Hi Dana1,
Thank you for your input! I will make sure to steer clear of having a double vapor barrier and will keep the rigid board to the exterior of the structure. I was wondering if there is a major benefit of using polyiso vs XPS or EPS? I know that the R-value of the iso is slightly higher but does it justify the higher cost? Any downside to using poly iso?

I read Building America Special
Research Project: High-R Walls Case Study Analysis and on page 32 was where I saw polyiso referred to as PIC. I am a energy efficiency Noob so I just figured this is what it was called. Thanks for letting me know so other people could understand what I was talking about!

Graham

Iso is lighter per unit R-value than XPS and tolerates higher temps (max operating temp of ~200C instead of ~100C) making it the premium choice for roofing insulation in very hot sunny areas (although EPS has a huge share of that market due to the lower cost.) Anywhere weight's a factor iso steps up.  IIRC it also has a much higher kindling temp. 

With fiber facing instead of foil facing iso is highly permeable, making it an excellent choice when you need drying capacity on the side of the structure it's mounted.  Foil facings are used to give it a Class-I vapor retardency and radiant barrier effect, making it perfect for exterior sheathing in cooling dominated climates, but a not-so-great choice for exterior mounting in New England unless it's 1/2-2/3 of the total R value.

Iso is also soft, and brittle much easier to ding-up/break in handling than XPS.  In the usual sheet-goods densities it doesn't have much compression loading capacity either (don't use it under slabs!) There are sprayed iso products too, in various densities- some prefer 2lb iso spray to 2lb polyurethane, but they're both rather pricey per unit-R.

Fiber-faced iso used in roofing isn't outrageous- 4x8 sheets of 3" (~R20) run on the order of ~$50/sheet. Some manufacturers (Atlas, Hunter) have iso pre-laminated to OSB on one side (like a one-sided SIP) for roofing apps in various thicknesses, and you may consider using that under the siding for high-R walls as well, since it allows outward drying, and comes with the sheathing already attached making it a significant labor-saver and an inch thinner than XPS.  Long-screwed into the structural studwall I think you should be able to make it work.  (Foam-seal all of the seams & joints as you go.)  If you use furring on the exterior and long-screwed through the OSB/iso chunk at every stud you have a ready-made rainscreen gap too, which keeps the OSB happy no matter how hard the driving rain.  (I'm considering doing this to my place when it's time to re-side & re-window, which could be awhile...  It's a major hack as a retrofit, but you can super-insulate existing structures this way without dramatically changing their character or losing interior floor area.)

Thanks for the info, but I am a little confused now, sorry ;). I would like to use 4" of some kind of rigid board insulation over 2x6 studs. Should I use foil faced iso or fiber faced iso or xps?
Should I go with 1 - 4" thick layer or 2-2" layers?
Do I need to use OSB under the exterior isulation or can I use metal or let in bracing?
The roof is going to be a very simple design of prefab trusses with an energy heel and blown in cellulose, so I am not going to use rigid board for the roof. Sorry for the questions. Thanks for taking the time to help me out! Enjoy your weekend!

Rules of thumb for your climate: 

If it's more than half the R value, you can use either permeable or impermeable rigid foam board on the outside. 

If it's less than half the R value you have to do the math to figure out if impermeable (foil/poly-faced) stuff can work safely.

Note that  4" of XPS (with or without facing) is semi-impermeable, and should only be used on the exterior if it's more than half the R value.  (Up to 2" is OK.)

If you use impermeable or semi impermeable insulation on the exterior, you need to avoid vapor retardent materilas & wall coverings (including impermeable paints) on the interior side to avoid a double-vapor barrier trap. 

If you use permeable or semi-permeable rigid board on the exterior that is less than half the total R value, you need to apply a Class-I vapor retarder somewhere on the interior third of the total R value.

Basically, you need to keep moisture from the interior room air away from any wood or fiber portion of the wall that stays below the interior room air's dew point for extended periods, yet still have a structure than can dry.  If the interior room air is 70F and maintained at 30% RH (for health & comfort), the dew point of that air is ~40F.  If your average temp for the coldest weeks of the year is 10F, the dewpoint occurs roughly halfway through the R-value. Daily temperature swings might make it condense & vaporize on the coldest days, but it stays dry in general. If you place vapor-retarder 1/3 or less into the insulation from the warm side it stays dry always.  But if you have an exterior vapor retarder the structure needs to dry toward the interior. See:



The higher the interior humidity, the closer to the interior the dew point occurs. Vapor retarders just under the gypsum are difficult to seal perfectly & maintain (every electrical & plumbing penetration, every picture hanging hook creates an air & vapor leak), so it's more reliable to have excellent outward drying capacity, which you get with fiber faced iso or unfaced EPS (or iso/eps laminated onto OSB. OSB is semi-permeable.)

What does this mean? You can put 3" of foil-faced iso on the exterior of a 2x4 16"o.c. studwall since the interior foil-facing is more than halfway into the  whole structure R.  (R20 iso/R12 studwall) But on a 2x6" 24" o.c. studwall you're approaching the 50/50 limit, bur if you bump it to 4" (~R25 iso/R20 studwall) you're again good.  But you use 4" of XPS you're at the 50/50 limit with a 2x6 wall again. But you can't use interior side vapor retarders.

But if you use fiber faced iso, it's highly permeable, and you can go as thin or fat as you like and it will still have excellent drying capacity toward the exterior, but interior-side vapor retarders are mandatory.

Quote: "I would like to use 4" of some kind of rigid board insulation over 2x6 studs."

You really need to decide on what you are going to put on the exterior of the house because the previously suggested fiber cement siding just won't work over 4" of foam. It is just too heavy and the foam provides no vertical support to hold it up. You would need screws or nails ~6" long. Think about the weight of that siding levered out 4" from the structural stud pulling down on that nail/screw. It will sag down and cut right through that foam if there is nothing to support it below. Now if you were to extend the slab or footing out past where the foam ends and then provided some 2x4 furring strips over the foam 2' on center and resting on the concrete, that might work. But now we are right back to why not just use a double stud wall and dense pack it with the lower cost but just as effective cellulose?

I am worried about the dew point being on the interior of the double stud wall and I don't like sacrificing an extra foot of floor space on each side. According to Dana1 in order to keep moisture out of the structure at least half of the R-value must be outside the framing. In a double stud wall this would mean that at somepoint inside the wall moisture would accumulate, especially during the winter months. I am not sure if there are any strategies to help with this and would love to hear them if there are.

As for the fiber board siding if I can't use it then I will switch to something else. I assume that there is some kind of siding that will work over 4" of external rigid board, or else other people would not have built this way. What type of siding is ideal for this situation?

Siding over foam:

We've designed many projects with rigid foam to the exterior, the best way we've found is to use 1x3 or 1x4 wood strapping on the outside face of foam, screw through with SIP screws (or other oversized head, smooth shaft long screw) to hold the foam to the sheathing. Then, nail up whatever siding you wish to the strapping. The strapping doesn't even need to be PT, it will dry just fine under there as long as there is a clear space between the siding and foam. You do need insect screens top and bottom to prevent critters from getting in there.

Once you have the strapping tight against the foam the shear and friction will support plenty of siding weight. If someone gets nervous, it's worth convincing them to build a simple mockup and let them see for themselves how strong it is or isn't.

By far the best guide to working with exterior rigid foam is the Canadian REMOTE / PERSIST construction guide, it has explicit instruction on how to attach windows, foam and work with the various roof and wall conditions. These techniques depend on the 2/3 1/3 rule (2/3 of insulation to the exterior of sheathing to keep the sheathing warm and prevent condensation on that sheathing).

Link to the Cold Climate Housing Research Center REMOTE Manual: http://sustainable.cchrc.org/docs/REMOTE_Manual.pdf

JLC article by Thorsten Chlupp: http://www.interioralaskabuilders.com/construct/files/RemoteWall.pdf

Green Building Advisor on these systems: http://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulation-out-your-walls-and-ceilings

gmink21................So can't you find someone in your area to at least give you an estimate for a sip wall package? Maybe for the roof also (again, depending upon your design) I'm probably repeating myself but a properly installed 6 1/2" SIP wall will perform as well, or better, than any of the complex combinations of materials that you're talking about. and I don't see how it will be more expensive. If you just can't find a contractor nearby who's at least willing to look at this option with you, just find the nearest SIP manufacturer, and ask them for a referral. I'm sorry, I forgot the location where you're building, but I do know that R-Control has numerous SIP plants around the country. They also have some excellent videos, and complete technical data on their website (As do most of the major SIP producers)

Gmink21,

I went through the same painful process when I started planning my house.  I settled on a modular house built with R40 SIPs.  We added a layer of polyiso to the outside for good measure.  Exterior walls = R47!

If your best friend is a GC, you should be able to get a wholesale from the factory.  My experiences was this... no other method of building came close on price.  Looked at site-built SIPs, ICF, double stud wall.  Modular/SIP won hands down.

Check out this article for some good ideas:  http://www.finehomebuilding.com/how-to/articles/energy-efficient-from-ground-up.aspx?collection=72770

My house is significantly larger than 900sf, but you can see exactly what I did, and how I did it here: http://www.gouingreen.com.

Regards,
Ed
Mr. Green Dreams

Double stud walls with cellulose have been well proven to be a durable, moisture tolerant and generally inexpensive wall system in cold climates. There are many examples all over cold climates holding up extremely well. The problems with moisture in this construction system were mainly due to the use of fiberglass batt insulation, which has luckily fallen out of favor.

In the experience of the consultants we talk to, the moisture worries with these types of buildings are vastly overstated if there is a well built and verified air barrier, and no batt insulation is used. The air barrier can be to the inside or the outside, both work well.

If breathable to the outside is desirable, then either OSB or air-tight drywall is used to the interior, breathable fiberboard or diagonal 1x board sheathing to the outside.

With breathable to the inside (easier and more conventional to construct), ZIP sheathing works well on the exterior sheathing, with latex painted drywall to the interior as a vapor retarder (do not use a vapor barrier except in extremely cold climates).

If you have a good air barrier, there is no mechanism to move bulk water through the wall except for leaks, so even though the exterior sheathing will be cold, there is no location for water to condense upon. Water vapor may diffuse through the cellulose during the coldest months, but it generally diffuses back out without ever condensing. Condensation requires a condensing surface, even if the dew point falls within the insulation, it won't condense in there, it needs a surface.

What we really need is to figure out better ways of finding potential water leaks from the outside during construction, the fears of condensation have been vastly overstated with cellulose insulation. It's really the beauty of cellulose, Robert Riversong talks a lot about its capacity to protect wood framing from moisture, unlike the spray foams which force any ambient water into the more absorptive wood fiber of the framing lumber.

SIPS
SIPS are a great product, no question, but R value = R value. If the infiltration rate is the same, and it should be somewhat easier to get a low rate with SIPS, the R value rarther than the specific material will determine the product with the best insulation value. 6-1/2" EPS SIPS will be about R-26 which will give the performance of a well built R-26 wall, be it double studded, SIPS, ICF or anything else. (It isn't quite that easy - there are moisture issues etc to consider)
Bob I

So my 3 choices for R-40 exterior walls are down to these:



1. Conventional Framing with exterior rigid board insulation.



2. Double stud construction with cellulose



3. SIPs



I have decided I am using ICF's for the walkout basement and pre fab trusses with blown in cellulose for the attic. If all three of these systems have the same R-value, wall thickness and are all air sealed to the same ACH rate will they have the same heating & cooling demands? If they do then price would dictate which system to go with. Correct?

Sounds logical to me.

Ed

They're sorta equivalent. The thermal mass of cellulose still counts at R40- just not as much as it does at R25. In Stowe the performance difference won't be enough to be too concerned about though, but even-steven on dollars & R-value, go with the higher thermal mass.

Prefab trusses with around a 18-20" heel will get you an r49 over the exterior wall. Whenever I used tall heel trusses I always subtract 1" off the span of the truss. I then run sheathing up the wall and heel of the truss to the underside of the overhang. This works well. Provides good ventilation without the cheap styrofoam baffles. I did an r60 cellulose attic a few years ago. Trusses 24" o.c. with 5/8" drywall. There is a slight bulge in the drywall between the trusses. Next time I will either go 19.2" o.c. or spray closed cell a few inches thick, then top with cellulose to limit the weight on the drywall