Posted By jaysuede on 15 Aug 2010 12:28 PM
I am looking for some feedback on radiant barrier insulation vs spray foam insulation. Living in the atlanta, Georgia heat this summer has led to some very high GA power bills the past few months. I currently have r-30 standard insulation installed by Woodman in February 2009. There are (2) 80% efficient builder grade Trion furnaces in the attic with Trane 13-sear units outside. These 2 Units power the 2nd floor, with a 3rd furnace in the basement powering 1st floor. Basement is unfinished. Last 2 months bills have averaged $550 with 3 units chilling approx 4700sf of conditioned space, with programmable Honeywell thermostats running 78degrees during day, 76 at night. I am on a mission to reduce my monetary contributions to GA power.
I have had quotes for both spray foam and radiant barrier, as I am looking to take advantage of the tax credit up to $1500/30% off materials up to $5000, with ultimate goal of saving 20-30% monthly on my power bills.
The radiant barrier company quotes are pushing radiant barrier as being as equally efficient as Spray foam Insulation saying it will reduce your bills by 20%+ per month. A few questions below:
1) Spray foam insulation...(1) HVAC company told me having 80% efficient units were not compatible with Spray Foam Insulation?
2) E-Shield Radiant barrier, has a radiant barrier on both sides with inulation in center with r-value of 11. This will bring r-value in attic from 30 to 41. Is the e-shield barrier with r-value insulation any better than the standard radiant barrier?
3) Several companies have told me that radiant barrier alone does not qualify for a 30% tax credit? You must combine this with adding more r-value insulation to your attic?
4) Depot offers radiant barrier for $1.44sf installed = total SF of roof x pitch of roof = total SF. Is this market rate for standard radiant barrier?
5) Does radiant barrier alone with adding additional r-value insulation reduce power bills by 20%?
6) Another company recomends standard radiant barrier, combined with additional blown insulation for best energy efficiency?
Trying to evaluate the investment of radiant barrier vs spray foam insulation, the tax credits involved, to determine the amount of months til payback occurs with reduced savings on GA power bills. Appreciate all feedback.
In order:
1: The efficiency of the furnace has no direct bearing on the insulation type, but there are alwasy issues with clearances between the exhaust stack and insulation that must be addressed. Make your HVAC folks explain in detail what their perceived issues are.
2: R14 + radiant barrier is far better than radiant barrier alone. But evaluate it in terms of dollars per square foot per unit R value.
3: I'm don't know if/where/now radiant barriers are subsidized (or not) near you.
4: I don't know what the market rate for installing radiant barrier is, but the VALUE of it will vary by how much insulation you already have, and the length/peak of your cooling season. The Oak Ridge Nat'l labs has a set of tables for a present-value per square foot (the most you should pay in order to break even within 25 years) for radiant barrier, adjusted for climate and existing R value:
http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html
According to their financial analysis, if the RB is placed on the attic floor, in Atlanta it's worth no more than 5 cents per square foot of attic floor space if you have R30 insulation. If placed on the underside of the rafters it could be worth as much as 8 cents/foot (of floor area, not roof area.) See tables 3 & 4 on that web page. Methinks the payback @ $1.44/foot of roof area will have a payback measured in centuries, even assuming ridiculously (unrealistically) low discount rates. The ORNL estimates of RB cost is only ~1/3 that at the HIGH end, but that could be 20 year old price data. But even if you triple or quadruple the present-value numbers to update for higher future energy pricing it's still well shy of $1.44/ft
25: Radiant barrier & attic insulation alone or in combination are unlikely to achieve a 20% reduction in your power bills. (more on why later) If spray foam is used, and done in such a way as to achieve a near-perfect air seal, maybe... Radiant barrier has NEVER lowered the total cooling bill of ANY real-world house by 20% all by itself, but it might approach that in a completely uninsulated but air-tight one-story slab-on-grade structure with a low roof pitch. Roof pitch higher than 2:12, 2 story with a full basement & R30 batts in the attic, fuggedaboudit.
6: These folks may be on the right track. If the existing insulation is R30 fiberglass between the attic floor joists, even a 3" overblow of cellulose on the batts will keep the batts from losing R when the radiant heat flux is high, or when the attic is cold in winter. Low density fiberglass is somewhat translucent to radiated heat, and ends up absorbing that heat in internal layers. Under a 120F+ roof deck the insulation 1.5-2" below the top of the fiberglass will be several degrees above the attic air temperature(!), which means the remaining depth of batt now has to insulated against a higher temp with less R-value(~R20-R24) . If you blow cellulose over that, the hottest point in the insulation will be the upper surface of the cellulose, which convection cools to within a few degrees of the attic ambient. If you then block the radiated heat transfer between the roof deck and the cellulose with radiant barrier, both the attic air and cellulose temperatures drop. If the cellulose is blown (or foam is sprayed) to a depth such that it's at least 3" above the joist edges it provides a significant thermal break to what is otherwise a thermal short-circuit through the insulation layer. But if there's room for more cellulose, spending the RB money on more cellulose is probably the better deal, both winter & summer.
Your cooling loads are a function of way more than the heat gain through the attic. In a 2-story house with an unshaded southern exposure the attic isn't likely to account for more than 1/3 of the cooling load, so even if you cut that contribution in half you're only looking at 15% or so of the total bill- best case. Realistically treating just the attic R levels would only buy you single-digit percentage reductions in the cooling bill. The attic insulation is often the easiest to remediate- but in terms of quick return on investment- AIR SEAL the place before dealing with R-value or radiant barriers. Air infiltration alone can account for as much as 40% of the total cooling load- which includes the very significant latent loads (humidity) that you get in GA, and cutting infiltraion by half or more is often realistically achievable.
If your foundation sill & band joist aren't well sealed (as in spray-foam insulated) and you have any dryer vents, door & window weatherstripping issues in the basement, any air-leaks at the attic floor/2nd-floor ceiling level has a HUGE stack-effect, sucking air in the basement, releasing it in the attic. Upper floor ceilings have many potential air leaks, starting with the attic door access, plumbing stacks/chases, balloon-framing with open tops in exterior or partition walls, electrical penetrations (especially the evil non-air-tight recessed lighting fixtures), etc. etc. etc.
Often the easiest/cheapest way to remediate air leakage at the top is to insulate the roof deck itself all the way down to the soffits, sealing the soffits and ridge vents. This may violate local code, but isn't as awful for your house as it might seem- attic ventilation is designed to purge the attic from moisture sourced inside the house to keep it from condensing and causing mold/rot conditions in the attic in winter. But with Atlanta's mild winters that risk is low, and in the summer it's sucking moisture into the attic & house all day and night, increasing the latent cooling load. With the attic inside the pressure envelope of the house the AC will keep the attic dry,even if it's on the hot side of the insulation layer. With the furnace and heating ducts up there, sealing the attic from outdoor air will reduce the losses from ducts & air handler as well. If you seal the attic venting, you'll likely have to provide ducted combustion air to the furnace though. It may still be best to air-seal at the attic floor. (And fiber insulation between the joists doesn't count as air-sealing- it leaks like an air-filter under a blower-door test. Foam sealing all big hole, caulking/taping the rest can get you there though without a full-on foam insulation job.)
Before buying $5000 worth of foam and only getting half the air-infiltration under control, a full-on energy audit with a pressure-door & infra-red imaging scan is worth it, to find all of the readily fixable air leaks. THEN start looking at where it's most cost-effective to insulate (a cellulose overblow in the attic is a likely bet). There are insulation contractors who specialize in air-sealing- find one (or two). Most run before/after pressure tests with blower doors to demonstrate that they have acheived or exceeded their contracted-for results. A lot of this stuff is DIY- not rocket science at all. Start with big leaks: mail slots, open flues or flues with leaky dampers, clothes-dryer & bathroom & kitchen vents with missing or inadequate backdraft control, etc. The largest typically overlooked leak is the foundation sill & band joists, but there are likely others easily detected with blower-door techniques.
When it's time to re-roof, if your roof pitch is low, or has a very large south facing pitch, using "cool roof" shingles/coatings will reduce peak cooling loads as-much or more than radiant barrier placed on the interior. With pitches higher than 2:12 it's not worth paying extra for, but California Title 24 compliant cool-roof shingles don't necessarily cost more than standard goods- they just tend to lighter in color than basic-black or deep browns/reds. (Not all "cool roof" materials are created equal- if it doesn't meet CA Title 24 specs for solar absorption & heat-emissivity keep looking.)
If your cooling ducts are in the attic and above the insulation, sealing and insulating just the ducts with 1.5-2" of closed cell or 3-4" of open cell foam can make a disproportionate difference to the bill. Pressure testing & sealing any HVAC ductwork (in the attic or elsewhere), can be worth 15-20% on overall system efficiency all by itself, and insulating at least the supply ducts even inside of conditioned space is usually worthwhile (up to ~R6ish, if inside of fully- conditioned space- more in semi or unconditioned attics/basements/crawlspaces.)
The first $1000-2000 of air sealing goes a long way, and well insulated wind-tunnel isn't much cheaper to heat/cool than an uninsulated wind tunnel. R30 is on the skimpy side (that's code-minimum for new construction in Atlanta, IIRC), but it's likely that even R60 in the attic wouldn't reduce your cooling bills by the amount you are looking for without air sealing. In any high efficiency building envelope air-sealing is job-1.
Then, look at the shading issues particularly on the E/S/W sides. Awnings or roll-down exterior shades over S facing windows, and exterior shades on the E & W sides can reduce sensible-heat gains through those faces of the building by about half. If you have any single pane windows/storms on those sides heat reflecting window films can also work. (But don't go overboard with the most-reflective versions if you hope to have usable daylight, eh? 35-50% heat rejection is enough to make a difference, especially if used in combination with adjustable shading. Also, don't use heat rejection films on double-pane sealed glass that gets direct sun or it'll likely cause the seal to fail.)
Last, not least- don't fall for any of the ceramic paint/coating scams (of which there are many.) While some might reduce heat-transfer through exterior walls, the greater heat gain will be through windows, not walls, and the fraction of the total cooling load getting in through insulated walls is quite small.
If this is an older timber-framed house with low-density batts or no insulation in the wall, retrofitting it with either blown cellulose or high-density fiberglass (like JM Spider @ 1.8lb/ft
3 density) is worth it (and subsidized). Using either would reduce the stack effect flows of exterior walls by more than 95%, whereas lower density blown fiberglass or standard density batts put barely a dent in it. If the walls are currently under-insulated the effect on comfort is considerable, even if it's only enough to cut the utility bills by 10% or less. Slow-rise foam cavity fill would reduce air infiltration by more than 99%, but you pay a lot for that level of tightness (and the risk of wall damage on finished walls are much higher with slow-rise foam than with blown-fiber.)
Don't worry too much about over-sealing the place- it's very hard to do as a retrofit. Any competent air-sealing contractor would tell you when you're approaching a tightness level that would require mechanical ventilation.