I did research on various topics and came up with three solutions for attic insulation and ventilation. I chose and purchased a Solar Attic Ventilator and have been looking at Radiant Barrier (for the heat) and Spray Foam Insulation under the rafters. I have gotten quite confused with conflicting advice. Can we/should we use spray foam with the attic ventilator? I would like to recover the attic space and am told that we can remove/eliminate the blown in insulation with the spray foam. Does the radiant barrier only work with blown in insulation or does it work with spray foam. Seems overkill but our attic is an oven and I hate to add more blown-insulation. Very confused as to how to coordinate any or all of these options and if we should seal the vents as suggested by some spray foam folks. Thanks!!!

I assume your home is in Georgia.  If so, then my guess is that the consensus of opinions on this forum will be that you should use spray foam between the rafters and seal the vents in the attic.  The debate will probably be between using open or closed cell foam.  I prefer to use closed cell for the higher R-value per inch and the added strength but there is some risk that it may hide future leaks in the roof.

A lot of topic touched on here...

At best you're saving ~5-6% in AC bills with the solar attic ventilator in your current situation unless you have insulation values lower than R19:

http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/

If you use spray foam in the rafters, decomission the attic fan and seal all soffit/ridge venting or it'll be a waste of money. Radiant-barrier still "works" with spray foam & EPS, etc, but it's relative value shrinks dramatically with the total R-value. Whether it's worth anything to YOU depends on the total R-value (and to some extent, the reflectivty & emmissivity of your roof. If you have a "cool roof" type of material, it buys you exactly NOTHING!) But it's not cost-effective in most of the US if your total r-value is R10+:

http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table4 (Figure your R-value and look for the nearest-equivalent climate city. The number in the box is the max per square foot you should pay to get a 25-year break-even on cost.)

There's no need to REMOVE attic floor insulation, unless by "I would like to recover the attic space..." you mean you need it to run at exactly the same temperature & humidity as the rest of the house. If it's used primarily as storage, spraying the rafters and leaving the blown increases the total R-value (a good thing!), but leaves the attic as semi-conditioned space- it'll run at temperatures somewhere between the conditioned space below, and the roof temperature (but nowhere NEAR what it had been.) The exact attic temp depends on the ratio of the R-value of the foam to the R-value of the blown insulation, the thermal mass of what's in the attic, etc. but it'll be many tens of degrees cooler than the blistering temps you get in an uninsulated roof deck.

It's much cheaper to add R-value to the attic floor. That won't raise the attic temperature even one degree, but it'll lowers the heat-flux through the attic floor to the rooms below. Whether that's cost-effective by itselve see:

http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table6

But note, best-case your "average effective-R-value" of radiant barrier is about R5/R6, and may not hit even half that during the heating season. Adding even R5 (1.5") of blown cellulose is comparable in both effectiveness (sometimes cheaper), but works year-round. Radiant barrier is only a GREAT deal when you have low R-values & no space for additional fiber insulation. It's cost is fixed, but it only reduces a fixed-fraction of the heat flux. At R30+ the heat flux is already quite low, and the "average effective R-value" of radiant barrier is disappearing into the test-data noise (it's value primarily theoretical & inferred.)

In most places in the lower 48 adding R19+ of blown cellulose is cost-effective as a DIY project even if your existing R-value is R25, since R19 of the material runs $0.25-0.30/ft^2 even at box-stores, and they throw in the blower-rental for free. In areas that subsidize it, going even higher works. R-values over R50 are usually pretty pointless in the south though- the heat losses/gains through walls & infiltration tend to be far higher than the roof once you're over R30-ish (but R40+ is worthwhile if you get a kickback.) 5.5" of half-pound foam between 2x6" rafters is only about R19-ish, so removing the blown insulation isn't really recommended. If you can get a 10-12" of foam up there, your attic will probably run near the conditioned-space temp whether you remove the blown stuff or not. But that much foam can be a pricey proposition.

As an addendum on radiant barrier, this is a bit dated (the details of the whys/wherefores are better known now), but still pretty accurate:

http://www.fsec.ucf.edu/en/publications/pdf/FSEC-DN-6-86.pdf

Study figure 3 carefully- this is an R19 fiberglass test with/without radiant barrier. Note that the temperature of the attic is only reduced ~6F with the addition of radiant barrier. But the temperature of the insulation 1" below the top of the fiberglass drops by over 10F degrees- it went from being HIGHER than the attic temp to several degrees LOWER. This is a known deficiency of fiberglass: It's somewhat translucent to the radiated heat, absorbing the heat & heating up in internal layers, which are insulated from the attic ambient. The translucency of cellulose is very low, and while the top layer of the insulation is also higher than attic ambient on hot summer days, it doesn't exhibit the same non-linearities of fiberglass. IIRC EPS passes some infra-red heat too, I'm not sure about spray polyurethane or spray polyiso.

But look at the map in Figure 5 for confirmation of whether it's even remotely worthwhile for you. Then consider that it's based on attic R-values of R19 or less- if your total insulation is higher those lines move south. Independent of insulation type the total performance is similar, only marginally different for fiberglass. (The wintertime performance of foams & cellulose in attics can be significantly better than low-density fiberglass though due to much lower convection internal to the insulation.)

I concur on the attic ventilation comments above. Spray foam is ok in a vented attic at the ceiling level. It is BEST at the roof level in an unvented attic. Our performance guaranteed specs call for 6" of open cell (which is R-23+) covering the rafters to eliminate bridging through the 2 x 6 dimensional lumber. That will give you a worst-case performance of +/- 10% temp variance from your interior temperature. Let me know if we can help.

Scott V.
[email protected]

What he said. If you do the roof level close off your vents. If done proper the attic should stay close to temp in the main house

I haven't seen any comments on the Tyvek Attic Wrap system - it's much more than a radiant barrier - it creates a ventilation duct between the rafters.

Any difference in insulation or other side temperature of the main roof surface as compared to the eves can lead to ice damns. That's why my attic is staying vented.

Ice dams are usually a heat leak problem (either from air migration or too low an R-value.)  The solution isn't always to ventilate- the leaking heat away before it can melt the snow- it's better to stop the heat leak.  (Ice dams themselves can be a good way to figure out where you have a heat-leak problem.)  Where possible, the better solution is to air-seal and insulate.

Ridge venting is usually blocked by snowpack when there's any decent amount of snow, so soffit/ridge venting only slows ice dams from forming when there 's scant snow to melt & freeze in the first place.  Ventilated roof decks can promote drying of condensation or leaked water (which is good) but aren't great insurance against ice-dams.  The air-gap of the Tyvek system probably adds up to an effective R-6, with VERY low air flow to keep the deck below freezing from heat leaking up from the house. (But the same miniscule air flow adds a path for waremer outdoor air to heat up and melt the snowpack from below on freeze/thaw days.)

The roof/attic R-value required to stop ice dams from forming depends on the anticipated snow load.  Snow behaves as insulation, so the R-value of the insulation relative to that of the snow needs to ensure that the temperature of the roof deck stays below freezing over the average daily temperature.

On southern aspect eaves there can be solar heated air rising up the wall to collect under the eaves melting the snow from below only to freeze later sometimes creating an ice dam.  In combination with heat-leaks up in insufficiently insulated exterior studwall this too can sometimes create ice dams where otherwise the risk would be low.  If the wall is well insulated southern wall solar eave-heating usually just causes a melt-gap between the roofing and the snow, with no ice dam.

Sometimes wind-washed ridges thin out the snow layer causing a solar-heating & melting of the upper portion of the roof, but that usually results in a thin sheet of re-frozen water under the snowpack, rarely and ice-dam proper.  But insulating even a small above the roof deck, including over the eaves (important to avoid localized re-freezing right at the exterior wall boundary rather than an extended sheet) will mitigate that too.  Typically 2" of EPS or XPS above the roof deck (even over the eaves) is sufficient to all but guarantee ice dams won't form  in my climate (New England ~7000HDD, up to 2' of rooftop snowpack typical, rarely more.).  Usually just an inch of XPS or ISO evens the roof temps out enough to do the trick.  Clearly YMMV.

Building Science Corp has some generic R-value recommendations for treating ice damming problems both with/without attic/roof-deck ventilation:

http://www.buildingscience.com/documents/digests/bsd-135-ice-dams/?full_view=1

Note they're only talking about an additional R-5 for the unvented vs. vented roof deck.  That's 1" of XPS. (I'm a bit surprised they didn't mention the insulated roof-deck over the eaves approach. Several roofing contractors in my area have been doing that for years with excellent results.)

> it's better to stop the heat leak

Yes, but nobody has come up with perfect insulation yet. Stop as much heat as possible and then vent to remove what will get through.

> On southern aspect eaves there can be solar heated air rising up the wall to collect under the eaves melting the snow from below only to freeze later sometimes creating an ice dam

Did you make this up? Warm eaves and cold roof is exactly opposite of the problem.

> only talking about an additional R-5

I read R10.

Stratification (warmer air rises towards the ridge) also adds to the warm roof, cold eaves problem and again, ventilation helps.

Sounds like unvented requires more insulation and more expensive insulation to achieve the same performance - at least in a cool, no hurricane climate.

>Did you make this up? Warm eaves and cold roof is exactly opposite of the problem.

Yes, I made it up- how could you tell? :-)

Warm eaves next to the wall from the solar heated air column next to the wall with colder eaves 6-12" out is an ice dam starter. Add a small heat/infiltration leak at the top of the wall and it just keeps building from there. (Spot-blown cellulose cavity insulation can slow the wall heat leaks down a lot if you can identify where that has been a contributing factor.)

Tyvek radiant-barrier might add up to R10- effective for the cooling season, but never during the heating season. The extra space coulda been filled with something better (like half-pound foam) that works year-round.

It just takes more (not more perfect or more-expensive) insulation (and decent air-sealing) to achieve the performance necessary to avoid ice dams. Retrofit fixes using ventilation increases energy use but are cheap to implement. Retrofit fixes using insulation runs quite a range from cheap to outrageous, but reduces energy use.

When you're at the point of re-roofing, sheet-goods over the roof deck isn't a dramatic cost adder, and IS a reliable fix. If roofing isn't going to happen for awhile, a couple inches of half-pound foam between the rafters as both air-seal & insulation isn't outrageous either, and usually does the trick (while adding ~R7 or so to the total insulation level.) (But I guess that's still quite a bit more expensive than opening up more ventilation to suck the heat out faster as a solution. :-) In many retrofit cases simply air-sealing the attic floor better and adding a foot of cellulose over the 2" of vermiculite & mis-applied R11 batting or whatever under-rated insulation they started with is first line of attack. Boosting the R-value to something over R40 by any method seems like a better way to go, and usually works.

Years ago in Arizona I had an attic ventilatin issue, it was so hot that the fan wouldn't last a year. Having only one small fan for an 1800 sq. ft. attic might have been part of the problem. After replacing the fan twice, I reconsidered having a fan suck air out of the attic, and instead installed a 24 inch "whole house" fan in the ceiling of our garage, using the same thermostat control of the old suction type fan. That sent air at considerable flow into the attic, and helped cool the garage as well. Don't forget to add vents at the garage doors, or somewhere, to allow air to get into the garage....