Hey guys
Need some opinions.
I am renovating a 1966 house. I got it stripped down to the studs. I have been looking at using foam in the walls. I have been reading on hear about 2# foam and 1/2 #. It has a really big attic and wa wondering if I should foam the rafters and pay the extra for the heat/air that moves up there. Just kinda confused of what to do.

Thanks in advanced.

Foam away.
I have advocated your exact plan for some time now. Foaming the rafters actually firms up the roof structure, and stabilizes the attic environment. The added cost of conditioning the attic air is minimual, and the benefits will outweigh this added cost. I have used the 1/2# foam for some years now, and am quite happy with the results. Also, if the house in on a crawlspace, consider spraying the rimboard area and the inside of the foundation. This, along with a good layer of poly vapor barrier, will produce a stable crawlspace environment.

I'm all for conditioned attics, too, but doesn't his climate play some role in the decision?

forgot to mention. The rafters are 2x6. Don't know if this is a problem. The house has a full basement with 2 craw spaces under 2 different areas.

Guys I am thinking of doing this (see picture ). My code calls for R-38. Can I do this and add blown in on the attic floor to reach R-38? Will that cause a problem in the attic? Thanks

I'm a bit concerned about putting the 1" of closed cell foam on the wall sheathing & roof exterior since it becomes a class-III vapor retarder at that thickness somewhere around the R-value mid-point in the wall, and at the exterior in the attic, which may create wintertime condensation issues inside the wall (even more so in the attic) depending on climate. In heating dominated climates it's better/safer to work the vapor retarders with the most-retardent materials toward the interior.

How many heating/cooling degree-days is it in your location? (See if you're in here: http://www.climate-zone.com/climate/united-states/ or give us the first 3 digits of your zip code.) In most heating dominated climates it'll work better if you seal the attic floor with the 2lb foam, pile cellulose on top of that, and use half-pound foam alone on the underside of the roof deck for a dries-toward exterior vapor profile.

You may opt to go further, do most of the insulation in cellulose, and put 2" of half pound foam on roof deck for a guaranteed air-seal, with a dries-toward-exterior profile. This will create bigger seasonal temperature peaks in the attic though, which may/may not be OK for you, depending on your intended use for the semi-conditioned attic space.

Ponder this bit when thinking about it:

http://www.sprayfoam.org/uploads/pages/4507/Vapor%20Barriers%20&%20Vapor%20Retarders.pdf

Then figure out at what average depth into your R-value the average wintertime dew point is. You'll have to make assumptions about your interior RH & temps will be look at a psycrhometric chart to figure out the dewpoint at that humidity level, the use your coldest-month average temps & average min temps (from the climate-zone.com or other) data to see whether you'll have sustained periods where the dewpoint is in the cellulose, with your stackup.

Beyond that, make sure you use EPS, not XPS on the outside of the OSB if you go ahead with the CC foam as depicted or you'l have rot issues in humid areas, since XPS is also a class-III vapor retarder. It may also be worthwhile to provide a true drain-plane & air gap behind the cement-fiber siding too, since it can wick & store a lot of rainwater. On the roof layup, if there's a way to allow a drain path between the lath & roof deck to allow leak water to escape, not just dry-in-place it would be good. (I'm sure Galvalume NEVER leaks, eh? ;-) )

As an alternative to EPS on the outside, fiber-faced open-cell iso board (usually sold as roof insulation) is also very permeable, with a significantly higher K-value. (~R6.5-R7/inch instead of ~R4/inch). It's more expensive, but not necessarily more expensive per R-value, especially when installation costs are considered. (Labor costs would be the same, so it's just the material cost difference.) In mixed climates it may be a cheaper/better alternative to the 1" of half-pound foam in the wall layup- go with 3.5" of spray cellulose, and 1-1.5" of fiber-faced iso on the exterior. By putting the higher R-value outside the studs you get the full benefit, whereas the inch of 2lbs foam between the studs is R6, but broken by R1 sections of stud for ~15% of the area, reducing it to R5. (You could spray an inch of 2lb foam completely encasing the studs on the interior side, but that ends up being somewhat more than 50% more matierial.) It may also be cost effective to put 1/2" foil-faced iso as a vapor retarder & air-barrier on the interior side of the studs for another full R3+, and you'll end up with a true R20+ whole-wall R-value.

Play around with this model a bit:

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

Note that added foam, (interior or exterior) retains its full R-value in the whole-wall calc, since it has no thermal bridging from studs.

Be sure to vapor-seal the floors of the crawl spaces and insulate those walls as well. If the crawlspaces don't extend below the frost line it's a good idea to insulate the floor perimeter with a 2 foot band of 2" XPS, if not the whole thing. If your subsoil is below 55F and you can take the hit in ceiling height, R5 and a thin-slab on the basement floor may be worthwhile too.

I am in Delaware. 19720 Zip code. Thanks for the info

19720 is pretty much Wilmington/Newark weather:

http://www.climate-zone.com/climate/united-states/delaware/wilmington/ (somehow this doesn't look right- they probably mis-keyed and put in weather data for Wilmington NC or Wilmington CA)

Try:

http://www.weather.com/outlook/events/weddings/setthedate/month/average/19720?from=36hr_outlet_wedding

or

http://www.climate-zone.com/climate/united-states/new-jersey/newark/

or

http://www.wunderground.com/NORMS/DisplayNORMS.asp?AirportCode=KILG&SafeCityName=New_Castle&StateCode=DE&Units=none&IATA=ILG

If you assume a worst case winter humidity (or control to) 70F/21C, 50% RH indoors...

http://www.av8n.com/physics/img48/PsychrometricChart-SeaLevel-SI.jpg

...then the dew point occurs at ~54F/12C.

The average January temp appears to be ~ 32F, so the crude-model sez the average dew point occurs roughly (54-32)/(70-32)=0.52 or 52% of the R-value in from the exterior or 48% of the R-value out from the interior. The most vapor-retardent layer should be no further out than 42% of the R value out and you've got the 2lb foam starting about R7-ish cellulose, or 7/17= 41% in from the interior so I think you're safe in the wall stackup as-drawn, since the dewpoint occurs within the foam.

In the attic you can tolerate the dew point occurring regularly within the half-pound foam, but not the cellulose. The 1" of 2lb foam is ~R6, then the 4.5" of half-pound foam figure ~R16 for a total of R22. The max you can totally-safely get away with as cellulose on the floor is then R22 x 42/58= R16, for a total of R38, just making your code requirement. If you put a dehumidifier in the attic and keep it at 60% RH or less you can heap it up further (as long as you don't reach the freeze point in the attic space on the coldest day of the year and break your dehumidifier hose/reservoir. :-) ) Alternatively, maintaining the cold-weather RH in the conditioned space down to under 35% brings the dew point down to ~41F, buying you quite a bit more margin, which puts the dew point even further outward in the insulation.

Still, if you can reliably seal the attic floor with an inch of 2lb foam as an air barrier/vapor-retarder you can skim-coat the rafters with half-pound foam as a perfect air-seal then blow cellulose to whatever R-value you like (which will be less money than what you have drawn. Less 2lb foam, due to less surface area, less half pound foam due to thickness.) It's still easier to design around the strongest vapor retarder on the interior, with a dries to exterior stackup in heating-dominated climates. If you go with the stackup as-drawn, pay attention to & control your conditioned space relative humidity. (Lower RH is always better for the building materials, but below 30% is less comfortable and less healthy for the occupants.) There's a lot of error in the actual application installation vs. the modeling of the materials- if you're going to put the vapor retarder on the exterior it's best to not design it very close to the margin, which means a lot more foam on the outside, which is why most people design it with the vapor retarder nearer the interior for heating dominated climates. The rule of thumb in Canada had been 1/3-2/3, with the vapor retarder no more than 33% of the R-value from the interior, but even that gets 'em into trouble sometimes. Canadian code now calls for a 6mil vapor barrier on the interior side there. That's overkill for DE, where stackups with NO vapor retarder works fine, as long as the interior side is at least somewhat more vapor-retardent than the exterior side, and there are decent interior & exterior air-barriers (even dense-packed cellulose is probably sufficient as an air-barrier in DE as long as outward-drying isn't much impeded by other layers.)