The preferable method is spray foam, closed or open. Foam boards are hard to seal, possibly creating unseen moisture problems. As well, the uncovered rafters may still be susceptible to moisture/freezing problems.

As pointed out in one Dr. Lstiburek's recent postings, to operate well, conditioned/unvented attics should have returns and supplies; and this, in turn, has created further complications with fire codes. The insulation has to be covered with incombustible materials and smoke alarms and shutoffs incorporated into the HVAC system.

I don't agree that you need to add supply & return
to the foam sealed attic. sorry joe.
only if the attic is to be used as living space
would hvac provision be made, and then
the load calc would include the attic living
space & system be designed accordingly.

nor do I agree that a foam sealed attic is
a conditioned attic. it is a semi conditioned
unvented attic.

Shanetill,
you can make an unvented attic with foam
sheathing. how much depends upon the
R-value of the foam boards. you would have
to meet the minimum requirements for
attic insulation in your area.
great attention to detail would be needed.
each piece of foam board at eaves would need
to be cut to fit tightly, and sealed into place.
I like caulk/backer rods & paint on mastic,
as see issues with cans of foam used
to air seal. at this eaves/soffit area it has
to be air tight, so you need to fill the gaps
with a material that will expand & contract
and last.
seal these pieces to the underside of the roof
decking, both sides of rafters & the top plate below.
then use the foam board to fill in rafter bays.
offset the cuts in the layers of foam board,
cut them to fit tightly & caulk at sides of
rafters as you install each layer.
sealing the joints would also be necessary.
finish the install with this final layer that will
also cover the faces of the rafters, just like
you would with spray foam insulation.

this final continuous
layer also seals to the soffit/eaves foam board
blocking.

know that like with closed cell spray foam
insulation, that the foam boards will trap the
water when you have a roof leak.
open cell spray foam insulation will let the
water out of the insulation.

your way is a LOT of work, a lot of foam board
& cases of caulk. but you can do it.
(if your labor is free)
if you are buying materials & paying for labor
the easier more cost effective install may well
be spray foam insulation.

run the numbers both ways.

getting into the ceiling joist bays to
seal off at the eaves...that is some
hard work! I tell you this from
experience.

best of luck.

I dont like the idea of that method whatsoever. One of the reasons spray foam performs so well (and allows them to use below code minimum Rvalues which they shouldnt) is that spray foam forms a continuous air barrier and in truss situations, covers the top chord framing members to reduce thermal bridging. Cutting pieces of rigid foam to fit in between framing members is a recipe for airleaks galore and relying on canned spray foam in this magnitude and application is very suspect.

Rigid foam board can work great at the roofline for non-truss roofs but it needs to be used as intended: in continuous, well sealed sheets. This usually means on top of the roof decking which can be a tricky but very high performance or below the framing members. These preferred methods, make it easier to air seal, which should be an approved tape, and reduces thermal bridging.

There seems to be a backlash against spray foam at roof decks among some high performance builds lately. Vented attics are tried and true. As long as you seal the ceiling plane, keep HVAC out and avoid an interior attic access, these attics can be super insulated with cheap fluffy stuff and roof leaks are easily detected and located without worry of the foam creating high moisture levels in the roof structural sheathing. The trick is avoiding steep vaults and recessed cans which leads us to use SIPS or foam at the roof deck.

Why are you wanting to switch? It's NOT simple to do properly.

"As long as you seal the ceiling plane, keep HVAC out and avoid an interior attic access"

the reason we use foam for an unvented
semi conditioned attic is because
furnace & ducts are almost always
in the attic.
oversized penetrations for recessed lights, supply
boxes, bath vent fans are hidden by trims,
grills & covers. (these are the most common
leakage sites..but not all leakage sites)

now if trades people would actually seal
the holes they cut...ahh that perfect world.
no more going to happen than homeowners
not adding huge amounts of recessed lights.

I've been hearing lately that homeowners
don't want attic staircase in garage...
because someone might break into the
house via attic...go figure. it has been a long
time since I've heard that one!

you'll note that I didn't recommend foam
to seal the foam boards in place, but caulk.

granted it would be a huge undertaking...
but could be done. I've seen it maybe twice
in my 15+ years. always small houses
with low pitched roofs.
these cajuns...LOL!

personally given the difficulty, spray foam
for the entire roofline would be the option
I'd chose given the amount of labor to
install foam boards.
making the final layer of foam boards
attached to the bottoms of the rafters
would reduce thermal bridging

here in my area we use open cell foam
which allows moisture to exit the foam when
the roof leaks.

just fyi

Ok I should have mentioned that I live in south central Texas the reason I'm doing it this way is because I got the materials cheap I am doing this all by myself. I have a metal roof that's my plans were to cut the sheets in 24 inch wide pieces and glue them in place between the roof trusses in the foam seal around them. yes I can contest this is a lot of hard work . I have allready done this to the whole downstairs wall in my house

This post was edited by shanetill254 on Sun, Dec 14, 14 at 20:16

here are some more picture you can see the far edges will not have floor just the sheet rock ceiling from downstairs.
I plan on taking eave vents out and sealing them whole and reinstalling the vent to cover were the holes were cut.
I am going to remove the vent baffling you can see in the pictures and caulk all gaps between the decking and roof joist.the reinstalling the foam.
Lastly I plan cutting a piece of ply wood to cover where it has been cut out for the ridge vent .
the whole reidge with the exception of 4 ft from each end is a working vent.

I think I paid 10 a sheet for the foam off craigs list

This post was edited by shanetill254 on Sun, Dec 14, 14 at 20:23

just wondering...where did you come by the
foam sheathing?
I'm sure you know the value of the sheathing,
I never see 4" foam sheathing except in the oil field.

do understand that caulk is a better sealing method
than foam. foam will keep the layers from fitting
tightly & not make as good of a transition from
foam board to rafters.

is metal roof on solid sheeted roof?

I'd love to see pics of the foam board..hint hint.

best of luck

''Free'' is usualy the most expensive kind of any project, from the poor heartworm + mutt that follows you home, to that windfall of exotic building materials âÂÂâÂÂtoo good to throw away'' that causes a complete decor redesign because it goes with nothing you have. Beware the trap of free materials.

Is the ductwork in the attic?

The roof is on 1x4's attached to decking
im not sure on how to upload photos will try to post a few

I don't agree that you need to add supply & return
to the foam sealed attic.

Unconditioned attics were "connected" to conditioned space by building practices that didn't emphasize tightness in the structure and in the ductwork and HVAC in the attic. But what happens when you close off the roof and soffit venting to make it unvented space, then tighten the ceiling and ducts? You then have greatly reduced air exchange between the attic and the conditioned space. In summer, especially, exchange with the air-conditioned air of the conditioned space below the ceiling will reduce moisture. If there's a greatly reduced exchange, the lighter moisture-laden air in the attic will drift upwards, possibly creating mould and rot problems in the wooden structure.

Once you create a formalized air exchange to eliminate the moisture problem, inspectors may require intumescent coatings on the foam and smoke detectors and alarms in the HVAC system. Code changes will be coming in this area, says Dr. Lstiburek.

BTW, the term "semi-conditioned" space has experts such Martin Holladay wondering what that means exactly. Sounds like conditioned space on the cheap to me.

hollysprings:
Beware the trap of free materials.

Indeed!

"But what happens when you close off the roof and soffit venting to make it unvented space, then tighten the ceiling and ducts?"

you don't tighten the ceiling plane.
the whole reason foam sealed attics are so efficient
is because you are moving the air & thermal barrier
from the attic floor to the roofline.

when that happens, the many holes in the ceilings
are no longer sucking air from the attic into the
living space.

thermal bypasses( like fireplaces that you
can see into from the attic)
no longer pull attic temps into the bypass.

ductwork & equipment in the attic is no longer
in the hottest part of the house.

the average duct system has 30% leakage.
why would you need to add a duct to the
attic...there is plenty of leakage.

80% of homes already have oversized hvac
systems...why open the door to having the units
"sized" to include the attic?
many hvac companies size units by rules of
thumb...these aren't the companies who will
actually learn to size for unvented attics...they'll
just add another few tons to the house.

Joe has really gone after ASHRAE's 62.2 ventilation
strategy. and good for him! he also realizes that what
works for cold climates (on which codes are based)
does not work in hot humid climates.
see for yourself:

http://www.buildingscience.com/documents/bareports/ba-1309-ventilation-system-effectiveness-and-indoor-air-quality-impacts

http://www.buildingscience.com/documents/bareports/ba-1310-supplemental-dehumidification-warm-humid-climates

I think these are the correct links...but I can rest easy
Worthy will correct me if they aren't ! LOL!

"conditioned space on the cheap" just ...wow.

Another reason for this discrepancy in viewpoints is that colder areas and those with meaningful topography often employ basements which means there are few good reasons to locate any HVAC in the attic other than point exhaust fans and ducts. I tend to agree with energy rater that there is probably enough leakage to handle these issues when even very tight duct work is located in the attic.

Even on our slabs, we go out of our way to avoid putting ducts up there. Using deep floor trusses and sometimes soffits creates a more compact system with less areas of duct leakage. But maybe that's a bad thing.. thanks a lot Joe. And thank you worthy for bringing Joe's recent writings to light.

One more picture for clarity i wish I could post more than one picture per post