I'm getting my ICF home built, and will soon have to make a choice on attic insulation. The walls are Fox Blocks, about R-25. I'm building in upstate SC, so it's a moderate climate. The house is about 1250 sf, and the windows are average double pane. A few problems have been eliminated: There is no rim joist issue, as I used a ledger bolted on the inside. The top plate has room for foam insulation inside and out. I had raised heel trusses made, so I have about 18" of insulation space out to the top plate. My question is- What's a sensible insulation value for this house in this climate? I want decent performance, but i feel I don't need to go crazy in this climate. Is there a standard ration between wall and ceiling R-values?

I don't know about standard ratios, but under IRC2012 the code-min for SC would become R38, and it's not a huge cost adder from there to hit R50 or even R60. With 18" of space you have the room for it. Using cellulose rather than fiberglass adds weight, but will perform better at the temperature extremes, both winter & summer.

The Building Science guys believe there's a long-term economic rationale for at least R50 if you do it with cheap stuff like cellulose. See the baseline recommendations for climate zone 3 in table 2, p10 of this document:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

Higher R attic insulation is generally a cheaper performance upgrade than many/most, and an R60 attic will have half the heat gain/loss of a current-code-min R30, and you'll probably be able to feel the difference at 2PM on that 100F day in July. If you gave it the full 18" of cellulose on day 1 the settled depth over time would be about 16" and run R60, according to National Fiber's chart: http://nationalfiber.com/docs/ExpandedBagCoverageChart0911.pdf

The material is pretty cheap, and going to R60 won't be anything like 2x the code-min R30- much of the cost to the contractor is setup and break down, and tying up the blower for the afternoon, which about the same whether they blow 9" vs. 18". The raw material cost for 1250 square feet of R60 is about a $1000-1200 (~2500 lbs @ ~40-50cents/lb) vs. ~$500-600 for a code min R30. Most of the other costs are pretty much fixed, so doubling it costs the contractor maybe $700-800 more (if he's paying top labor rates), marked up it might be at most an extra buck a square foot to go from code-min to R60.

Setup and break down time is the same either way, active blowing times double, but with a 2 stage blower they can move a whole lot 'o 1.2-1.3lb density cellulose in an hour with just a couple of guys working it. A Krendl 1300 could deliver R60 in your place in an hour if the guy filling the hopper can keep up, whereas it might take 2.5-3hours of blowing time to get there with a low end rental single stage blower, but in neither case is it more than a full afternoon's job for a coupla guys with a truck big enough to handle a ton or so of cellulose and a blower.

So, since you have the space to fill, get it quoted at a few different points, say R30, R45, R60 to see what it takes, but most of the time the marginal costs of the next R10-R20 goes way down from whatever the minimum charge is. (And blowing fatter attic insulation is a far better bang/buck than going with upgrading to higher performance windows when you do the math, even if performance gains are pretty paltry above R45 on an annual energy savings point of view.)

Posted By jdebree on 07 Dec 2012 05:10 PM
I'm getting my ICF home built, and will soon have to make a choice on attic insulation. The walls are Fox Blocks, about R-25. I'm building in upstate SC, so it's a moderate climate. The house is about 1250 sf, and the windows are average double pane. A few problems have been eliminated: There is no rim joist issue, as I used a ledger bolted on the inside. The top plate has room for foam insulation inside and out. I had raised heel trusses made, so I have about 18" of insulation space out to the top plate. My question is- What's a sensible insulation value for this house in this climate? I want decent performance, but i feel I don't need to go crazy in this climate. Is there a standard ration between wall and ceiling R-values?

It looks like you are in climate zone #3. Under previous IECC recommendations, the attic minimum was R30, and this was upgraded in IECC 2012 to R38. You would need to check with your county building department or check the internet to see what requirements are minimum for your county. The IECC 2012 requrements of R38 are probably a reasonable target. However, it is pretty cheap to add extra attic insulation. Since you've already spent money on ICF construction with R25 walls, you could consider going as high as R49 to reduce summer cooling and winter heating costs. So the minimum of R30 or R38 is determined by code, and anything more than R49 would probably be a VERY long payback. I am in climate zone #6, and I computed that going from R38 (code requirement at time of construction) to R60 with loose cellulose insulation will take 28 years for payback.

You could also use something like BEopt, a free download, to evaluate your question quantitatively, both in terms of energy and financial cost.

Because of when I initiated my permit, I'm grandfathered in to IRC 2006, but that doesn't really matter to me. The entire house exceeds code by a wide margin. I believe my main concern would be A/C, since we both prefer a cool house. In the winter, we generally keep the thermostat around 60. In the summer, we keep it at 80, but would prefer it a little cooler. I plan on using a mini-split system, and a small one at that, so I want a tight, well-insulated house. It's interesting to consider that at nearly 60 years old, I have never lived in a fully insulated house. I lived in CT for my first 30 years, and none of the houses I lived in had ANY insulation! It will certainly be a revelation to live in a tight, insulated, modern home.

I'm going to get some local pricing for the attic insulation. I'm not going down the road of figuring out the payback; at my age, there are probably a number of elements of my house that won't pay out while I'm still on the green side of the grass. I want a strong, quiet, comfortable house with the lowest possible maintenance. I expect that my summer electrical bill will be comically small compared to my uninsulated, single pane window, leaky-ductwork-in-the-attic home in FL.

I was thinking R-48 or more, and it sounds like somewhere between R-48 and R-60 makes sense. While we're at it- is there any logic to doing your own blown cellulose with a rented machine? I'm doing all of my own work except excavation and concrete work. And what will I need for a ceiling structure to support R-60? My trusses are on 24" centers, and I am going to cross-strap 1X 4 furring on 12" centers. Drywall will be 5/8". Will that hold up without sagging?

It's certainly possible to do open blow R50+ attics with a single-stage rental blower, but it's a lot of material and it would probably take 2- amateurs more than a day to get 'er done.

The whole "payback" discussion is a bit silly- there's not a good short-term rationale for even making code-min if looking strictly at net-present-value of utility cost savings- it's all 25 years+. But there's a perceptible comfort difference between an R30 attic and an R50 attic during the cooling season.

With the 1x goods 12" o.c. the 5/8" gypsum should be able to handle the load, provided the dead-weight loading of the truss chords are still well within spec. You're getting on to ~2lbs per square foot of load @ R60, plus another ~2.75lbs per square foot for the 5/8" sheet rock, so you'll be close to 5lbs/ft dead-load on the truss chords. Hopefully the good folks who designed the deep energy heel trusses factored it all into the truss spec.

There are lighter weight high-strength 1/2" sheet rock solutions that are specified to work with 2.2lbs/square foot insulation even mounted directly to 24" o.c. trusses, but I'd be more comfortable taking that down to 16" o.c. furring, if not the 12" o.c. you're talking. Fastener spacing specs also become important when at higher per square foot dead loads, but I think you're more than good if it's 12" or 16" o.c. See:

http://www.usg.com/rc/data-submittal-sheets/panels/sheetrock/sheetrock-interior-ceiling-sag-resistant-submittal-WB2195.pdf

Posted By Dana1 on 11 Dec 2012 03:58 PM
...snip...
The whole "payback" discussion is a bit silly- there's not a good short-term rationale for even making code-min if looking strictly at net-present-value of utility cost savings- it's all 25 years+. But there's a perceptible comfort difference between an R30 attic and an R50 attic during the cooling season.
...snip...

If the goal were strictly comfort rather than energy savings and payback times, then probably better to focus on windows, doors, and infiltration rather than increasing ceiling insulation. You'll "feel" coolness in the winter or heat in the summer more from an R-3 window that you are sitting next to than an R-30 ceiling. Heavy drapes, or better yet, cellular shades with side seals. will really help reduce body radiation to the cold window surfaces in winter, and the side seals will reduce the "reverse radiator" effect of cold windows in the winter. Likewise during the warm season, treating solar radiation through the windows is critical for comfort. So window treatments are part of the low-hanging fruit for comfort.

Storm doors can help reduce infiltration around outside doors, again important for comfort.

Finally, if comfort is really the driver rather than energy savings, then avoid high-efficiency, modulating, condensing hot-air furnaces as they blow air that often feels a bit chilly when operating at low power. (Low exhaust gas temperatures are important for good thermal efficiency, but a side effect is low air temperatures to the house.) The discomfort can be avoided by moving furniture around or blocking duct outlets, but avoided entirely using a lower efficiency, hot-air furnace. (For me, efficiency and payback are important, and I'll stick with the high-efficiency furnace and the chilly outlet temperatures.)

The cost of window upgrades can be quite a bit more expensive than the material cost of an "extra" R20-R30 of cellulose. Yes, windows count for winter comfort, but far less so in SC than in CO. (99% design temps in SC are in the 20s & 30s F.) Cooling season comfort is more of an issue here than heating season comfort, and taking the attic-R up a notch really helps. (Shading west facing windows on the exterior can count too.)

This place is being heated & cooled with modulating inverter drive mini-splits, not gas-burners, no ducts. Output air temps on mini-split heads are typically north of 110F if it's keeping up with the heating load, and the variable blower speeds are very quiet, without a huge wind-chill (unless it's running flat out and you put it at seated head-height in your breakfast nook, which would be a mistake. :-) ) In an SC climate it'll be extremely efficient, with comparable (or even lower) operating cost to condensing gas, but even if it's a bit more it'll be low enough to be a "who-cares?" sort of deal.

Posted By Dana1 on 11 Dec 2012 05:18 PM
...snip... Cooling season comfort is more of an issue here than heating season comfort, and taking the attic-R up a notch really helps.

My roommate in college in Atlanta (about same latitude as South Carolina) would take issue with that statement. He was from West Palm Beach, Florida, and when winter came to Atlanta, he would always say, "God didn't mean people to live this d**n far north!" In the Atlanta climate, you could live without A/C (which we did in some dorms and apartments), but not without heat.

I think jdebree will finally be living in comfort in his ICF home in South Carolina.

I don't know about living here without A/C. Last summer, it was over 100 for a week, and set an all-time high of 107 degrees.

My windows are double-pane Marvins- nothing special, but the best we could find without going wildly over-budget. Solar gain in the summer will be minimal due to the number of trees, large overhangs, and an attached garage on the west side.

I'll look into the high-strength 1/2" drywall. Lighter sounds better to me!

Lee, the "...here..." in my "cooling season comfort is more of an issue here than heating season comfort..." wasn't strictly about being located in SC. It was intended to mean the situation under discussion: Increasing attic-R to well above code min in SC.

With a code-min R30 attic it's comfortable as long as the heating system keeps up, (except when sitting next to code-min windows during the 99% condition.) But during the cooling season taking the ceiling temps down a few degrees with an R50+ attic is an increase in comfort, even at the same average room temp. And since the cost for the upgrade is less than a grand, it's a decent investment in comfort even if the financial payback is several decades away, or even forever NPV-negative when viewed strictly on energy costs.

In other terms: During the heating season the attic will never be below the outside temp at the 99% condition, with a delta-T of maybe 30F, 35F at most from the room temp, and a ceiling cooler than room temp isn't a huge comfort hit during the heating season. (Floors cooler than room temp can be pretty uncomfortable though, and radiant floors warmer than room temp are the ultimate in heating comfort.) During the cooling season the direct solar gains through the roof will put peak attic temps a few 10s of degrees above the 90-95F 1% design temp (which may be increasing over then next decade or two), for a larger delta-T from room temp to attic than the 99% situation, and a correspondingly larger delta-T between ceiling & average room temp. And a warmer ceiling than room temp IS a comfort hit during the cooling season. Also, the coldest ceiling temps during the heating season will occur during hours when the occupants are asleep and don't care, whereas peak ceiling temps during the cooling season occur during hours when most people are up and about.

Bottom line- going over code min on attic insulation affects cooling season comfort far more than it affects heating season comfort, and this will be true even in much colder climates than SC, with much lower 1% design conditions. Comfort is about keeping the occupants comfortable, and not strictly about the average room temps, and the direction of radiated heat incident on the humans matters.

Our old, completely un-insulated house in FL was an interesting study in hot weather comfort. The original roof was a 2/12 pitch, and for several reasons, I built a 6/12 pitch roof over the original, rather than remove it. The A/C ductwork was in the original attic. The new roof essentially shaded the old one, and at peak temperatures, there was about a 40 degree drop in temperature in the old attic. Not only did our electric bill drop dramatically, but we were able to raise the thermostat a few degrees with the same perceived comfort. I always assumed it was because the A/C didn't have to work as hard, but I now see that the house felt cooler because there was less heat being radiated through the ceiling. When I did this roof-over 30 years ago, the idea of saving energy or increasing comfort never even crossed my mind.

That house also had jalousie windows, and in the winter, we had to tape plastic over them to keep the wind out! I look back, and its hard to believe that we lived there for 6 years without any heat or A/C before we finally had a central system installed.

But during the cooling season taking the ceiling temps down a few degrees with an R50+ attic is an increase in comfort

My math says ~.5 degrees (as compared to R30) - unlikely to be significant on a radiant comfort basis.