So you've put down an attic floor, it's "staying down" and now you want to insulate to R30 but not under the roof sheathing.

Assuming you want to use the flooring for storage, your only choice is under the flooring you just put in. So pull up the flooring and fill up the space. But even the highest R Value fg batts will only give you a nominal R28.5 if installed perfectly.

You don't mention your climate. But R30 is only considered adequate, according to Energy Star, in the deep south and southern California.

Spray foam, if used, is applied on the attic ceiling surface, not under the flooring.

The quote we received initially for foam was for ceiling installation not roofline. All the insulation contractors I have talked to agree that it would be wasteful to foam the roofline after seeing our floorplan. We are zone 4 in central NC. Shouldn't we be able to achieve at least R-38 with foam with an available 10" in the ceiling joist?

Just to clarify, we are just finishing framing, so the ceiling drywall in not in yet. We have to do rough ins then insulation before drywall, so I was thinking we could just install insulation from the bottom up. Is this possible?

Foaming the floors does seem like a waste.

When it comes to payback calculators there are a lot of variables. I know your climate, I can't remember your heating source. For NG - just do the batts.

I know you don't want to rip the floors up but what are you planning on doing up there? Finishing someday? Is your ductwork and airhandler up there?

I have some area that is floored and I have R-38 batts. In my post construction retrofitting, I have put an inch of rigid foam down on top.

While r-30 is the minimum, as batts with all the framing, it is pretty bad. But not $500 extra a year bad. Again it depends on your heating source.

Ten-inch ceiling joists are not typical. So, yes, you have plenty of room to spray in closed cell spray foam (ccspf)--which has R Values up to 6.5 per inch--more than sufficient to reach R38.

Putting down an attic floor before you insulated limited your options.

However, spraying 6" ccspf to reach the desired insulation level will leave you with a gap between the bottom of the insulation and the drywall. Using other types of insulation, this would be a mistake that degrades thermal efficiency. Since ccspf should be airtight, it just creates some extra cubic feet of conditioned space. (And costs a heck of a lot more than other methods would have had you followed the usual building order.)

This post was edited by worthy on Sat, Dec 22, 12 at 9:12

Wrong order of operations. In your case, I'd do a closed attic instead of a ventilated one. It will let the attic be in "conditioned" space.

I'd do a closed attic instead of a ventilated one

Since it's framed like a residential floor with 2x10s rather than 2x8s or less and has flooring, the OP is half-way there.

My favourite rooms in my 19th Century homes were the attics, quiet and far away from the madding crowds below.

Attic Retreat

Yeah, make the attic conditioned space by placing the foaminsulation against the roof decking. It will be more energy efficient and easier at this stage. And probably not cost a dime extra if you're already talking about doing any kind of foam.

As folks have already said, you have two choices:

1. Pull up the attic floor sheathing and install loose fill or dense-pack insulation there in order to have a non-insulated attic. Will low temperatures matter to whatever you plan to do in the attic?

2. Insulate the roof by adding insulation (batts or spray) on the underside of the roof sheathing and add rigid insulation to the top of the roof sheathing if you desire added insulation value. This will give you an insulated attic and allow you to have normal living activities there, including storage without worry of low temps in the attic if you are in a cold zone.

If all your ducting and plumbing lines are under your first floor slab, these will not be an issue for however you decide to insulate. But if you have lines in the attic you will need a strategy for them, depending on your choice of attic insulation.

By the way, I believe you may get greater insulation values by using dense pack celluose rather than fiberglass batts, if you go in that direction. It takes a trained professional and special machine to install properly.

Good luck with you project.

so even though the walls & ceilings are not
sheetrocked the attic is decked?
interesting.

normally, when insulating attic floor with foam
the foam is sprayed against back side of
the sheetrock ceilings.
since your attic floor is decked, spraying against the
decking from inside the hosue would be how you'd
do it.
so to do that..first the perimeter of the ceiling
would be sealed. you want to seal the top plates,
wire, pipe penetrations. then fill the ceiling joist
bay with foam.
to get the full depth of
the joist you'd over fill the joist bay, then
trim off excess prior to installing sheetrock
ceiling.

this would be option three. and the one I'd chose
recommend.

everyone...house is North Carolina.
ducts in crawlspace, so equipment is either
in closet inside living space, or in crawlspace.

if the roof is high, and not used for ducts
then it is more cost effective to seal
& insulate attic floor.

OP, check into what foam is used in your area.
we use open cell in hot humid climates.
are you sure the ceiling joists are 2x10?
if so, that gives you 9.5" for insulation
with open cell..that would be about R-36
depending upon R-value per inch of product.

shop foam insulators wisely. you want a full
joist bay fill, not an 'average' fill.
what they mean by 'average' is that if you
add up low spots & high spots of foam it
'averages' out to xxx. you want full depth
everywhere. excess is cut off so that sheetrock
install is smooth.
ask to see jobs of the foam companies in progress.
ask to talk to former customers.

once completed..blower door test the install
to make sure it has stopped air movement between
attic space above into living space below.

I gotta ask....why floor the attic so early
in process?
and where are heating system & water heaters?
tell us locations, what size & what fuel type please.
how will attic above foamed floor of attic be vented?
do you have recessed lights? what provision
has been made for recessed lights & insulation plan?
are recessed lights Insulation Contact Air Tight
ICAT??

knowing the answers to these questions may
change answers to previous questions.

best of luck

Our ceiling, with attic over, will be blown in dense pack cellulose insulation. In fact the guy was installing today. We had the drywall installed on the ceiling and the insulators drilled 3" holes in the attic deck and blew it in. Our attic turned out big so we could finish off for living space but do not plan this until down the road. Then we will insulate the rafters.
This is the same plan for the garage ceiling as we have unfinished space over it as well. The guy insulating said this is common to do, also the building inspector agreed with this altho installation varies with installers. The building inpector was very impressed with the degree of care in the blown in install. So if you go this route get references.

Nothing beats doing a retrofit job on a brand new house!

LOL. Tell it like it is Worthy!

wonder what OP decided to do.

with cellulose you want to make sure you
seal EVERY crack between attic and living space.
the fine particles of borate treated newspaper
really get into living space.
hopefully recessed cans are ICAT, not just IC.
that the cut in the sheetrock is sealed not
just at recessed light penetrations, but hvac
supply boxes, returns, bath vent fans, stove
vents, electrical and plumbing penetrations.

lots of folks with cellulose in attic complain
of dust..not realizing what they are breathing
in every day.

frogs (family room over garage) and attic
space that shares walls with extreme attic
temps are always a problem to heat & cool.
this is one of the advantages of foam insulating
the roofline for unvented attic. if not then
foam/foil sheathing attached to attic side of the
shared walls is needed. the sheathing should
go from top plate to bottom plate & then between
joists/truss to sheetrock of ceiling below.
this are the two ways to escape the heat/cold
gain of rooms with extreme attic temps on sides,
top and bottom of these rooms.

best of luck.

Just thought I would update you all...
First, we decided to use foam for the ceiling and stick with sprayed in fiberglass in the walls, and that dropped our HVAC load by 1 ton and saved some $$. So we then had the manual j calcs re-done if we foamed the ceiling and exterior walls which dropped the HVAC load another ton, saving more $$. We plan to foam the ceiling and exterior walls. So the extra money spent in foam insulation is close to the $7-8000 we saved on our geothermal HVAC by dropping from 6 tons to 4 tons.

Now I don't need airtight can lights, and the house should be more airtight in general. I think we will be very happy with this decision in the long run.

You can see pics of our progress on the January building thread.

Thanks for all of your input!

did you install the foam as I recommended?
if not the attic will not be air tight
at eaves.
could you post a like to the thread you mention?

best of luck

Yes, perimeter will be sealed well. Walls are 2x6 and wil be filled a minimum of 3 Inches, not average fill. Ceiling will be 6" minimum fill. The performance value of the icynene foam is equivalent to r38 in ceiling and r19 in walls. Granted some spots will be thicker than the minimum fill so actual performance may be a bit higher. This is sufficient for us to get a sizable rebate from our electrical company at completion.