to have a Class-I vapor retarder on the warm-in-winter side of the stackup, along with near-perfect warm-side air sealing.

Dana (or others !)
Spoke to a lumberyard today. The 1" SIS is about a buck a sf. To use R5 foam and 7/16 OSB its .69/sf. I know the labor is less on the SIS but if there isnt a good net cost advantage to use SIS why experiment. Im thinking for the sake of cost and knowing what Im getting of doing this.....
The timberframe is the structure so I want to do 2x6-24 with 7/16 OSB with R19 and 1" of R5 (approx) foam board taped. I will be satisfied with the overall R value on this as well as knowing the products track records better.
My question is Im looking to get around R5 on the foam sheathing. What should I use for 1" to get the vapor barrier issue solved and no issues with sweating that gets me around R5 ? The lumberyard suggested "blueboard" at that price would do both vapor,R5,and not create sweating issues? I will be either R19 or R21 inside with T&G interior.
Thanks for your advice !
Mark

"Blue board" is Dow's version of XPS, R5 @ 1" thickness, and very commonly used for this application in New England (as is Owens Corning's "pink board".) At 1" it's semi-permeable (Class-III vapor retarder), with no vapor trapping issue. Go for it!

Dana Thanks for your reply ! Am I safe just taping it or do I need house wrap too? Cant wait for spring to get it done ! Mark

It kinda depends on the anticipated longevity of the tape, eh? Some sort of tape/caulk/mastic seal is good, but a housewrap or semi-permeable felt isn't a bad idea for the long term.

Would love to find a tape that Im confident in so I can use this as a vapor barrier. Time for more research !
Anyone know anything on the tape options out there ?

Mark

Typical tapes for foam sheathing are Venture red housewrap construction tape and Dow Weathermate tape.

http://www.google.com/search?&q=venture+housewrap+tape

EastMark
One line in your thread raises warning flags: using t&g interior. The danger in the various exterior foam boards is moisture - condenstaion on the inetrior of the foam as others have warned. But it goes a little further - moisture travels on air, so when you cut down on the air moving through the wall you cut down on the moisture. The air movement within the insulated stud cavity will substanitally reduce the effective R value of the fiberglass (some call it filterglass) insulation. Your assembly - studding (I assume between the beams) tends to be leaky anyway since the beams will dry & spaces will open up between the studs and the beams, but boards slow down air flow very little. Poly will help, but you'll be putting lots of holes in it so it will not be very effective. I strongly recommend sheathing the interior wall with something; drywall would work well & be as cheap as anything. Put it up, tape the seams, seal the edges, then nail your t&g boards over that. More work, but well worth it!
Bob Irving

Amazing amount of good info here. Thanks for all the replys!
The walls will be outside the Timberframe rather then between the posts. Exterior would be log side then foam (taped) then 7/16 OSB then 2x6 with R19 or R21 Fiberglass and finally T&G pine interior. Does that stack up concern you Bob ? or others ? Theres no chance of spacing opening at the posts enveloping it this way. Maine is the climate.
Thanks again for everyones thoughts and suggestions. Ive learned alot here.
Mark

If you don't have a WHOLE lot o' foam (2/3 of the total R value), you'll need an interior side vapor retarder/air barrier.  T & G pine is both air & vapor permeable, which would allow humid interior air to reach the cooler edges of the studs & fiberglass.  A sheet of 6 mil poly under the T&G pine paneling would be right- foam/caulk around all electrical & plumbing penetrations.  Lap & mastic-seal the seams where the poly overlaps.

Any reason you're stuck on batting vs. blown (cellulose or fiberglass)?  Either would give you higher real world R-value (no voids, compressions or gaps), and cellulose would have lower convection within the fiber, hygric buffering to wick any moisture that did find it's way in away from the framing , and a decent amount of thermal mass.  Blown fiberglass would have higher R value than batts or cellulose.