I'm in the design/planning stage of building my first home. I was thinking for our climate zone in Lagrange, Ga (Zone 3 - Hot and Humid) it would be great if I could find a extruded polystyrene foam with foil backing to use behind my Hardie Artisan Lap siding with 1x4 furring strips.

I had the intent on just going with something like Tuff-R Polyisocyanurate foil backed, until I saw some test result which show how R-Value is affected by temperature to various different types of rigid insulation that's on the market.

• http://www.energyvanguard.com/blog-...a-Constant

The results show extruded polystyrene foam from 130 deg F at R20 and as the temp drops increasing to a R25 at -10 deg F. If the test are legit, wow that's some nice performs in a very wide temperature range.

Extruded polystyrene (XPS) loses R over time due to leakage of it's HFC blowing agents. After 50 years in the field it's performance is comparable to the beaded expanded polystyrene (EPS) foam of equivalent density.  Thed blowing agents for XPS that give it an initially higher R are a mix of HFC, the predominant being HFC134a are powerful greenhouse gases (~1000-1400x CO2).   EPS and polyiso are blown with pentane (at ~7x CO2).  With sheet EPS most of the pentane is already gone by the time the product hits the distributors yard, but with foil-faced polyiso it takes a handful years. EPS is cheaper per unit-R than XPS, so you might as well use it, if you're insisting on a consistently linear derating curve.

There are many manufacturers of EPS with foil-facers in the US, but I'm wondering why you feel the foil facer is important?  Foil facers are powerful vapor barriers, and can interfere with the drying capacity of the wall assemblies.  A single foil facer facing a 3/4" gap can add another ~R1-R2 of average performance, but has no thermal value if applied flat up against an adjacent layer.  In a GA climate a foil facer on the exterior side of the wall assembly requires that you make the interior side sufficiently vapor open to dry toward the interior.

The performance crossover per-inch between polyiso and EPS occurs at about 8C/46F (mid-foam-depth temp), and the crossover between polyiso & XPS occurs at about 10C/50F:






If you design your stackup with a layer of EPS or XPS over a primary layer of polyiso such that the mid-winter temp at the outer facer of the polyiso averages ~45-50F you will optimize the overall performance per inch.  In LaGrange GA the mid-winter binned hourly mean outdoor temp is about 43F, so it doesn't take much EPS/XPS to get there. eg #1:

Assuming 2x6 construction with  R20 fiber, and R1 for the gypsum + sheathing, if you add an inch of polyiso you have about R27 from the interior paint to the exterior facer of the iso.  If you want to define the mean mid-winter temp of the facer to be 46F, and the conditioned space temp at 70F, that's R27/(70F-46F)= R1.125 per degree. So to ensure a 46F mean temp at the facer with an outdoor mean temp of 43F, it take R1.35 x (46F-43F)= R4.1 of EPS or XPS. 

That's 1" of EPS, outside of the 1" of polyiso which yields a center-cavity R of ~R30, and a whole-wall R (after thermal bridging of the framing ) of about R24, which is a high-performance wall for that climate, but not outrageously overboard. BSC suggests R20 whole-wall as a starting point for zone 3 climates. (See Table 2, p10 of this document.) 

eg #2:

If you dial back to 2x4 framing with R13 cavity fill with R1 gypsum+ sheathing you're looking at R14 center-cavity. With an inch of exterior polyiso that comes to R20. Defining the exterior of the iso to be 46F you then have R20/(70F-46F)= R0.833 per degree, and would only need R0.833 x (46F-43F)= R2.5 on the exterior of the iso, which can be made with less than 3/4" of EPS, or 1/2" of XPS, ending up at about R23 center-cavity, and about R19 whole-wall after thermal bridging, which is in the "right" range to think about from a long-term cost/benefit analysis.  Bumping the EPS to an inch gives an even bigger pad for preserving the performance of the polyiso, and brings it into the low R20s whole-wall.


In Miami all-polyiso is the clear annual thermal performance winner, but it's worth simply splitting the thickness 1" EPS & 1" polyiso in a LaGrange GA climate on 2x4 framing, or settling for  just 2" of unfaced (for better drying capacity) EPS and NO polyiso on 2x6 framing, which would also deliver low R20s whole-wall performance:

http://www.greenbuildingadvisor.com...-beats-r-6

At 2" unfaced 1.5lb density "Type-II" EPS has a vapor retardnecy of about 1.5 perms, which is plenty for managing summertime moisture drives into an air conditioned building with standard latex-paint being the most vapor retardent interior side layer (about 3-5 perms), and sufficient exterior R to keep the sheathing from loading up with moisture over the winter in that climate. Unless you're going for Net Zero Energy or PassiveHouse type performance levels, the 2x6 w/cellulose + ~2" of exterior EPS is going to be your best bang/buck, and would provide a highly moisture-resilient stackup.

General Exterior Specifications

• Hybrid Style - Cape Cod / Colonial / Bungalow
• 1 ½ Story Home with combination of concrete siding products
• Unconditioned attic space to be unfinished and utilized for storage areas wherever possible
• Raised Monolithic Slab somewhere around 21” above Grade and Brick around Foundation slab
• Estimated Conditioned Space – 1900 Sq. Ft. first floor / 900 Sq. Ft. second floor

Targeted Home Budget somewhere in the $100 to $110 per Sq. Ft.

Property Environment/Conditions

• West Point lake 500 Ft Away
• Building site 30’ above 100 year lake flood level
• Lot Grade is sloping on left and right side of property ( No Problems with drainage)
• heavily wooded area – Mix of 75+ year growth of Mature Pines and Hardwoods / Ground covered with fallen Pines due to pine beetles ….
  
• Signs of termites …. Found a few live 3” diameter or so Sweet Gum trees showing termite trails at base of LIVE trees.
• All trees within 60 ft. of edge of building pad to be removed, including Stumps, roots and any organic material which would attract termites.

Photo of 1 Acre building lot and my Buddy.

Questions

1. For the Lagrange, Ga area how do you think the ROI curve would fare between the following 3 wall configurations below?
2. What would be the Pros & Cons of the following configuration?
3. If your where looking for most bang for the buck, but you where willing to add a few more dollars in the wall assembly to increase the long term durability of the assembly what would you change or add?

Exterior Wall Configuration - 1

• 5/8 Drywall
• 2x6 16" O.C. w/Cellulose
• 7/16 OSB Sheathing
• TYVEK Commercial Wrap D
• 2" unfaced 1.5lb density "Type-II" EPS
• 5/4 x 4" Pressure Treated Yellow pine Furring strips
• JamesHardie Board Artisan lap siding - 4.55 lbs./sq. ft

Exterior Wall Configuration - 2

• 5/8 Drywall
• 2x6 16" O.C. w/Cellulose
• 7/16 OSB Sheathing
• TYVEK Commercial Wrap D
• Two layers of 1" unfaced 1.5lb density "Type-II" EPS staggered
  
• 5/4 x 4" Pressure Treated Yellow pine Furring strips
• JamesHardie Board Artisan lap siding - 4.55 lbs./sq. ft

Exterior Wall Configuration - 3

• 5/8 Drywall
• 2x6 16" O.C. w/Cellulose
• 7/16 OSB Sheathing
• TYVEK Commercial Wrap D
• Two layers of 1 1/2" unfaced 1.5lb density "Type-II" EPS staggered
• 5/4 x 4" Pressure Treated Yellow pine Furring strips
• JamesHardie Board Artisan lap siding - 4.55 lbs./sq. ft

My Reasoning behind some of the Items in list

1. 5/8 drywall for more thermal mass, straighter walls and sound deadening.
2. 2x6 for more insulation, higher wind loads and greater potential to install beefy overkill lateral bracing.
3. Studs on 16" O.C. for hanging heavy objects like Artisan lap siding.
4. TYVEK Commercial Wrap D because its tougher and water dislikes it even more than the regular stuff .
5. Two layers of foam staggered to cover cracks when EPS begins to shrink, plus you recommended EPS. 
6. 5/4 x 4" Pressure Treated Yellow pine Furring strips because bugs dont like green stuff and the extra thickness is for that darn heavy Artisan lap siding attachment.
7. JamesHardie Board Artisan lap siding cause its heavy and pretty.

Issues I cant seem to get out of my head

• EPS not having a durable protective impermeable skin to serve as rain screen and no UV protection .... But you seem to prefer a sheathing that can breath in this area which makes sense. Once siding is on, only the bugs will see it.
• Free loading Termites shacking up with the EPS to stay warm?
• Furring strip timber screws sagging from weight once that darn heavy siding is in place. Tightening up on the timber screw spacing should prevent sagging furring strips.
• Forces pushing on external siding causes furring strips to push into the EPS and furring slips down shaft of timber screws causing waves in wall cladding.
• Maybe I should use Type-IX EPS for the higher compressive resistance (25psi)

Keep in mind, sometimes I worry over things that I shouldn't be worrying about.
I can handle criticism without having a fit, so please bring it on and let me know when I wrong.

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How are you transitioning between ••••Raised Monolithic Slab somewhere around 21” above Grade and Brick around Foundation slab•••• and the siding?

I will draw up a cross section soon and post it.

It's going to be a tricky custom detail..... I want the home to appear as though it has a crawlspace when actually its a monolithic slab. I'm hoping to get the brick flush with the back side of the siding. The brick being around the foundation wall will also add durability, character and get the wood framing higher off the finish grade, so maybe the termites wont destroy the wood framing. I have some measures I think will take care of the termites.

Another advantage I think will occur is the slab surface will contain a lower moisture content and why I want this, is I'm thinking about gluing solid hardwood down in some areas on the first floor. I don't like engineered hardwood floors for the simple reason they are not durable.

The ROI is impossible to calculate without knowing both your current and projected future energy costs, as well as your local material & labor costs.  If you're heating with $5/gallon propane and cooling with off-grid PV & batteries it's a different net-present value than if you're heating & cooling with highest efficiency ductless heat pumps at 10.5 cents kwh grid power (the current average price in GA, according to EIA data).

With any type of rigid foam, two layers with seams-staggered is worth it for the shrinkage issues cited.  The seams of EPS can  be sealed with fiber-reinforced duct mastic to improve long term air tightness, but yes, it will shrink some in the first couple of decades.  There is no advantage to going with higher density EPS in this type of application (There's some advantage to making the upper layer of EPS under a torch-down roof out of the denser stuff to make it more walk-able without damage, and if insulating under footings Type IX or Type X is usually called for.)

In termite country it's worth the upcharge for using borate-loaded EPS.  See: http://www.performguard.com/downloads/brochure/PerformGuard-SS.pdf http://colonialgreenproducts.com/ep...istant.pdf  (there are others- google 'em)  It's also worth using a copper-clad sill gasket or copper flashing (despite the thermal bridging of the copper) at the top of the foundation stemwall.  Using "borate only" cellulose is definitely worth it for wood-boring insect resistance, and widely available (all damp-sprayed "stabilized" cellulose uses only borate fire retardents), or even a "ant & termite" formula with extra borates (harder to find.)  Borated kill the gut flora that termites/ants/wasps need to be able to break down the wood fibers they injest, eventually killing the host insect. And since these insects cannibalize their weakened members, they end up taking it into the nest.

Even with fiber-cement siding timber screws 24" o.c. are adequate for 3" foam thickness and 4" wide furring.  (See Tables 1 & 2.) The screws need to be long enough to penetrate the stud by 1.5". Pancake heads are preferred to distribute the pressure, which keeps it from splitting the furring over time/weight/wind-load.

At even 2" of foam it's likely that the windows will be your biggest energy use issue.  Minimize the amount of area on the west-facing side to keep late-day gains low, since late PM gains during the heat of the day drives peak loads.

Kneewall attic spaces are notoriously hard to air seal, expecially if you need to access them for storage. Even though it's more expensive, it may be worth insulating and air sealing at the roof deck rather than the attic floors and kneewalls.   If you put 2" of EPS above the roof deck you could then fill the rafter spaces with cellulose, and go un-vented on the roof, and still meet the IRC prescriptive minimums for exterior-R on unvented roof assemblies.  If you're going to do a 2" foam-over on the roof, it can be made continuous with your wall-foam to minimize thermal bridging, even with eave & rake overhangs if you use exterior bracketing to continue the roof lines.   (There are several variations on the theme.)




I'm skeptical that this can be built for $100/ft of floor area without substantial DIY-discounted labor.  It's possible to take much of the sting out of the foam budget by using reclaimed roofing foam (insulationdepot.com or local sources), but you won't be able to get borate-loaded foam that way.  But with copper flashing at the top of the foundation wall and back-filling the foundation with compacted sand can minimize the overall termite risk.

Posted By ricky_005 on 05 Feb 2014 11:01 AM
I will draw up a cross section soon and post it.

It's going to be a tricky custom detail..... I want the home to appear as though it has a crawlspace when actually its a monolithic slab. I'm hoping to get the brick flush with the back side of the siding. The brick being around the foundation wall will also add durability, character and get the wood framing higher off the finish grade, so maybe the termites wont destroy the wood framing. I have some measures I think will take care of the termites.

Another advantage I think will occur is the slab surface will contain a lower moisture content and why I want this, is I'm thinking about gluing solid hardwood down in some areas on the first floor. I don't like engineered hardwood floors for the simple reason they are not durable.

The reason I ask is that it might be possible to rest your strapping on the brick for some structural support to alleviate some of your concerns. The trick will be to maintain thermal, moisture and termite breaks!

I did some rough estimates in excel and the numbers didn't look pretty. Once I do a wrought floor layout to tune in the square footage it will most likely fall between 2,200 to 2,400 square feet.

At some point here soon I'm going to have to get a more realistic idea what projects I will have time to DIY and what projects I will have to hire some cheap labor to Help me get the DIY project completed in a reasonable amount of time.

Items I will be DIY

• Electrical above slab only
• Light Fixtures
• Plumbing above slab only
• Plumbing Fixtures
• HVAC
  
• Interior Trim work
• Painting interior and Exterior
• Tile work
• Hardwood Flooring

I hope by doing all items listed above will offset the price per sq. enough to hit a target between $100 to $110 sq. ft. I will have it all worked out in my excel sheet and plug in a contingency number to make sure I'm covered.

I know by doing a 1 1/2 story home theres going to be some serious issues plugging the holes up stairs. I have not dealt with that aspect yet ..... I am hoping to get by with fiberglass and cellulose in the attic area ...... I am having some doubts about it working out well, will have to study it and see if I can come up with some cheap slick ideas.

But I'm thinking if I encapsulated the roof with ridged foam its going to blow my budget, I will look into it though.

@FBBP
I plan on using old fashion anchor bolts spaced rather tight. May use all-thread rods in a few locations to tie the top sill plate and rafters to the slab. Its way beyond building codes for inland but I like it.

I have 3 options on the top course that I can think of right now

1. standard modular brick flush with inside face of siding which will require very thick foam will have to work out the flashing detail and insect screen.
2. standard modular brick with top course on edge which will work with any thickness of foam but brick will be outside the face of the siding. I don't like the looks of it, but it would be the cheapest route.
3. 2" thick Thin brick veneer but that has some issues to...

Given some time I may end up having to settle with option 2 because of the cost of option 1 ....... There again I may settle for metal strapping for sill attachment .... and get the appearance of option 1. But strapping just doesn't look as good as a 1/2 bolt sicking out of a concrete slab. Guess I should look at strapping details a bit harder from an engineering aspect to see if they can perform as well as and 1/2" Anchor bolt. I suppose if I have the strapping placed close enough on center I could obtain the same results.

Any ideas which are structurally sound that would not jeopardize the shear strength of the anchor bolts to get the brick face flush with the inside of siding (2" of Foam sheathing) would be greatly appreciated.

To limit the thermal bridging at the foundation and preserve the performance of your ~R19-ish wall it's worth putting 2" of borate loaded EPS on the exterior of the stemwall, another inch between the stemwall & the slab, as well as an inch of (not necessarily borate-loaded) EPS under the entire field of the slab, with a 6mil poly vapor retarder between the slab & foam. Where the foundation-foam meets the wall-foam copper Z-flashing is in order, with the flashing extending to the outside of the foundation's exterior fiinish layer (brick, in your case.)



With 2x6 16" o.c. it's just fine to cantilever the bottom plates as much as 2" off the foundation if need be to align your exteirior planes. If the brick makes the cantilever requirements too severe, you may want to consider a fiber-reinforced cementitious EIFS finish that is designed to adhere to EPS.

In your climate the stem-wall and slab-edge foam have a thermal performance rationale, but sub-slab foam is primarily for moisture control, with thermal issues being somewhat secondary (but still significant in a higher-R house.) With an insulated slab with poly under it you limit the potential of moisture getting under the finish flooring and creating issues, and since the slab temp rises & fallls with the room temp, stabilizing that room temp with it's substantial thermal mass, but since inside the thermal envelope of the house it's temperature is always above the dew point of the adjacent air- it won't take on moisture, winter or summer. Without the sub-slab foam portions of the slab can easily be below the dew point of muggy hot summer air. By keeping it both air-tight and water-vapor tight under the slab the concrete and thermally somewhat isolated from the subsoil, the slab and any layers on top of it stay dry all year round.

If I could get used recycled foam cheap enough, I would consider doing the under slab installment.

As for the stem wall do you think it would be just fine as long as its shaded from the summer sun?