Hello,

I'm building a new 1600 sq ft home in Atlanta GA. I've been doing a lot of research on wall construction. Is 2x6 walls with 2" XPS or EPS on the exterior a good investment / worth it in the south?

My wall idea is;

"Outside to inside"

2" xps/eps, tyvek, 7/16 osb, 2x6 walls filled with dense packed cellulose, 1/2 gypsum board.

Is the above wall system worth it in the south? Can anyone see anything else I should do to the wall system.

I plan to use mini-split systems in the home.

I'll be using attic trusses and think my best option for insulation is maybe 2" closed foam on the roof deck and then dense packed cellulose. Any ideas on this option? I could maybe get 2" of xps on top of the roof deck. I'm using metal for the roofing.

My other idea was to put 2" of xps on the roof deck, then use dense pack cellulose under the roof deck. Would this be a better option then using 2" of spray foam under the roof deck? I like this option better as spray foam is pricey here. The dense pack cellulose is cheap. I can do 8"+ of cellulose. I would just space 2x4's in from the trusses to gain more space for the cellulose.

Using the above idea the roof would look like:

Metel, 2" xps, osb roof deck, attic trusses filled with 8"+ of dense packed cellulose, 1/2" gypsum board.

The roofer/truss guy is trying to get me not to use osb and instead use 2x4 purlins on the trusses to attach the metel. He say's it'll save me over $2000. Could this be an option with cellulose?

The roof would look like:

Metel, 2x4 purlins, attic trusses filled with 10"+ of dense packed cellulose, 1/2" gypsum board.

Another idea might be to use 2" of xps instead of osb. In this idea the 2x4 purlins would basically hold down the xps and give something for the metel to hold to.

Metel, 2x4 purlins, 2" xps, attic trusses filled with 8"+ of dense packed cellulose, 1/2" gypsum board.

Thanks in advance for any and all input.

What is a good investment (for you personally) depends greatly on how long you will live there.

In a US climate zone 3 location like Atlanta, there is a long-term financial rationale for whole-wall R values in the ~R20 range. (See the climate zone 3 row of Table 2, p10 of this document read at least the whole first chapter. Those Rs are "whole assembly", with all thermal bridging acounted for, not a center-cavity value.)

Without the foam a 2x6 16" o.c. wall with fiber cement siding and cellulose cavity fill comes in at about R13-R14, if 24" o.c. it's more like R14-R15. Adding 1" of foil-faced polyiso on the exterior held in place with furring through screwed to the studs 24" o.c. with pancake-head timber screws, and using short rink-shank nails to hang the siding (short enough to not to create a thermal bridge through the foam) will get you the R20 range.

An easier/thinner wall would be to drop back to 2x4s 16" o.c. , which is about R10 without the foam.  Then 2" of EPS (R8.4) or 2" of polyiso (R12-R13) would get you there. EPS and polyiso are blown with relatively benign pentane, whereas XPS is blown with HFC134a.  XPS loses most of it's blowing agent in the first 50 years of service (and at 1400x CO2 has a significant global warming impact), at which point it's R value is about the same as EPS.  EPS loses it's pentane within weeks, and is labled at it's fully-depleted R value.  Foil faced iso hangs onto it's pentane for about 15-20 years, but it's fully depleted R is about R5.7/inch.  Also, EPS increases in performance at lower outdoor temps (about R4.5/inch at a center-foam temp of 25F, as opposed to it's labeled R4.2/inch) whereas polyiso's performance it lower than labled at the cold temperature extremes (the exact curve of which is complex), but in an Atlanta climate will always outperform the EPS.

The 2x4 + 2" approach is an easier build than 2x6 + 1", since the total wall thickness is the same as a no-foam 2x6 walls, which makes the window & door detailing easier.

Rather than dense-packing, damp sprayed cellulose has slightly higher R, and is cheaper.  The key difference is in the air-retardency, but as long as you are caulking the sheathing to the framing (recommended in either case) and detailing the sheathing as the primary air barrier (don't forget to caulk under the bottom stud plate, and between the top plates, etc.) the marginal hit in air-retardency of the lower density cellulose has no consequences.

On the roof assembly, it is far cheaper to put R50 in cellulose at the attic floor than insulating at the roof deck.  (Also, code requires a structural roof deck, not just rigid-foam + purlins.)  If insulating at the roof deck you can put as little as R5 under the purlins with the rest as fiber on the interior without needing an interior side vapor retarder. Alternatively, and inch of closed cell foam on the interior side of the roof deck with the rest as fiber works, and is easier to make air-tight if your foam installer is good at the details.  But it'll still be 2-4x the cost of R50 open blown low density cellulose on the air-sealed attic floor, with a vented roof deck. 

The only time you'd ever want to air seal and insulate at the roof deck is if you were putting ducts & mechanicals in the attic, but it may be cheaper to go with and insulated-sealed bottom floor crawl space for the mechanicals instead of the insulated roof deck approach.

BTW: The utility regulations in Georgia is in a state of flux over compensating small distributed generators (and third-party ownership of rooftop solar that would make it easy is currently explicitly disallowed) but there is a lot of political pressure pushing back on the Georgia Power monopoly to accommodate more distributed green energy.  At some point well before 2025 it is likely to be both financially viable an financially beneficial to put PV solar on the roof,  but not if the roof has sub-optimal orientation & shading factors.  It's worth at least considering these aspects before breaking ground to be sure you don't paint yourself into a corner.  According to NREL estimates, only about 25% of existing residential & commercial rooftops in the US are suitable for PV, but on new construction you can usually choose whether to join the 25% or be relegated to the 75%.

Posted By jonr on 29 Dec 2013 10:16 AM
What is a good investment (for you personally) depends greatly on how long you will live there.

The plan is to live here for life. But at minimum 15-20 years (until the kids leave).

Posted By Dana1 on 29 Dec 2013 12:35 PM
In a US climate zone 3 location like Atlanta, there is a long-term financial rationale for whole-wall R values in the ~R20 range. (See the climate zone 3 row of Table 2, p10 of this document read at least the whole first chapter. Those Rs are "whole assembly", with all thermal bridging acounted for, not a center-cavity value.)

Without the foam a 2x6 16" o.c. wall with fiber cement siding and cellulose cavity fill comes in at about R13-R14, if 24" o.c. it's more like R14-R15. Adding 1" of foil-faced polyiso on the exterior held in place with furring through screwed to the studs 24" o.c. with pancake-head timber screws, and using short rink-shank nails to hang the siding (short enough to not to create a thermal bridge through the foam) will get you the R20 range.

An easier/thinner wall would be to drop back to 2x4s 16" o.c. , which is about R10 without the foam.  Then 2" of EPS (R8.4) or 2" of polyiso (R12-R13) would get you there. EPS and polyiso are blown with relatively benign pentane, whereas XPS is blown with HFC134a.  XPS loses most of it's blowing agent in the first 50 years of service (and at 1400x CO2 has a significant global warming impact), at which point it's R value is about the same as EPS.  EPS loses it's pentane within weeks, and is labled at it's fully-depleted R value.  Foil faced iso hangs onto it's pentane for about 15-20 years, but it's fully depleted R is about R5.7/inch.  Also, EPS increases in performance at lower outdoor temps (about R4.5/inch at a center-foam temp of 25F, as opposed to it's labeled R4.2/inch) whereas polyiso's performance it lower than labled at the cold temperature extremes (the exact curve of which is complex), but in an Atlanta climate will always outperform the EPS.

The 2x4 + 2" approach is an easier build than 2x6 + 1", since the total wall thickness is the same as a no-foam 2x6 walls, which makes the window & door detailing easier.

Rather than dense-packing, damp sprayed cellulose has slightly higher R, and is cheaper.  The key difference is in the air-retardency, but as long as you are caulking the sheathing to the framing (recommended in either case) and detailing the sheathing as the primary air barrier (don't forget to caulk under the bottom stud plate, and between the top plates, etc.) the marginal hit in air-retardency of the lower density cellulose has no consequences.

On the roof assembly, it is far cheaper to put R50 in cellulose at the attic floor than insulating at the roof deck.  (Also, code requires a structural roof deck, not just rigid-foam + purlins.)  If insulating at the roof deck you can put as little as R5 under the purlins with the rest as fiber on the interior without needing an interior side vapor retarder. Alternatively, and inch of closed cell foam on the interior side of the roof deck with the rest as fiber works, and is easier to make air-tight if your foam installer is good at the details.  But it'll still be 2-4x the cost of R50 open blown low density cellulose on the air-sealed attic floor, with a vented roof deck. 

The only time you'd ever want to air seal and insulate at the roof deck is if you were putting ducts & mechanicals in the attic, but it may be cheaper to go with and insulated-sealed bottom floor crawl space for the mechanicals instead of the insulated roof deck approach.

BTW: The utility regulations in Georgia is in a state of flux over compensating small distributed generators (and third-party ownership of rooftop solar that would make it easy is currently explicitly disallowed) but there is a lot of political pressure pushing back on the Georgia Power monopoly to accommodate more distributed green energy.  At some point well before 2025 it is likely to be both financially viable an financially beneficial to put PV solar on the roof,  but not if the roof has sub-optimal orientation & shading factors.  It's worth at least considering these aspects before breaking ground to be sure you don't paint yourself into a corner.  According to NREL estimates, only about 25% of existing residential & commercial rooftops in the US are suitable for PV, but on new construction you can usually choose whether to join the 25% or be relegated to the 75%.

Dana,

Thanks for all your help.

Finding installers for wet blown cellulose is impossible around here. I plan to do the dense packed myself as I can't find any local installers that do anything but loose blown cellulose and/or fiber bats. I can rent the correct machine (not the cheap kind at lowes) locally to dense pack cellulose.

Locally 2x4 studs are around $3 a stud. 2x6 are only $4.20. The price difference from 2x4 to 2x6 is only a couple hundred bucks and since I'm doing the insulation the extra wall thickness is not much of a difference.

I plan to use vinyl siding over the EPS. My plan was to cut down 4x8 sheets of osb to 4" wide strips to attach the siding to over the EPS.

It would look like:

vinyl siding, 7/16 4" wide OSB, 2" EPS, tyvek, 7/16 osb, 2x6 wall, 1/2 gypsum.

If I used the 2x6 studs and went with the EPS on the outside how much would I need for my dew point (to protect my osb)? Would 2" of dense EPS be okay or would I need more since the R value is lower? If I used EPS I can do it 2 ways. One is to just get 2" think and the other would be to get 1" thick sheets and alternate the seems. Which option would be preferred if cost was the same? If using option 2 I could get 1" and 1.5" to give me a total of 2.5".

The reason for using attic trusses is so I can use the attic for bedrooms at a later date if needed. The price difference from regular 6/12 trusses and 7/12 trusses with a usable 14' wide room was $3500. So for this reason I have to insulate the roofing deck. I'm planning to use mini splits for heating and cooling so there will not be any mechanical's.

If code doesn't allow the use of purlins then I'll have OSB sheeting. So from there what is the the best way to insulate. I'd prefer to stay away or at least use as little spray foam as possible. I still worry about the off gassing (I know they say it's safe but I still worry.)

If I don't put any XPS or EPS on top of the roof deck could I just use dense back cellulose under the roof deck? Do I still need a vapor barrier or anything else?

Would the following system work?

Metal, OSB Sheeting, Attic Trusses w/ dense packed cellulose, 1/2 gypsum.

I'll also note that I plan to use a light colored metal roof. Probably galvalume or a light colored painted metal.

The way my property lays did not allow me to position it to best use the sun since I wanted a walk out basement. I plan to build a 40'x60' building on the property that I hope to try and position so the roof would be in a good position for solar. Can you point me to good info on the roof position and angle (roof pitch) for using solar. My home is positioned west to east. The west side (front door) will have a 10' covered porch. The east side (back) is only about 30' from the tree line where I have tall pines that provide a lot of shade.

Thanks again for everyone's help and input.

From a dew point control point of view, with vinyl siding (which is inherently back-ventliated) you wouldn't need ANY exterior foam, even as far north as US climate zone 5(!). With unvented siding 2" of EPS would be sufficient for dew point control on 2x6 w/cellulose even in zone 5, but not in zone 6.

See: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

The exact angle of the roof pitch doesn't much matter for PV solar, as long as the ridge axis is within 45 degrees of a true east <-->west and mostly unshaded to the south. A bit of shading on the E or W side won't affect output much, but SW and SE shade can cut into it a bit, and S side shade would cut into it a lot.

With #15 felt (rather than #30) on the roof deck and the metal roofing on purlins the roof deck is vented to the exterior, and you don't need venting on the interior side, as long as you have at least wallboard with latex paint on it as the interior side vapor retarder. The OSB + #15 felt together run about 1 perm, the interior paint about 3 perms, so the roof deck can dry in both directions, but won't load up enough during the cold winter months to matter. It is also protected by the moisture buffering capacity of the cellulose.

Rental blowers from box stores are typically 1-stage cheapies, and they don't usually rent with dense-packing hoses. It's possible to fabricate up a reducer for the 2.5" nominal hose down to 1" and use an 8-10' somewhat flexible plastic hose for your dense packing, and with a 1-stage blower you'd be able to hit 2.8-3.2lbs density, but not 3.5 (a standard dense-pack number). But even 2.8lbs should be good enough density to eliminate settling in your climate with 2" of foam over the exterior sheathing to limit the seasonal moisture cycling. On the roof it could have settling issues if you do a sloppy job, so be sure to dense pack the corners and ends VERY well. Read up on it online, and check out online videos of how to dense pack. It's not rocket science, but there are a lot of details to get right.

Posted By Dana1 on 29 Dec 2013 03:25 PM
From a dew point control point of view, with vinyl siding (which is inherently back-ventliated) you wouldn't need ANY exterior foam, even as far north as US climate zone 5(!). With unvented siding 2" of EPS would be sufficient for dew point control on 2x6 w/cellulose even in zone 5, but not in zone 6.

See: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

The exact angle of the roof pitch doesn't much matter for PV solar, as long as the ridge axis is within 45 degrees of a true east <-->west and mostly unshaded to the south. A bit of shading on the E or W side won't affect output much, but SW and SE shade can cut into it a bit, and S side shade would cut into it a lot.

With #15 felt (rather than #30) on the roof deck and the metal roofing on purlins the roof deck is vented to the exterior, and you don't need venting on the interior side, as long as you have at least wallboard with latex paint on it as the interior side vapor retarder. The OSB + #15 felt together run about 1 perm, the interior paint about 3 perms, so the roof deck can dry in both directions, but won't load up enough during the cold winter months to matter. It is also protected by the moisture buffering capacity of the cellulose.

Rental blowers from box stores are typically 1-stage cheapies, and they don't usually rent with dense-packing hoses. It's possible to fabricate up a reducer for the 2.5" nominal hose down to 1" and use an 8-10' somewhat flexible plastic hose for your dense packing, and with a 1-stage blower you'd be able to hit 2.8-3.2lbs density, but not 3.5 (a standard dense-pack number). But even 2.8lbs should be good enough density to eliminate settling in your climate with 2" of foam over the exterior sheathing to limit the seasonal moisture cycling. On the roof it could have settling issues if you do a sloppy job, so be sure to dense pack the corners and ends VERY well. Read up on it online, and check out online videos of how to dense pack. It's not rocket science, but there are a lot of details to get right.

I plan to rent a real cellulose blower and not the cheap kind from Lowes.

So your saying use 15# felt and not 30# on the roof as 15# will allow the roof the breath to the exterior? If I don't use any purlins would I still be okay? Basically if I just have the 15# felt over the OSB then just screw the metal roof right to the OSB roof deck. The metal would still allow air to move as it has raised humps formed into it correct?

Is it better/really a gain to bring 1"-2" of spray foam into the roofing system over just having 10" of dense packed cellulose. If you feel having 1"-2" of closed spray foam under the roof deck is a big + and worth the possible off gassing worries I'm open to the idea. If I don't put foam over the roof deck I won't be doing anything for thermal bridging so in that case isn't it worth putting 2" of foam on top of the roof deck? Or is worrying with thermal bridging just a waste and not something I should worry about with the roof system?

If it was your home in the south how would you do the wall and roof system? 2x6 studs and usable attic trusses are a for sure thing. How would you build the rest of the wall and roof system? Money isn't really the issue, obviously I haven't found a money tree but I think you see what I mean. What I'm trying to understand is where is a system a waste and/or not a waste in the south. It's hard to find info on building green/energy efficient in the south. Most info is on building in colder climates. I also want a safe home (I want to try and keep toxic high VOC stuff out of the home). My goal is to build a safe, comfortable, high efficient home but not waste money unneeded.

Also what is your thoughts on this question? If I used EPS for the wall system I can do it 2 ways. One is to just get 2" think EPS and the other would be to get 1" thick sheets and alternate the seems. Which option would be preferred if cost was the same? If using option 2 I could get 1" and 1.5" to give me a total of 2.5".

#15 felt runs about 1-2 perms when bone-dry, about 5 perms when sopping wet, so with a purlin ventilation gap it's a vented roof, just vented on the exterior.  It's roughly as vapor-open as latex paint, but it's variable. But #30 felt is under 0.2 perms always, on the border of being a Class-I vapor retarder, a true vapor barrier. Without the purlins it's not sufficiently vented- it needs at least inch or so of air gap for it to work.

I'm not crazy about using expensive spray foam, and the code is actually a bit wrong on it's prescriptives for unvented roofs, but under a galvalume roof your north facing pitches will be at some risk with just dense-packed cellulose unless it's reasonably vented See the zone-3 locations in Table 3 of this document  and read the whole thing until you get the true gist of the details and terms.  That document is less about the thermal bridging, but rather the moisture accumulation in an unvented roof deck.  I'd be inclined to putting an inch of foam on the exterior even WITH the purlins if going with a high solar reflectance finish like galvalume.  It might be better to just use a more absorbtive finish for the steel.

With a couple inches of foam above the roof deck you would be fine from all points of view, and that would be my personal preference if cost were no object.  But in a trussed roof with 2x4 or 2x6 rafter elements and 10-15" of cellulose the thermal bridging is taken care of by the cellulose, with but a few diagonal elements poking through the cellulose.

Staggered sheets of 1" foam with the seams sealed on both layers on the  wall sheathing is the preferred method since foam will shrink & flex over time. With overlapping layers any shrinkage has a thermal break (the other layer) over it. But if it's cheaper to do it in one 2" layer, use a bead of low-expansion "Windows & doors" type can-foam between the edges as you put them up- it'll remain flexible long enough to matter.

There are several vendors of reclaimed roofing foam in my area (including insulationdepot.com, who will ship larger lots nationally, for a price)- if you can find a similar resource near you the rigid-foam cost can be cut by 60-80%.  The quality of the reclaimed material varies, and is usually priced accordingly, but 2" roofing iso in pretty-good shape is run-of-the mill,  typically between $10-15 for a 4x8 sheet in my neighborhood.  I did my basement in 3" polyiso at $20/sheet, with maybe 5% of the lot dinged up to where you couldn't use the whole sheet, the rest nearly-perfect. Roofing EPS is somewhat cheaper per inch, comparable per-R.

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