One question, I believe I will need around 6-8" of foam to properly insulate my roof deck. My roof deck has 2x4 rafters. My question is are there any issues with the foam completely covering the rafters on the underside of the roof deck. All of the photos, I have seen of this application show the foam stricly between the rafters, and I am guessing to achieve the 6-8" it will completely cover the rafters.

Bryan

That's actually what you want. All thermal bridges to the outside should be covered with insulation. With 8" of foam you'll have 4 to 4 1/2" foam over the rafter. The combined R value at the rafter would be about 20, ~4 for the rafter & sheathing, and ~16 for the foam.

Mac is correct. Covering the rafters is a good thing. Go for it.

I think in those pics they shave the foam off flush w/ wall framing for drywall and no other reason.

That would be correct. On roof decks they don't trim it unless it's going to be a cathedralized ceiling.

In fact, trimming half-pound foam reduces it's air-barrier function sligthtly.  The "skin" of the rough foam has far fewer open cells than the cut-foam.

BTW: In my previous post somehow I was thinking "2lb foam", and it's vapor retardency issues.  With a 1-2" air-sealer coating of half-pound foam it doesn't change the outward drying capacity very much. But depending on the roofing material itself there isn't much drying capacity (in some areas vapor-retardent roofing felt is mandatory), so it never really HURTs to design it for the dewpoint to occur within the foam, but it's not absolutely essential in every case.

Hello everyone. This is an amazing discussion. Many thanks to Dana1, dmaceld, and the other experienced and knowledgeable posters here. I live in Amarillo, Texas. We are at a relatively high elevation for Texas of 3500 feet. We have very low humidity, but we have both cold winters and hot summers. I am interested in foaming the roof decks of both of my homes here. I have obtained two quotes and recommendation from foam insulation contractors here. Both recommended removing the blown in fiberglass insulation which we performed with a rented VersaVac over the Memorial Day holiday. One recommended installing 5 inches of OC foam and the other has asked me to choose between 5 inches of OC and 2 inches of CC -- noting that he would install the OC and that the CC would be more expensive. I have been told that they would cover the 2x6 attic rafters in the homes with approximately 2in of foam regardless of whether the cavities are filled with 5in OC or 2in CC.
Should I use OC or CC?
Should I have them install more than the 5 inches of OC or 2 inches of CC?
Do I need to install some type of vapor barrier, such as a barrier paint on the undersurface, if the OC is used?
Thank you in advance or any answers that you provide -- you guys are amazing.
John

Here's the answer I gave on this same question in the ICF forum 4 days ago in the thread about insulating truss space.

Posted By newbiejohn on 05/25/2009 10:03 AM

OPEN cell or CLOSED cell and why...

What type of RValue should I be striving for in this install with a ICF home?

Open or closed - you will not get a consensus on that issue. If you haven't already read the thread in the general forum, residential, on spray foam on the underside of the roof. It's 5 pages long with 95 responses!!! You'll get a lot of good explanations there as to the benefits and drawbacks of both.

R value gets a lot of differing opinions also. I say, at a minimum go with what the Energy Star requirement is for your area. That could be upwards of R 40 or more. You may limited by what the contractor can do in one or two passes. In my case the contractor was able to apply ~ 10" of Icynene in one pass, but not without problems. That gives me about an R 40. Keep in mind that the true R value at the end of the job will be an estimate because the surface of the foam is about as flat as the foam on a latte.

In hot-dry climates like the TX panhandle it almost doesn't matter as long as you don't go out of your way to create a vapor-trap with two vapor retarders (eg. CC foam + foil or vinyl wallpaper). The drying capacity in either direction is pretty good, and saturation of structural wood w/mold complications are relatively rare as long as you have drying-capacity in either direction. Air sealing is by far a higher priority in a mixed-dry climate than vapor permeability control, and allowing both exterior & exterior side drying is useful.

Short answer- don't put any interior vapor retarders up. Longer answers, See:

http://www.buildingscienceconsulting.com/designsthatwork/hotdry/index.html

http://www.buildingscienceconsulting.com/designsthatwork/hotdry/bestpractices.htm

Yes it's cold, at 3500' in TX, but it's not Saskatchewan-cold, where proper use of vapor retarders can make-or-break it. Average daily highs in Amarillo still break freezing by quite a bit through the winter- even the average daily temps are still above freezing, even if cold-snaps can go for days at much colder temps:

http://www.wunderground.com/NORMS/DisplayNORMS.asp?AirportCode=KAMA&SafeCityName=Amarillo&StateCode=TX&Units=none&IATA=AMA

Compare the average winter temps with someplace that most would consider actually cold like Minot ND:

http://www.wunderground.com/NORMS/DisplayNORMS.asp?AirportCode=KMOT&SafeCityName=Minot&StateCode=ND&Units=none&IATA=MOT

With vapor-control/condensation issues the averages are far more important than an acute winter day or week. In Minot (as in most of Canada) proper placement of vapor-retardent materials like CC foam, and the importance of interior-side vapor retarders is greater than in the high-dry not really THAT cold TX panhandle.

Bottom line, half-pound (OC) foam is probably the better value in your place. As for how much...

http://www.ornl.gov/~roofs/Zip/ZipHome.html

Plug in the first 3 digits of your zip code. I put in 791 for Amarillo and got R49 as the recommended R-value for attics in new construction. 5" of foam is only good for about 1/3 of that. For retrofit on existing construction with a currently installed R19 it's recommended that it be bumped to R38, which would be ~10-11" of half-pound foam. Should you have left the blown FG in place? Maybe... (too late now. :-( ) If 10" of foam is affordable, go there. If not, 5" of foam on the rafters and ~10" of cellulose on the attic floor will get you closer to the recommended R49-ish.

I've never quite understood the willingness of foam contractors to recommend removing older insulation from places that won't affect the installation of their material. This is not a unique story by any means. The "...because you don't need it anymore..." rationale just doesn't have any science to support it. R-value controls the rate of heat transfer- more is better. Only in areas where condensation scenarios are likely when there is a split thermal envelope (rafters + attic floor) with air leaks from the interior to the mid-point (attic interior) would that be of any concern, but that's not in Amarillo (and not a huge or insurmountable problem in most other places either.) To be sure spraying onto a clean dry fiber-free surface gives reliable results, but that would only require pulling the old stuff back 18-24" from the edges (to be replaced after they're done) not vacuuming out the whole space.

Just to add...

The DOE generic recommended cutoff heating season heating degree-days (HDD) for putting up some sort of vapor retarder is 4000 HDD, and Amarillo averages 4258 HDD. Being drier than many areas in the US with 4000HDD climates you can probably skip it, but if you're at all concerned, vapor-retardent paint would more than suffice. The 4000HDD is a convenient round number chosen to be on the safe side for any US region. Local & regional humidity conditions can skew the picture quite a bit- I'm pretty sure you're "safe" without it, whereas you might consider vapor retarders more seriously in more humid areas like coastal VA which has a similar annual HDD numbers but 2x the annual rainfall of Amarillo.

http://www.climate-zone.com/climate/united-states/texas/amarillo/

http://www.climate-zone.com/climate/united-states/virginia/wallops-island/

I’m in the midst of constructing an energy efficient home in the Chicago area (~6500 HDD). The foundation (full basement) and above-grade walls are ICF with triple glass windows. To reduce air infiltration, the ceiling of this ranch style house is constructed with 11-7/8” deep I-joists with a perimeter rim board and sheathed with 5/8” OSB which serves as an unfinished attic floor. The rafters rest on a 2x4 plate over the floor sheathing at the perimeter. Steel straps tie everything to the concrete walls.

I’ve read with interest this thread concerning spray foam under-roof insulation hoping to confirm the viability of my plans to insulate the ceiling of my house. The attic is ventilated, unconditioned space to be used for storage. There are no cathedral or pan ceilings anywhere in the house.

I have insulated the ceiling from below with about 3” of 3 lb polyurethane closed cell insulation (R7/inch). The foam seals the entire ceiling and the ceiling to wall connection. There are only a few penetrations of this layer for plumbing stacks, a range hood duct, and a whole house fan with automatic, insulated doors with gaskets. All electrical wiring and fixtures are installed below the foam layer to virtually eliminate air leakage to the attic. I’ve filled the remaining joist space below the poly-foam with unfaced fiberglass insulation (R25) to bring the total ceiling R-value to about R50. The ceiling will be finished with ½” thick painted gypsum board.

I’ve used the WUFI heat and moisture flow modeling software to determine the risk of condensation forming on the interior surface of the foam for the case of a cold Chicago winter with what I think are satisfactory results (minor amount of condensation which dries mainly to the interior after several weeks.) By keeping the interior humidity low during the coldest winter months (HRV ventilation w/humidistat), I believe that no condensation will occur. Adding a vapor barrier paint to the drywall (perm = 1.0) also gives good results from the simulator, even after a two year simulation period.

Any reaction to my overall approach? Will it work? Should I use a vapor barrier paint throughout or, possibly, just in high humidity rooms (baths)?

If I understand this correctly, the ceiling has about R21 of foam on the cold side, and R25 of fiberglass on the warm side-in winter (a 45%/65% ratio)/ This dries-toward interior stackup is less than ideal, but at a ratio that can work in Chicago's climate, but not much colder. I suspect your model has it right.

Key to keeping the attic condensation/infiltration under control is careful treatment of the "... few penetrations of this layer for plumbing stacks, a range hood duct, and a whole house fan with automatic, insulated doors with gaskets". Air leakage at penetrations can pump a lot of moisture into the attic (as well as draw a whole lot of heat out of the house.) It should be possible to limit sustained condensation conditions within the fiberglass in check by keeping the wintertime interior RH to under 35% (which you would be doing to reduce glazing condensation anyway.)

Avoid vapor retardent paints on the ceilings or you'll create a vapor trap. Putting vapor retardent paint beween the interior and the ICF walls isn't necessary and may even cause paint-peeling & other issues trapping moisture in the wall. The concrete is a large hygric buffer, and EPS is moisture tolerant, so it doesn't much matter if there's vapor-leakage through the gypsum- the dew point will occur in the highly moisture-tolerant concrete or EPS. Controlling the ventilation in high-humidity areas with a humidistat would keep mold from gaining hold on any furring strips or other cellulosic materials in the walls/ceilings.

I'd question the utility of a whole-house fan here. You have a well insulated high-mass structure which will moderate sensible-load interior temps considerably, as long as you keep the windows closed and manage the solar gains through glazing well. Nighttime ventilation in humid areas can add considerable latent-load to the structure, and the latent-loads may be the BULK of your AC load most of the summer. Some amount of compressor-based conditioning of the interior will likely be necessary to keep the indoor humidity under 60%RH, and the savings from nighttime ventilation with the whole house fan modest (and further offset by wintertime infiltration losses, since most leak air, some worse than others.) Summer is the high-humidity season: May-September are the highest rainfall months in Chicago, and the average RH is over 65% June-August when the average temps are over 68F. (See: http://www.climate-zone.com/climate/united-states/illinois/chicago/ ) You might consider an ERV instead of HRV to manage the latent-loads, along with a dehumidifier or small AC unit controlled with a humidistat. Whole house fans & nighttime ventilation schemes tend to work best for drier cooling-season climates with reliably cool nighttime temps (like the western slope of the Rockies.) They can do OK with sensible loads, but can increase latent loads considerably for both diminished comfort and lesser-than-advertised power savings.

Tyvek attic wrap system serves as a radiant barrier and turns an attic into semi-conditioned space - the attic becomes sealed and air flows from the soffits to the ridge vent through the rafters. So you get lower shingle temperatures, lower attic temperatures (warmer in winter), no ice dams. All without expensive spray in insulation.

Has anyone used it?

Thanks, Dana, for the critique of my ceiling insulation plan. It confirms my own concerns about creating a vapor trap if a vapor retardant paint is applied to the drywall. I also concur with your assessment of the utility of a wholehouse fan. It has become increasingly clear that it will rarely be used. Since it has already been purchased and installed, it will be there on those occasions when it may be useful. The use of an ERV instead of an HRV is not clear to me, however. Both provide necessary ventilation for healthful living, but the HRV is critical for reducing interior humidity levels in winter. In summer, I plan to use a small central AC unit (1-1/2 ton) and an AHU with a variable (low) speed blower to wring out excess water vapor from the conditioned air space. I understand that an ERV will minimize the need to do so, but doesn't the benefit of an HRV in winter exceed that of an ERV in summer? Most of what I've read puts Chicago in a geographical region calling for an HRV rather than an ERV.

An ERV returns nothing like 100% of the moisture in winter- it's more like half. The ventilation system will stilll have a significant drying effect in winter. The degree of summertime benefit to an ERV in Chicago climate may be somewhat marginal, but the fact is, summer weather there is FAR more humid than winter weather, which is why the whole-house fan use is less than ideal :

http://www.climate-zone.com/climate/united-states/illinois/chicago/

The monthly rainfall numbers, combined with the monthly average temp numbers tell the relative-humidity story: July & August are two of the highest precipitation months, and THE highest cooling-degree-day months. January & February get significantly less than half the precipitation of July & August. The winter dryness of the incoming ERV air will overcome it's humidity-scavenging capacity-

I doubt your dehumidifier would run much (or ever) in winter, even with an ERV instead of an HRV. But it'll run a LOT in summer if you regularly use a night-time whole house fan cooling strategy instead of AC.

Still you have less than 1000 cooling degree-days, an ERV won't deliver the same energy savings as an assist to mechanical dehumidification that you see in hotter, more humid environments. It's definitely not critical.

OK you have confused me. As I understand it, a supertight house conserves humidity as well as heat. So why would you want an ERV putting even 50 percent of the moisture back in a 1600 SF home in the winter? This is particularly puzzling when the goal in the bath or kitchen is to handle high humidity events like showers or boiling potatoes.

South central pa is right on the dividing line between ERV and HRV. But I want the windows open at every opportunity, or what's the point of living above a babbling brook? In the winter, it is easier to add humidity than substract it, particularly if you have a wood stove going. Or so it seems to me.

The key here is how dry is dry when an HRV is making .35 air changes per hour. Anyone have any experience?

This past winter, before I installed the HRV, the humidity in my well-sealed and insulated 2200 sf ranch w/full basement reached 60%. High enough to cause condensation on the triple glass windows! When the HRV was connected up and running continuously at low speed, the humidity dropped to about 40% with no appreciable increase in natural gas consumption. By contrast, my conventional construction home not far away, built in 1991, had interior RH in the vicinity of 20% (no supplemental humidification.) A well-sealed house does indeed trap moisture which makes mechanical ventilation mandatory. The question is: what will happen when we move in and really start producing moisture -- cooking, showers, etc. I assume that I would be able to maintain the indoor relative humidity to 35% in January, as Dana1 recommends, using an ERV, but how much heat will be lost vs. using an HRV? How do the increased heating costs compare with the cooling cost savings obtained in the summer using an ERV? Do the HRV/ERV recommended zones shift appreciably when applied to super-insulated homes?

Thanks, Clark, If an HRV didn't reduce humidity excessively in Illinois, I wouldn't expect a problem in wetter and warmer Penna. As I noted before, we'll get fresh air the old fashioned way in the summer. I believe it was Thoreau who said that life is not to measured by R values and air changes per hour, but rather to be enjoyed by listening to the birds sing and smelling the honeysuckle

The primary difference between an ERV & HRV is the dessicant wheel. While this adds to the load of the blower it adding a few percent to the electricity used by the device (all else being equal), it's efficiency in terms of recapturing energy is similar. You'd be trading somewhat lower efficiency in winter (by having to run it longer to stay under 35%RH) for somewhat higher efficiency in summer (by reducing the latent load without compressor based dehumidification.) It may take a fairly sophisticated model to tell EXACTLY where the crossover based on very local weather data. I was looking at Chicago as being drier in winter and more humid in summer as possibly tipping the balance in favor of an ERV despite the sub-1000 CDD climate.

Had R values & ACH parameters even been invented in Thoreau's time? ;-) I like tossing open the windows from time to time too, but even in my central MA climate/neighborhood the hours where that makes it more comfortable are relatively few (and yes, the dehumidifier cycles run longer when I do.) With moderate solar-gain management the number of hours I run the AC each year is fewer than 100.

I believe it was Buddha who said Seek not nirvana on the Psychrometric Chart.

Sprayed Icynene foam can offer real advantages.  I have been in an attic on Jax beach in summer with the bottom of the roof deck sprayed with Icynene.  It was just a few degrees warmer than outside.