I will probably go stick construction due to a desperately tight budget, but I want to make it as well-insulated as I can. I plan to put 2 layers of 1" foam board over the sheathing, then furring strips, and Hardi plank. In drawing the outside corners, by the time you add 2" of foam plus the furring, it doesn't give you anything solid to attach the corner boards to. You would either have to have wide corner boards, or wider furring at the corners in order to have something solid to attach the corner boards to. How is this usually dealt with? I know there are cases where even more than 2" of foam is applied, so there must be a way to handle it.

Wider furring at the corners would be my first choice.

In my experience 'desperately tight budget' and 'James Hardie' don't go together. I hope you do better than I did, it's 3x the cost of vinyl around here....

Posted By galnar on 21 Jul 2010 10:15 AM
In my experience 'desperately tight budget' and 'James Hardie' don't go together. I hope you do better than I did, it's 3x the cost of vinyl around here....

...and 2" of foam isn't a heluva lot of R (R8 if EPS, R10 if XPS, R12-13 if iso.)

Going with lower cost siding and spending the difference on thicker foam might be the better long-term investment.

With 2x6",  24" o.c. stick-built framing and low density cellulose (preferred) or standard density fiberglass batts (if perfectly installed, with foam air-sealing at the sheathing) cavity fill, in conjunction with 3" of exterior iso would yield a clear-wall R of ~ R40, which would be pretty good for most of the lower 48 of the US, and wouldn't need an interior vapor retarder. (In cooler parts of Canada or the very coldest parts of the upper midwest of the US some vapor retarder might be necessary in that stackup though.)   In $/R it might be cheaper to go with 2x4" construction with R15 (high density batt or blown fiberglass) cavity fill and 5" of EPS for an ~R35 clear-wall value.  Either stackup would run ~ 10" between finish-wall and the outside of the exterior siding.

If the plan had been for 2" of EPS or XPS on 2x6"/R19 batts the clear-wall R would be under 30, or under R25 if 2x4" framing.  R25-30 would be GREAT in moderate climates like coastal CA (and if the rest of the design is done right could even eliminate the cost of heating/cooling systems in that type of climate), but more is better where the temperature averages & extremes are more severe.

Dana, for thicker foam board on the exterior, what is the preferred method to attach siding in the outside corners, since most of the corner is foam? Would you use wider furring strips (maybe 6" wide or so) in the corners that are long screwed into the framing corners of the house? This would allow the corners to be trimmed out and nailed, plus room to nail the horizontal siding.

use "Z" furring

My build is going to be in upstate SC, a fairly moderate climate. I was thinking 2" of foam on the outside, well sealed, and 2X6 wall studs, with either batts or blown-in insulation. I'm going to be doing the work myself, so I know it will be done properly, and with a degree of over-kill. As for the Hardi planking, the materials aren't that expensive, and again, it will be DIY. Much of the cost in Hardi seems to be for the install.

I don't mind going thicker on the external foam, and it isn't that expensive, but at what point does it become flimsy for attaching furring and siding? I figured 2 layers of 1" foam, taped, would make an adequate thermal break, and a pretty effective seal.

LP makes an manufactured wood siding - LP Smart Side which has a 50 year warranty & costs about the same as Hardi Board. Shown in the house fgeatured in the new issue of Solar Today - see solartoday.org/video. LP's web site shows several houses where it is mitred at the corners - so no corner boards.

Alernatively, you can run a few pieces of strapping 2'-3' long horizontally to the corner & cut the vertical pieces around them to give you nailing for the corner boards.
Bob

Posted By jdebree on 21 Jul 2010 05:29 PM
My build is going to be in upstate SC, a fairly moderate climate. I was thinking 2" of foam on the outside, well sealed, and 2X6 wall studs, with either batts or blown-in insulation. I'm going to be doing the work myself, so I know it will be done properly, and with a degree of over-kill. As for the Hardi planking, the materials aren't that expensive, and again, it will be DIY. Much of the cost in Hardi seems to be for the install.

I don't mind going thicker on the external foam, and it isn't that expensive, but at what point does it become flimsy for attaching furring and siding? I figured 2 layers of 1" foam, taped, would make an adequate thermal break, and a pretty effective seal.

Go with blown insulation if you can- it's far more likely to meet spec than ANY batt installation.  As a DIY project dry-blown cellulose is probably the easiest, but seek out "borate only" or "sulfate free" materials for the project (or dry blow goods sold as wet-spray cellulose, which are all sulfate free.)  Most of the cheaper goods in the US still use sulfated fire retardent, or a mix of borate & sulfate.  This isn't normally a problem, but should the stuff ever get wet it can corrode copper plumbing & wiring (even hot-dipped nails, given enough time.)  You'll get a true ~R20 clear-wall R out of dry blown cellulose in 2x6 24" o.c. construction, R18 if 16"o.c.  With standard density batts you'd get about an R less than that, but only if installed PERFECTLY- any gaps or compressions contribute to a rapid degradation of performance due to convection currents inside the cavity. Blown goods typically outperform batt goods at "equivalent -R" by 8-10%, often more.  Low density fiberglass also loses R value at high temperature differences- far more than high density blown fiberglass, or any density blown cellulose. 

Cellulose in a 2x6 wall at 2-2.5lbs/ft³ density (typical low-density cavity fill) adds as about as much thermal mass to the equation as 1.5-2" concrete skinned wall would, which gives another nice performance boost in places like SC where daily swings in outdoor temps are often go between being well above then well below conditioned space temps ~180 days/year.

Furring has to be through-screwed into the studs to work, and Hardi is fairly heavy stuff.  The type & density of the foam make a difference on how thick you can make it before compressive-strength issue come into play, but going wider on the furring distributes the pressure. At outside corners you may consider using real planking  or cut down OSB to be able to make the siding tight at the corners.  Iso has lower compressive strength, but gives you 50% more R per inch of depth compared to EPS.  XPS typically has higher compressive strength, but is only 5/6 the R value of iso. 

In a SC climate  2" of XPS sheathing (R10) + 2x6" cellulose cavity fill would deliver a ~R30 clear wall, which should be enough. Alternatively you could go with 2.5-3" of EPS, which is likely to be less money.  1" XPS sheathing is available in 2x9' and 2x10' sheets, which makes it easy to get a full coverage on the exterior with vertically oriented sheets, minimizing tapeing & cutting labor (no horizontal seams.)  Be sure to use Hardi-compatible screws for attaching the furring, or  they'll corrode- look at the Hardi spec sheets for screw lengths and types + fastener spacing.  Also, refer to this for specs on use with furring: http://www.jameshardie.com/pdf/USTB_Attaching-James-Hardie-Products-Over-Rain-Screen.pdf   I suspect you'll be looking at 4-5"  screws to get a sufficient bite into the studs through 3/4" of furring, 2" of foam, and 1/2" of structural sheathing.

From R30 clear walls, to bring it to near-PassiveHouse levels in your climate, insulate the foundation walls (or slab-edges down to 18" below grade) with 2.5-3" of borate-loaded EPS (for termite control on the exterior) or 2" of  XPS on the interior (foam-sealing over the foundation sill band joist) and put ~ R5-R7 of XPS under the slab, and R40-45 attic insulation + radiant barrier or heat rejecting "cool roof" materials.  Using air tight methods and thermal breaks on all framing, routing all utility penetrations underground, and optimizing the glazed area & orientation can get you the rest of the way there., but it sounds like you'll be at least 80% of the way there with the insulation package you have sketched out.

Wow- Thanks for the detailed info! Considering our current house in FL has NO wall insulation, and barely R-19 in the attic, I should be able to make quite a change in efficiency when I build the new place. One of my concerns is properly sizing the A/C to reflect the way the house is built. If I have a regular A/C guy come in, he's probably going to way over-size the unit. We keep our house very cool in the winter (~60) but I need my A/C in the summer.

Posted By jdebree on 22 Jul 2010 04:14 PM
Wow- Thanks for the detailed info! Considering our current house in FL has NO wall insulation, and barely R-19 in the attic, I should be able to make quite a change in efficiency when I build the new place. One of my concerns is properly sizing the A/C to reflect the way the house is built. If I have a regular A/C guy come in, he's probably going to way over-size the unit. We keep our house very cool in the winter (~60) but I need my A/C in the summer.

If you design the shading & overhangs right and use heat-rejecting CA Title 24 compliant "cool roof" materials you'll mostly be dehumidifying, not cooling the place. Even a 1-ton central air system might be oversized, but some amount of mechanical dehumidifcation would be required when outdoor dew points are well over 60F, which would be much of the time from May through September in SC.)

It's probably worth springing for the PassiveHouse design tools  (or at least the demo version) if you're still in the design stage of this house.  In climates like yours getting it close to zero heating/cooling energy is far easier than in much colder climates.  Greenville SC climate is pretty similar to Sacramento or Redding CA climate (only much higher humidity), and PassiveHouse designs have worked with only ~R25 walls in more temperate Berkeley CA, which makes me think ~R35 clear walls might work for you.  The monthly average temp for January in Berkeley is ~45-50F, but for you probably more like 35-40F.  Your average summer temps are 10F higher too- you'll need more R than in Berkeley, but not 2x more.

Attention to details to make it air-tight, high-R, and using active ERV rather than random air-leakage to keep the air fresh are key, but it can work. 

ERV recovers a large fraction of the dehumidification as well as exchanging sensible temperature with the incoming air stream, as opposed to HRV, which only does a sensible heat exchange between incoming and outgoing air. In drier climes there's no benefit to ERV over HRV.  If you're going superinsulated air-tight, specifying the ventilation and dehumidification scheme will be a bigger factor than the conventional central AC approach.  Keeping it under 60% RH indoors (at any temp) in summer makes it more comfortable and eliminates mold hazard, but getting it under 50%RH is important if you have dust mite allergies. 

In winter keeping it around 30-35% and using cellulose insulation to buffer the humidity protects the framing & sheathing from accumulating enough moisture from the interior air condensing in the wall cavity to cause problems. At 60% RH indoors in winter there is some potential for condensation from vapor diffusion from the interior with  R10 sheathing, R20 studwall, but that goes away almost completely at ~40% RH in your climate.  If you bump that up as high as R20 on the exterior there's zero concern.  In winter the outdoor air is relatively dry (much lower dew points), so just adjusting the ventilation rate of the ERV (or putting it under dehumidistat control when temps are below 40F) would be enough, and take use less total energy than running the dehumidifier.

Posted By jdebree on 22 Jul 2010 04:14 PM
Wow- Thanks for the detailed info! Considering our current house in FL has NO wall insulation, and barely R-19 in the attic, I should be able to make quite a change in efficiency when I build the new place. One of my concerns is properly sizing the A/C to reflect the way the house is built. If I have a regular A/C guy come in, he's probably going to way over-size the unit. We keep our house very cool in the winter (~60) but I need my A/C in the summer.

Call Bob Morgan with Bayonet Plumbing & HVAC, he helped us acheive our 2010 "Best Energy Efficient Home" Aurora Award

Just to throw a wrench into the works- What happens if you have the windows open a lot? We like the windows open any time the outside temp is between ~55 to 80. Does this have any different effect in a more efficient envelope than in a typical drafty old house? I'd rather have fresh air and hear the birds singing than keep the house at exactly 72.6 degrees year-round.

As for A/C, I may go with a couple of mini-splits. We keep our FL home at 80 degrees in the summer, and it night it gets stuffy due to the A/C not cycling much. Maybe we should add a dehumidifier to our current home?

IMO, open windows are fine if you make sure that the house sees low humidity at least part of each day to stop mildew.

Posted By jdebree on 25 Jul 2010 01:45 PM
Just to throw a wrench into the works- What happens if you have the windows open a lot? We like the windows open any time the outside temp is between ~55 to 80. Does this have any different effect in a more efficient envelope than in a typical drafty old house? I'd rather have fresh air and hear the birds singing than keep the house at exactly 72.6 degrees year-round.

As for A/C, I may go with a couple of mini-splits. We keep our FL home at 80 degrees in the summer, and it night it gets stuffy due to the A/C not cycling much. Maybe we should add a dehumidifier to our current home?

When it's 55F out it's not humid enough for outdoor air to create an indoor humidity problem.  At 70F indoors, 55F outdoors, the relative humidity (RH) of the air you won't be higher than 50% as it warms to room temp.  Anything under 65% RH is pretty low hazard from a mold POV.

There will be times when it's 80F outdoors that you could run into issues though.  If the outdoor dew point is 65F, bringing it into a 70F room would raise the RH to about 80%. But if the same air is brought into a 75F room it'll only raise the RH to 65%.  The dew point is somewhat independent of the outdoor temperature, but forms a lower bound for the outdoor temp. Air cannot have an RH over 100%, so it's dew point will always be lower than it's temperature.  But objects in a room lower than the dew point of the surround air will condense water from the air (the droplets on the outside of the ice-chilled drink, for instance.) 

[edited to add]

Adding a dehumidifier to your current home wouldn't be bad idea.  80F is pretty comfortable at 40-45% RH but is pretty miserable at 60%RH+.  Air-sealing the house to limit air infiltration would be job-1 though, which would limit the rate at which outdoor humidity creeps in after the sun goes down, and lowering both daytime & nighttime indoor humidity during the cooling season.  With a tight enough house (not necessarily super-tight) a single standalone dehumidifier can usually keep up with the whole house humidity load at night.

I recall that some mini-splits had built-in dehumidification capability. Not sure how it worked.

Posted By jonr on 27 Jul 2010 04:06 PM
I recall that some mini-splits had built-in dehumidification capability. Not sure how it worked.

It's usually a matter of the internal controls running them at  low CFM, low coil temp when the humidity needs to be lowered, as opposed to the optimal highest-efficiency point that the unit is capable of.

Higher SEER units are typically run at higher temp, higher flow, which makes them less effective at removing humidity even if it's doing a great job on the sensible temp.  You don't get much dehumidification out of a 50F coil, but at 40F you can.   Running the coil at a lower temp comes with a hit in efficiency, but in a tight well insulated low solar gain house,  it's pretty much all latent load- running it in dehumidifier mode it'll still use less electricity than air-conditioning a lesser insulated leakier house. 

A mini-split with humidity control wouldn't be a bad option here.

Being concrete block, our current home is pretty tight, but the doors and windows are horrible. Single pane, and worn out to boot. If I weren't getting ready to build elsewhere, I'd definitely put some time and money into this place.

Posted By jonr on 27 Jul 2010 04:06 PM
I recall that some mini-splits had built-in dehumidification capability. Not sure how it worked.

the mini-split in my office keeps humidity between 30-40%