Your roofer is living in the past. Elk & Certainteed standard warranty applies to both vented & unvented roofs. Cool roof rated shingles will outlast darker shingles, whether it has a warranty or not.

If you take a 25 year dark gray/black asphalt shingle and it craps out in 22 years due to the slightly higher average temps of an unvented-insulated roof you're still money ahead on the heating-energy savings. But a high-E, high-relectance CRRC rated shingle will as a rule last longer, and provide cooling-energy savings to boot.

We do this every week and I have a hot roof in my 1921 balloon frame. The losses through an unheated slab are not that dramatic, but perimeter losses can be significant and must be addressed.

We install radiant ceilings - finishing a job in a newly constructed SIP home tomorrow - so this is something to consider before filling the joists with cellulose.

Posted By Dana1 on 01 Sep 2011 12:12 PM
Your roofer is living in the past.

Unfortunately, I think many are.  When i proposed sealing the attic and foaming the underside of the roof deck he pretty much freaked and said it shouldn't be done.  He suggested insulation on top of the roof deck with an added nailer for the shingles.  But, what is the purpose of insulating the roof if he just wants to pump 1200 cfm of unconditioned air through the attic anyway?

Posted By BadgerBoilerMN on 01 Sep 2011 02:15 PM
We do this every week and I have a hot roof in my 1921 balloon frame. The losses through an unheated slab are not that dramatic, but perimeter losses can be significant and must be addressed.

We install radiant ceilings - finishing a job in a newly constructed SIP home tomorrow - so this is something to consider before filling the joists with cellulose.

Badger, what is your advice on a retrofit to adress perimeter losses on an uninsulated slab-on-grade?  Excavate beyond the frost line and rigid foam with a reflective barrier?

You can go straight down or down (say 1 ft) then out (ie, horizontal foam, may be cheaper digging). Cover exposed foam with fiber reinforced stucco direct to the foam.

Just curious, has anyone cut V grooves in a concrete slab to add radiant with 3/8" pex where you just can't tolerate a higher floor? Perhaps with aluminum plates (to allow equivalent performance with wider spacing, less cutting).

Posted By jorshw on 01 Sep 2011 03:50 PM

Posted By Dana1 on 01 Sep 2011 12:12 PM
Your roofer is living in the past.

Unfortunately, I think many are.  When i proposed sealing the attic and foaming the underside of the roof deck he pretty much freaked and said it shouldn't be done.  He suggested insulation on top of the roof deck with an added nailer for the shingles.  But, what is the purpose of insulating the roof if he just wants to pump 1200 cfm of unconditioned air through the attic anyway?

Are you sure he was talking about insulating the roof deck from above, but leaving the gable ventilation in place?

Adding insulation above the roof deck can be a good approach to an unvented attic.  If left vented it still has some value, since you won't have a 120F roof deck radiating onto your ducts & air handler, which will be surrounded by 90F air, not 110F air, but it's not half as useful as a sealed-attic.

With R15-R20 above the roof deck you can then safely seal the attic.  You can also add unfaced R21 batts between the rafters directly against the roof deck for higher R.  A ~3-4" Hunter or Atlas nailbase panel would get you there:

http://www.hpanels.com/images/stori...eld-NB.pdf

http://www.atlasroofing.com/tab...ion_url=58

If doing your own with standard OSB (no nailbase),  tape/foam seal the seams of the iso, and stagger the seams of the OSB with that of the iso by at least a foot for better air-tightnes.

A vented version would take 4.5", but would run your shingles 5-10F cooler:

http://www.hpanels.com/images/s...ent-II.pdf

http://www.atlasroofing.com/tabbed.php?section_url=51

Furring over standard  iso  without nailbase, through-screwed to the rafters with OSB screwed to the furring can sometimes be cheaper as a vented solution- depends on how complex the roof lines are and your labor costs.  Foaming/taping seams for air tightness counts too.

A vented nailbase approach has the additional advantages that shingle nails won't compromise the integrity of the iso, and the nailbase itself will dry toward the vent cavity should the shingles ever leak rather than rot in place- it's a more resiliant assembly.

deleted double post

Update....

Attic: We cleaned probably a 2000lb load of junk and recyclables (old bath mats, broken glass, dead light bulbs, stacks of cardboard, etc) out of the attic this past weekend so I took some pictures so you can see more or less what is up there. 2x8" rafters, 1x12"+ roofing boards, exposed duct work, etc.

http://quantumrallysport.files.wordpress.com/2011/10/img_0336.jpg

http://quantumrallysport.files.wordpress.com/2011/10/img_0338.jpg

http://quantumrallysport.files.wordpress.com/2011/10/img_0339.jpg

http://quantumrallysport.files.wordpress.com/2011/10/img_0340.jpg

Exterior Walls: had to tear into an exterior wall to fix a leak where the water main enters the house and learned a bit more about the construction as a result. The house mostly has brick exterior walls--and I have not tapped into them yet so can not comment on their construction. However, the front wall has about a 25ft section comprised of the following layers from outside to inside: Paint Wood Siding 2x4 stud cavity with fiberglass batt 1.75" concrete Plaster Paint At least some of the interior walls have a similar construction ---Plaster over concrete--- as i learned while mounting some things on a couple of walls.

Radiant floors over existing slab and forced air for cooling (the way God intended it to be done). I design these for a living, but every good radiant floor heating system - new or retrofit - starts with a proper heat load analysis. Once you button up the house a sealed combustion condensing boiler or condensing water heater would be just the tickets. Warmboard is not recommended for slab-over installations.

I you intent to keep using them, the metal ductwork should be sealed at every joint & seam & beaded edge you can reach with duct-mastic. As is they leak far more air than you might realize.  I saw a bit of aluminum tape on one of the round Ls, which probably helps. But replacing the tape with duct mastic is more reliable on those. (When the galvanizing is fresh you can sometime get decent long-term results with FSK tape on the flat section seams & joints, but not so much in it's current condition.) The supply ducts should be insulated to at least R6 (1" of cc foam works well as insulation + sealer) in an insulated-unvented attic, R8+ if the roof deck is uninsulated.  It looks like you have reasonable access now- duct mastic is cheap, you can probably do it all for $50-75 worth of material (available at the box-store home-center chains.) A stiff paintbrush or a putty knife works, don't skimp- lay it on at least 1/8" thick, and work it into the joint where there are obvious gaps.

The masonry gable end can be insulated on the interior with rigid foam, if you intend to take a sealed & insulated attic approach. (Tape the seams, spray foam the edges to prevent convection around the rigid foam.)

With 1.75" of concrete and plaster on the interior you have a decent amount of thermal mass inside the 2x4 studwall insulation, which is good!  Is there exterior wood sheathing and a gap between the sheathing and the exterior masonry?  Or is the front wall just wood-frame (no brick veneer)?

We have not torn into a wall with the brick veneer yet so we are not sure exactly what is there. The front wall is wood frame with siding.

Thanks for the advice on the duct work, I think 1"cc there might be the first thing we do. Got one estimate for 5.5" of open cell foam in the rafters and rigid foam on the gables for about $8k which seemed high to me for open cell. He didn't want to do CC for some reason...

Open cell is fine on the roof deck if you have a bit of R above the roof deck as well. At 5.5" most open cell is still a bit too vapor permeable- more than 3 perms, and there can be some wintertime moisture accumulation in the roof deck. Between 0.5-1.5 perms is about right. (2" of closed cell is ~ 0.6 perms, and about the thickest you could go on the inside.) A combination of 1-2" of closed cell with the rest being wet-spray or dry blown cellulose (blown behind netting works out to a slightly higher R than an all open-cell solution.

Closed cell foam at 2" would run about $2-2.25 per square foot in my neighborhood, (a 1" flash-foam would be ~$1-1.20/foot) and at 5.5" open cell would run about $2.25-2.50 per square foot. With gables and the sloped roof a 2400' house would have about 3400-3500' of roof area, so yeah eight grand is the neighborhood. A flash-foam for air-sealing & moisture protection with the rest done as cellulose in netting would still be over five grand, but would outperform the all open cell solution both thermally and in terms of protecting the roof deck.

The IRC spells out R15 minimum of low-vapor permeance air-impermeable foam outside the fiber layer at code-minimum R values in your area (US zone 4A), but with only 5.5" of rafter depth you'd be at about half code min, so going with 2" of ccfoam you'd have plenty of margin, and at 1" you'd be just shy, but still probably OK.

If you went ahead and spray-foam insulated the ducts and spot-sealed the attic floor you could leave the attic vented and put 15" of cellulose up there, which would be a lot higher R for a lot less money, but you wouldn't have any storage space, and air-sealing & insulating the attic access can get tricky.

If i do CC foam in the attic (not living space) do i need to add an ignition barrier to meet code? Will the drywall/plaster ceiling of the living space below count?

Posted By jorshw on 31 Oct 2011 06:01 PM
If i do CC foam in the attic (not living space) do i need to add an ignition barrier to meet code? Will the drywall/plaster ceiling of the living space below count?

In your case, nope.  Exceptions in the code exist if it's a tight crawlspace or attic with only limited service-access and no air communication between the attic and the conditioned space, in which case the plastered ceiling would count, but not for something used as a storage space.

If you put the foam on the roof deck and apply wet-sprayed 3"+ of JM Spider or wet-sprayed cellulose in the same rafter bays the Spider or cellulose would count, but on the sloped roof it would have  to be blown in mesh.

How do "they" know what I use the space for. If it is not a living space, it should be exempt.

If you decide to go with the conditioned attic I would install a Space-pak with hydro coil supplied by a Vertex water heater. Then you clear up the attic space for storage or? The perimeter insulation is needed no matter what type of heat is used just more so with radiant.

Ready to go... 3" of CC cell going in next week at $1-sq/ft

Next step--replace a couple of rotted windows.

You can't buy the material plus maintenance on the equipment for 3" of CLOSED cell foam at $1/square-foot. Are you sure you are't being quoted for open cell? ($1/ft would be on the low side even for open cell @ 3" depth in my neighborhood.)

$1 sq/ft per inch of thickness....

got it.

It's going on the roof deck, between the rafters?

If yes, filling rafter bays with some cheap stuff is worth it. The thermal bridging of the 3" of rafter that extends through the foam is an ~R2.6 thermal bridge in your R18 roof, which undercuts performance considerably. Assuming a 10% framing fraction (counting the rafters, ridge & soffit framing) it works out to ~ R13-R14, not more. Compressing cheap unfaced R11 batts or blowing cellulose into the remaining 2.5" gap between the foam and interior half-inch gypsum or OSB about doubles the R of the rafter element bridging through the insulation, since it's the. Assuming the same 10% framing fraction it would then hit around R21-R22, a 50% boost in whole-assembly R. If you're not closing it the bays in right away you can take your time on that one- getting it air-tight and insulated is the first most-critical aspect, but it's a relatively cheap performance boost.