I have a regular old two story house.  The second story is unfinished and need drywall.  So, I have begun looking to insulating better.  I have a metal roof and radiant barrier.  I have recently had a quote for insulating at the roof line, converting the space to unvented attic or semi air conditioned space.  My research shows that it can be much more efficient.  I have access to every nook and cranny and it can effectively be done like a new build.  My question is that I have been told I only need about R21 at the roofline for open cell foam insulation.  This goes against everything I know to be true.  I thought I needed R38...I am in Houston, TX.  I have been told you only need R38 if you are insulating the cieling and keeping the attic vented because those temps are going to be much hotter.  Basically, with a regular insulation job, I need to insulate my air conditioned space from an attic that could be 140 degrees, but if I move my insulation to the roofline and seal that attic space, you don't need that much insulation because its not ever going to be but 110 outside at the max.

So...whats the scoop?  Do I need R38 at the roofline or R21 if I convert my attic to a sealed, unvented system?

I guess the first question is why is the attic at 140? The roof plane is radiating into it as it is being heated by the sun. Will moving the insulation to the roof plane change the amount of energy that the roof is collecting from the sun? I suspect not, in fact the roof will likely end up warmer as it no longer is being convection cooled by the venting from below.

I have no idea what the actual temperature is...I was using it as a number. The attic in a vented space is significantly hotter than the outside air temperature, which was my point and the reason for moving the insulation from the cieling to the roof line.  My roof is a metal roof, so frankly, it can get to 300 degrees for all I care as long as I can keep it out of hte attic space...which I think is teh goal here.  Thats why Im asking though.

For unvented attics insulated at the roof deck (aka "compact roof", in many building codes), they usually spell out R30 (center cavity) in places where R38 is required when insulated at the ceiling. Houston code spells out R30 min if at the ceiling (see: http://documents.publicworks.houstontx.gov/documents/divisions/planning/enforcement/2000irc_ch11amend.pdf ) when the window to wall ratio is 25% or less, and only hits R38 when the ratio is higher, or the insulation does not extend out over the eaves. (see N1102.1.1.1 Specific insulation
requirements, on p.5). R21 center-cavity at the roof deck doesn't meet code, but adding 2" of interior rigid XPS would, or adding 2x3 furring laterally to the interior of the rafters and giving it another 2.5" of spray foam would. (If the roofing wasn't already up, putting 2" of foil faced iso above the roof deck and mounting the steel on purlins/furring through-screwed to the roof deck giving it a minimum of 3/4" of air gap would let the steel get hot, but keep the roof deck cooler, but that's not in the cards, eh?)

Alternatively, putting only R21 at the roof deck and another R20-R30 at the attic floor gets you there, but you may be asked to prove it to the code inspectors. The attic air will have the same dew point as the fully conditioned space air, but the temperature will run above where you keep the conditioned space, yet still well below where it had been running. If you already have attic floor insulation, do NOT let the foam-installers insist that it be removed (and charge you for it). Even if your ducts are in the attic, having the additional R at the attic floor lowers the total load more than any duct-gains of being only partially inside the thermal envelope of the house. Even if you only have the 1980s code-min R19 batts on the attic floor, adding R21 at the roof deck would meet code from a total-R point of view. (But adding an over-blow of cellulose on top of the original insulation all the way up to the joist tops is likely g to be a cost-effective code-plus.)

BTW: The peak heat flux at the roof deck isn't about the outdoor air temp, but the direct solar gain of the roofing. With dark composite shinglesand a low-pitch roof it's easy to have roofing temps 80F+ over the ambient air temp, and roof deck temps over 160F. That's temp you're insulating against, not the outdoor air temps. The solar gain and infra-red emissivity of your roofing can be huge factor, more significant than the absolute R value of the roof, or any radiant-barrier band-aids. But it is what it is, unless you're willing to use a paint on "cool roof" finish to your metal roof. Insulating at the roof deck will raise the peak and average temp of the roof deck, but not by a large amount. The large majority of cooling of the roof is via re-radiation of the absorbed solar gain back at the sky, but also by air convection on the exterior. Attic ventilation is only a second order effect, since the attic interior is isolated by ~R1 of roof deck and roofing felt or underlayment. Over-ventilated attics (particularly power-ventilated attics) often run cooler because they're sucking cool dry air from the conditioned space, which in-turn pulls outdoor air into the house- the attic runs cooler, but at the expense of a higher loads on the AC, and lower living-space comfort.

My plan was to remove all insulation in the attic...meaning the stuff in my cieling AND the batting stuffed between the roof rafters (yes, its there for wahtever reason), and instead have spray foam on my roof decking between the rafters. What I am now most concerned with is why when talking to several installers, including the one that gave me a quote, they all insist that R21 is more than what is needed for proper insulation at the roofline. I was told I need to remove my cieling insulation because its faced batting with the vapor barrier facing the room, and that it will keep the cieling from passing air to the attic.

I will be removing my insulation myself if needed. But based on what you are saying, these installer are incorrect about how 'much' i need sprayed. So, to say im confused is a fair statement.

Plus, I ran across this late last night.

How much R-value is Enough?

In unvented attics with cathedralized insulation, we recommend 6" of foam installed to the underside of roof sheathing having an R-value of 24. This is based on the excellent research done by Department of Energy (DOE) “Build America” program. Pulte Homes has insulated entire subdivisions having unvented attics with cathedralized insulation with an R-value of 21. As a result of the incorporation of this technique, Pulte was able to guarantee the energy costs in these homes.

http://www.advancedinsulationinc.com/newconstruction/sprayfoam/default.htm

So...what gives? As for meeting code requirements, Im well outside of the city, I dont have to meet anything in regards to that, im more using the code as a guideline, but I cannot seem to find a code that addresses the type of insulation system I am needing?  Im very confused now but I want to make sure I know what Im doing here, because once its done...its done and its going to be expensive.

Leave the insulation in the ceiling, as it enhances the thermal performance of the house at low/no risk or cost. Monitor the humidity in the attic with a $8 temperature & humidity meter- if the relative humidity doesn't exceed 60% at temps of 70F+ you won't have a mold issue.

Facers oon batts are VAPOR barriers, not AIR barriers, and if it's kraft faced rather than foil it's not a particulary powerful vapor barrier (0.4 perms , as opposed to 0.01 perms) at that. "Vapor" in this parlance always refers to water-vapor, not air, and a foil or kraft facer with a bunch of slits in it is still a pretty good vapor retarder, even though it's anything but air-tight.

The odds of your ceiling being truly air-tight OR vapor tight is quite small, and if you air-seal the roof deck & soffits with foam the dew point of the attic air should track that of the conditioned space. In a Houston climate, a vented attic is just an invitation for humid outdoor air to get in and condense on surfaces that are closer to the air-conditioned temperature on super-steamy days (or on cool air-conditioning ducts in the attic on most summer days.) No matter what side the facer is on, not removing the attic floor insulation is still very low risk. But the fact that the facers had been on the conditioned space side (the ceiling below) means that you had been at higher risk of condensation on the facer itself, a risk that becomes lower when the attic is sealed rather than vented.

So, how should this insulation have been installed in the cieling? No facing at all? Facing touching the drywall or facing up towards the attic space? I have no drywall upstairs yet, so I have no condensation issues yet.

Posted By agdodge4x4 on 28 Feb 2012 12:54 PM
So, how should this insulation have been installed in the cieling? No facing at all? Facing touching the drywall or facing up towards the attic space? I have no drywall upstairs yet, so I have no condensation issues yet.

In Houston's cooling dominated climate, with batts as the only insulation the vapor-retardent facer should be on the attic side (up, facing the attic space), not the ceiling (gypsum) side.

If there are batts beween the rafters there needs to be at least a 1" (preferably 2") gap between the roof deck and the facer along with soffit-to-ridge venting, and a semi-permeable or permeable air-barrier on the side facing the attic.

The moisture drives on the gulf coast are predominantly from humid summertime outdoor air with high (70F+) dew points.  In air conditioned spaces the interior air's dew point will usually be 55F or less, since the cooling coils are colder than that, condensing moisture out of the air.  If the vapor retarder is (incorrectly) placed on the interior side and the interior is 75Fwhen the outdoor dew points are in the high-70s (as happens often in Houston) you WILL get condensation on the vapor retarder and cool edge of the insulation, and anywhere the temperature of the material is below the dew point of the outside air.  With the vapor retarder on the exterior, that moisture stays outside the fiber layer, and the moisture content within the fiber layer is the same as the fully conditioned interior space, dried by the air-conditioning.

In cold climates the moisture drives flip- wintertime outdoor dew points are very low during the winter months, but in the conditioned space the dew points are usually 38-40F, the dew point of 30-35%RH/70F air.  But attic temperatures can be much colder than that,  air leakage or vapor permeation that gets through the ceiling can end up being absorbed by the wood structures in the attic unless amply diluted by drier outdoor air from the attic ventilation. This is the primary rationale for vented attics, long mis-applied to humid cooling dominated climates.

Mind you, a square inch of air leak moves more moisture than a whole ceiling's worth of vapor diffusion through latex paint, which increases the need for attic ventilation in cold climates even if batts with facers or poly vapor barriers are used.  But with an air-tight foam-sealed roof deck the volume of air pulled up through those air leaks drops off to near-zero.

In US climate zones 3 & 4 in general it's better to have only semi-permeable vapor retarders (latex paint qualifies, at ~2-5 perms), but on both the interior & exterior sides of the assembly.  Kraft facers are semi-IMpermeable- a class-II vapor retarder, and the ideal side for the facer changes with the seasons.  But it still allows SOME amount of drying in both directions, unlike foil or poly.

See:  http://www.buildingscience.com/docu...mendations

It is interesting because my houses roof rafters had batting installed and the facing is towards the attic.  However, the insulation in the cieling has the facing on what would be the gypsum side.  I also have joists that are 12" OC.  Every person that has looked at my house that is knowledgable has made mention of the builder being 'from the north'.  And, I know for fact that that is absolutely true.  No big deal, just expensive to build for the extra lumber.  Insulation doesnt matter because Im going to have it redone as an air conditioned/semi conditioned non vented attic with spray foam.

It's pretty common for cool-climate builders to make that fundamental error on vapor retarder placement when moving south. In US zones 3 & 4 it's usually of little consequence, but in the sticky gulf coast zone-2 it's asking for trouble to put the vapor retarder on the "wrong" side (if not the sure-fire recipe for disaster that having it on the wrong side in cold zones 6 & 7 would be.)