Hello, new to forum and am completely overwhelmed with info. I am currently renovating a single-store, brick home constructed in 1966. -HVAC units (2) are located in a ground level room -3975 sq ft living area (detached garage) -duct work located in attic -split system -currently have about 5" of batt insulation on attic floor -2 gable vents and 2 turban vents -house located in San Antonio, TX Trying to figure out if I should open cell foam attic ceiling (and remove batt) aka conditioned attic space OR leave batt in place and add additional blown-in cellulose plus added insulation on duct.

It's fine to pull the batts back so that you can hit the soffits with open cell foam to seal them and flop it back in place. Don't let some foam guy tell you it's not code-legal- it totally IS, and it does improve the overall house performance. The part that isn't code legal is arithemetic like:

attic-floor-R + open cell roof-deck R= code-minimum

If insulating at the roof deck, the roof-deck R needs to hit code-min. But that doesn't mean the attic floor insulation has no beneficial effect- quite the contrary!

Don't let them tell you that 5-6" is "as much as you need" either. IRC code-min for US climate zone 2 (San Antonio's zone) is R38, which is 11" of half-pound foam. "As much as you need" in foam-guy-speak really means "...is the limit of what is cost effective on energy savings in less than 10 year when using this expensive premium product." Blowing more than 5-6" in one lift is both a fire hazard (it heats up in the first few hours as it cures), and results in poor adhesion, and uneven thicknesses, which is why the manufacturers specify a max thickness per lift. Tying up the rig & crew while they wait the requisite cooling period is expensive, which is the other reason they'll argue that "it's enough".

That said, splitting the R between attic floor and roof deck is safe, and while the dumb arithmetic isn't valid, it's still better than stripping the attic floor insulation out, especially if you only install R20 at the roof deck. If you're going to re-shingle in a few years, adding a few inches of rigid foam above the roof deck would be a better way to bump that up closer to code-min, since it thermally breaks the rafters. (2.5" of rigid polyiso up top + R20 ocSPF between rafters outperforms R38 between joists or rafters, despite a lower center-cavity R.

Turbine vents depressurize the attic, and in many cases will suck as much conditioned-space air into the attic as it does outdoor air. That cools the attic, but it raises the cooling load for the house. It's only an energy saver if the attic-floor/conditioned space ceiling plane is VERY air tight, which can be difficult to achieve as a retrofit, especially when the ducts & air handler are in the attic. With a vented attic any duct leakage increases the air infiltration for the house, and it's pretty tough to get the ducts & air handler sufficiently air tight too.

From an overall performance point of view, the ducts and air handler should be fully inside both the thermal (insulation) and pressure boundary of the house, at which point duct leakage is of much lesser consequence, and direct heating of the ducts & air handler under a hot roof add about a ton, sometimes more to the total cooling load.

Bottom line, insulate at the roof deck, and blower-door test the attic for air sealing before they break down the rig and drive away. (A theatrical smoke machine in the attic with the house pressurized by a blower door makes it easy to spot the leakage points from the exterior.)

If you have a lot more time than money, meticulously air-sealing the attic floor (and blower-door testing to find & fix all the leaks you missed) and blowing cellulose over the batts to a total batt + insulation depth of a foot gets you to code min, but doesn't do nearly as much for the ducts & air handler situation.

Dana1, thanks for responding. The open cell foam will be applied to the attic ceiling only (closing off soffit vents, gable vents and turban vents). No foam on attic floor. Installer indicated the batt had to be removed from attic floor to eliminate moisture barrier. Thus creating an envelope effect. The air handlers (I have two units) is located in a separate room on the ground level (ducts in attic). The bid to applyfoam PLUS added cost to remove batt - is getting expensive.

would it make more sense to install ridge vent, close off gable and turban vents, apply additional blown-in insulation on top of existing batt and re-insulate ducts? I am trying to obtain great results while not breaking the bank (duh!). A new roof may be in the future (2-3 years).

The installer is simply dead-wrong about the attic floor insulation creating a moisture trap /hazard. Offer to sign a waiver absolving him of the non-existant moisture hazard if he is still balking. But make sure he'll sign on to leakage testing, if he is sealing up the place, since the largest source of moisture in attics in your climate is summertime outdoor air.

If the ducts and air handler are in the attic, it's very difficult to adequately air seal both the ducts & air hander AND the attic floor sufficiently to hit the performance you'd see with the roof & gables air-sealed & insulated.

Turbine vents usually end up adding to the heating & cooling loads. They may cool off the attic, but much of that cooling is by the conditioned air that is drawn up from conditioned space when the attic is so severely depressurized. No matter which way you go for insulating, they should be decommissioned.

If the gable vents are big enough and you DON'T have soffit vents, that's fine. In your climate any form of attic venting adds more moisture to the attic than it removes. If you go with a ridge vent you MUST put in soffit vents, and the cross section of the soffit vents should exceed that of the ridge vent by at least 50%, so that the attic isn't depressurized by very much relative to the conditioned space ceiling below.

If insulating at the attic floor, find somebody with a duct-blaster to test and fix all of the duct & air handler leakage, and a blower door to find & fix all of the attic floor/conditioned-space ceiling leakage. Fixing all of the leakage at the attic floor is dead-easy on new construction, but can be a major PITA as a retrofit, especially when you have all the register boots to seal to the ceiling.

In your climate it's safe to insulate at the roof deck with cellulose blown in netting, and you can add gussets & 2x4s to make the insulation depth anything you like, not just the rafter depth. Before insulating caulk all of the rafters to the roof deck, and seal as best you can at the soffits. To be strictly code-legal to the letter of the code you would then have to add R5 above the roof deck when you re-roof:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

An inch of foil-faced polyiso and long-nailing the shingles would do it. But there is no particular problem if you used just cellulose under the roof deck, which is sufficiently protective of the roof deck that it doesn't really need the exterior foam. This is true for climate zones 1 & 2 only- in colder areas you need to follow the IRC prescriptive levels. Damp sprayed cellulose would be great, but if you want to try a DIY approach with a rental blower, that's pretty cheap too. Just be sure to search out "sulfate-free, borate only" cellulose since a roof leak into sulfated fire retardents is corrosive to metals (and it stinks.)