I have a shop (about 19x21 floor dimenions, not attached to another building) that I use for general shop stuff (welding, repair, some woodworking, misc).  I'm about to replace the shingles in the very near future (about 6/10 pitch I think), and am considering putting 2" pink polystyrene sheets rated at R10 (XPS?) between the roof deck and the shingles (actually have 1/2" CDX or OSB under the shingles, but above the XPS).  The interior of the building is not finished (the "attic" space is open - no ceiling - I can see the underside of the roof decking).

I'd like to insulate so that I can afford to heat it in the winter (I live in northern SD).  I've been reading lots of forums regarding installing the XPS under the shingles, and other approaches - like spray foam under the decking, installing a ceiling and adding insulation there, etc.  All the answers I've read seem to depend on the local climate.

For our climate (HOT in the summer, COLD in the winter), what would you guys recommend?  Is the XPS under the shingles going to significantly age them prematurely?  Where should my vapor barrier be?  Does none of this matter since I don't "live" in the shop (so I'm not out there running showers, and don't generally keep it permanently heated?).  (maybe above frost for a week or two for certain projects).

I greatly appreciate anyone's input and time.

mp1010

On roof with pitches over 2:12 the vast majority of the shingle cooling will be from exterior convection and re-radiation, not conduction through the roof deck, and shingle life is basically unaffected. (The shingle color & whether it's on a N vs. S facing pitch will be a larger effect.) At 6:10 you're fine, but even were it a very low-slow roof, using California Title 24 compliant "cool roof" shingles would be adequate, since they reject much of the solar gain, but are high enough emissivity to cool by radiation.

With the roof deck exposed to the interior, all vapor retarders need to be on the exterior. At 2" the XPS itself is class-II vapor retarder.

R10 isn't much insulation for SD winters though. Using cheaper material like EPS and a secondary non-structural OSB nailer-deck above to bring it up to ~R20 (5" of EPS), and putting kraft-faced R8-11 batts on the interior (facers toward the interior, if faced) should work in your climate. The asphalt-loaded kraft facers are moderately vapor retardent but wouldn't interfere much with drying-toward the interior. If unfaced, holding them in place with a permeable or semi-permeable air-barrier like housewrap improves the performance of the insulation, since it then doesn't convect heat & moisture between roof-deck & room. (~3" of wet-sprayed cellulose blown behind netting would also work, if there's a contractor in your neighborhood.) Then you'd have a ~ R30 assembly, which would protect you better from the summer heat as well. R10 on a roof deck is nearly nothing in the blazing July sun, where the shingles will run 140F or more, but R30 can keep the interior face down to something reasonable- it won't feel like you're standing under a radiator. And the heat loss in winter will be 1/3 of what it is with R10. If it's reasonably air tight and the walls are insulated to more than R10, with an R30 roof earth-coupling alone might keep it well above 20F except during cold snaps, making it easier to bring up to temp when you want to use it. (Insulating shutters over the windows when not in use make a difference too. Foil-faced EPS or iso is pretty lightweight, and it's not too tough to fashion snug-fitting interior shutters to slip into the window openings, to pop out when you want the daylight.)

With at least 2/3 of the R outside the roof deck the roof deck is pretty safe from indoor humidity in your climate, especially since the shop will be cooler than your household comfort temps most of the time, and pretty dry. If ALL of the R value is above the roof deck, it's temp & humidity will be whatever the interior humidity is- if it isn't condensing on the shop equipment, it won't be condense on the roof deck.

Thinking about it over lunch, 5" of EPS and a layer of OSB would require 6-7" screws, which gets to be a rather unwieldy & slow installation. When labor aspects and the cost of the nailer deck are added in it's probably cheaper/easier/better to go with the panelized iso with pre-applied nailbase, where you can get to ~ R20 in only 3.5-4", and 5" screws, which is much more manageable.

See:

http://www.hpanels.com/2009/pages/pdfs/Lit_Prod_Color/H-Shield-NB.pdf

http://www.atlasroofing.com/tabbed.php?section_url=58

http://www.specjm.com/products/roofing/enrgy3.asp

IIRC ~3.5" thick 4x8 sheets of R20 nailbase-iso run ~$100/sheet (~ $3 per square foot), but I haven't quoted them recently. If you did R20 with XPS sheets of 7/16" OSB for a nailer deck you'd be at ~ 4.5" additional thickness, and ~$75 for a 4x8' section (~ 2.50/foot) but more installation labor.

I sure appreciate all your time writing that down, and thinking it over. I'm not totally sure what R20 nailbase-iso is, but at $100/sheet, it adds up! It's just a shop and the extra "conditioned" storage in the attic would be nice, but not sure it's worth the extra cost involved with insulting the roof - vs - attaching plywood to the bottom of the rafters and making a ceiling (and putting R19 kraft faced insulation over that - 2 layers). Everything is on a 24" center - if it matters.

I'm estimating to get a good roof, and get the ceiling insulated, I could do it for $1600-$2000. (Shingles are $75/sq; 7/16 OSB over the current deck planking at $217 (10.85/sheet) (add some tarpaper/felt - didn't price this). Then buy 1/2" CDX for the ceiling (will this sag if attached to the bottom of rafters that are 24" on center???) for $180.31 (13.87/sheet), and R19 Kraft Faced insulation at $325 - then double this to get R38 (hmm.. I suppose I don't want 2 layers of Kraft facing?).

I've begun placing R19 Kraft Faced insulation in the walls, and the shop always seems to be well earth-coupled - even without any insulation, it's almost always 20degrees warmer inside in the winter than the outdoor temp.

Do you see any big mistakes with this approach?

Thank you very much!

so you're looking at a stackup of :

Shingles /felt paper/roof deck/rafters (ventilation space)/ cdx / R19 + R 19 / air


Shingles /felt paper/roof deck/ R19 + R 19 in a rafter-bay / cdx

Or something else?

When insulating on the interior-only, it has to be PERFECTLY air-tight and the roof deck ventilated to the exterior or you'll end up with condensation potential at the roof deck & colder portions of the rafters. If unventilated it's safer to go with at least some of the R on the exterior.

BTW: At -10F R19 batts are only delivering ~ R10, due to convection within the insulation. Air-impermeable insulation tends to work better (FAR better at the temperature-difference extremes.)

fresh quote as of today from my local 84 Lumber salesman on ATLAS ACFoam nail base insulation (non vented):

2"x4'x8' R-9.6 $29.11 each

3.5"x4'x8' R-19.1 $49.89 each

psammy

Posted By psammy on 16 Jun 2010 03:22 PM
fresh quote as of today from my local 84 Lumber salesman on ATLAS ACFoam nail base insulation (non vented):

2"x4'x8' R-9.6 $29.11 each

3.5"x4'x8' R-19.1 $49.89 each

psammy

Those are good prices!

When I last checked, local distributors are charging slightly more than that for fiber-faced 3" x 4' x 8' roofing iso WITHOUT nailbase.

R19 batts from a box store run about a buck per square foot. R19 iso nailbase at $50/sheet is a bit over 50% more but it's well worth it IMHO.  It wont' have nearly the compressions & gaps, and won't lose nearly as much R value at high delta-T.

That's what I thought! I was expecting a lot higher price.

psammy