Good morning.

I am trying to settle an argument between many online conversations (and also learn for future build) about which wall is to perform better for the amount of a.) labor and b.) cost.

Both options are 16" O.C. framing.

Option 1:
Siding
1x3 firring strips
House Wrap
2" Rigid Foam (EPS or XPS in this case is fine)
*optional secondary house wrap layer due to concern of moisture behind foam against sheathing*
7/16" OSB
A 2x6 Framed wall with R-23 5.5" Mineral Wool Insulation
No Vapor Retarder or Air Barrier for that matter
1/2" Drywall

I would suggest that the 1st option may perform better due to the foam preventing thermal bridging, however it would cost more in the foam insulation, and slightly less in mineral wool and 2x6 lumber.  The labor would be quite a bit more intensive to detail not only the foam install, but the firring strips, window installs, and the sort.  House wrap over the foam is a requirement in my theory because of detailing window openings.  I would not do the house wrap under the foam *only*.  I would consider two layers to sandwich the foam.

Option 2:
Siding
House Wrap
7/16"OSB detailed to a degree, at least osb seams caulked
2x8 Framed walls with R-30 7.25" Mineral Wool Insulation
CertainTeed Smart Membrane
1/2" Drywall

This option would build easier, the materials would be cheaper overall due to no foam.  The Membrane would add additional cost but not very much, it would add more time than no membrane.  However, I believe the membrane can be detailed to serve as an air barrier allowing improvement there.  Personally I am on the fence between which option would be better. 

I do not want to introduce other options in this variable necessarily...such as staggered 2x4 with cellulose (creates too thick of wall for the comparable R-Value), or more costly methods like spray foam ($$$).

The biggest issue with the 2nd option - the 2x8 wall - is lots of thermal bridging which will lower performance. (plus your wall consists of 20% (+) R-7 wood, rather than insulation). Using two 2x4 walls with 1"-2" between will help. And, unless you are installing plastic siding, you should use a rain screen (strapping).

Bob - The issue with staggered stud walls is it limits down to one type of insulation - blow in cellulose (or FG). The cellulose, even at dense fill, is less R-value per inch than Mineral Wool, and it's not as appealing for that reason. It normally takes a pro to blow in dense cellulose (properly, where it doesn't settle and to prevent cavities) compared to the average builder to install mineral batts. If using Cellulose, is there a need for a membrane, such as the same used for mineral wool?   I do see value in double 2x4 wall if you make it thick enough. This limits the interior floor space significantly though if trying to keep a minimal sizes house.

Interesting to know that other sidings need a rain screen, I thought it was only needed for certain brands/types (and of course if fastening over foam).

All in all, you are suggesting the 2x6 with 2" of foam is better than 2x8 wall?  And that a 2x4 staggered wall is even better than the 2x6 w/ 2" foam? In terms of cost and labor?

A wall with the proper amount of foam is better insulation than anything else in that it keeps the whole wall warm. But it's not the least labor, nor the least expensive - that's a double wall with cellulose. (Actually, FG is cheaper, but it's lousy stuff). And you can use mineral wool in any wall where you can use cellulose or FG. Whatever wall you chose, thermal bridges are a big deal.

A rain screen provides an air space so that the siding can breath. Plastic obviously doesn't need to breath, and there is plenty of air behind it anyway.

Bob, my locale would not have blow in mineral wool. I can only special order mineral wool in batts (up to 7.25" thick) which fits nicely between a 2x8 wall. How do you mean it can be used anywhere cellulose can? A double 2x4 wall would also be staggered, right? That messes up the batt placement on top of the fact the wall would be >7.25" thick, or am I missing something...?

If you are building a 7-1/4" wall with solid plates and staggered studs, then the studs would normally be staggered so they don't touch. Studs in a typical double wall do not need to be staggered as the air space between the walls is all the thermal break you need. In the first scenario, the solid plates form a thermal bridge. (as does the subfloor and floor joists) To be clear, the best way to achieve a thermal break is with a solid layer of foam on the exterior, but without the foam it's a matter of choosing the best path with the least thermal bridging. Best, but not the least expensive way to make up for the loss of space from thicker walls is to increase foundation size slightly.

If you are building a 7-1/4" wall with solid plates and staggered studs, then the studs would normally be staggered so they don't touch. Studs in a typical double wall do not need to be staggered as the air space between the walls is all the thermal break you need. In the first scenario, the solid plates form a thermal bridge. (as does the subfloor and floor joists) To be clear, the best way to achieve a thermal break is with a solid layer of foam on the exterior, but without the foam it's a matter of choosing the best path with the least thermal bridging. Best, but not the least expensive way to make up for the loss of space from thicker walls is to increase foundation size slightly.

A SIP wall will out perform any of those methods in one simple process

Where do you find straight 2x8 studs? Down here, 2x8 material is all structural southern yellow pine (SYP) at full 8' or longer lengths which would require cutting every stud to the proper length. 2x4 or 2x6 pre-cut studs are typically spruce-pine-fir (SPF) which is generally much straighter than SYP and require no end cutting. Unless you have pre-cut 2x8 studs in your area, you would need to factor the cutting time into your equation. And the fact that the walls would likely not be as straight.

Can you get damp sprayed cellulose in your area (cellulose with dry starch adhesive that is activated by a small amount of water added via nozzle at the end of the spray hose)? That is probably the most commonly used wall insulation method used down here (Arkansas) and the best bang for the buck.

Have you looked at a Mooney Wall approach? With 2x6 exterior studs and ripped 2x4 inside strapping, that would give you a 7" wall cavity with minimal thermal bridging. But you really need a spray-in or dense packed cavity insulation with this approach. Also with the Mooney wall you could increase your exterior 2x6 stud spacing to 24" on-center which further reduces cost and thermal bridging. I would keep the interior strapping at 16" for drywall support. Just make sure if you install your drywall run horizontally that you have a strap at the joint between the sheets. Also with the Mooney Wall, wiring is simplified since you don't have to drill holes through the studs - just attach the wiring to the face of the 2x6 studs between the 2x2 strapping.

http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm

A 2x8 wall filled with 7.25" of R3.7/inch fluff like cellulose comes in at about R19 "whole-wall" after adding in the performance of the sheathing, siding, gypsum & air films etc. and adjusting it for the thermal conductivity of the framing.

Bumping it to R30 rock wool instead of ~R27 cellulose it comes in a bit shy of R21.

If you bump the spacing to 24" o.c. and make a bunch of other detailed revisions to get the framing fraction down to 17% instead of the typical 25% for 16" o.c. framing it comes in around R21 whole-wall for cellulose, R23.5 for R30 rock wool

A 2x6, 16" o.c wall filled with R3.7/inch fluff (R20 total) with 2" of exterior EPS comes in at about R24 whole-wall. With R23 rock wool it's a bit over R24.

If the spacing is bumped to 24" o.c. for a 20% framing fraction (not to tough a design hurdle) it comes in bit over R25 with cellulose, R25.5 with rock wool.

Unless you are in US climate zone 5 or lower having only R8.2 of foam on the exterior, IRC code would require something more vapor tight than interior latex as a vapor retarder. If using 2"/R10, XPS, discount it to it's fully depleted R8.2 @ 2", since that's about where it will be in a few decades when most of the HFC blowing agents have escaped (doing their climate damage.)

An interior air barrier is not a separate layer, it's a matter of detailing the drywall to be air tight, and that is CRITICAL to keeping the sheathing dry if you are in US climate zone 5 (or higher). This is true with either 2x8 or 2x6 + foam construction, particularly with rock wool, which has very little moisture buffering capacity compared to cellulose.

Detailing the exterior sheathing as the primary air barrier is also critical to getting the performance out of the rock wool.

If doing the furring on foam, use 1x4s, not 1x3, which tends to be significantly more twisted, dog-legged, and has a much higher propensity to split.

At 2", unfaced EPS is about 1.5 perms, which is about 2x as vapor open as dry 7/16" OSB. With a rainscreen gap has at least some capacity for the OSB to dry toward the exterior. Most XPS will be 0.6-0.8 perms at that thickness which is about as tight as dry OSB, with only a limited drying capacity, rainscreened or not.

cmkavala, Isn't thermal bridging a factor with SIP's?

SIPs have top & bottom plates, as well as framing around windows etc as well as splines, but the framing fraction is substantially lower than any framed structure. The marketing fluff doesn't always reflect the true U-factor/whole-wall-R, usually only the R-value of the foam itself, sometimes even exaggerated by using the higher performance of foam tested with a mean temp of 40F (which is fine if your average wintertime outdoor temp is +10F), but it can be a quick and straightforward way to build. Sheet metal clad SIPs have thermal bridging issues that can lead to condensation in truly cold climates, due to the higher thermal conductivity of metal compared to OSB. A serious gotcha with OSB clad SIPs in cold climates has to do with adequate long term air sealing of the seams- you have to take it VERY seriously in US climate zones 5 & up.

Posted By Dana1 on 19 Dec 2016 04:17 PM
SIPs have top & bottom plates, as well as framing around windows etc as well as splines, but the framing fraction is substantially lower than any framed structure. The marketing fluff doesn't always reflect the true U-factor/whole-wall-R, usually only the R-value of the foam itself, sometimes even exaggerated by using the higher performance of foam tested with a mean temp of 40F (which is fine if your average wintertime outdoor temp is +10F), but it can be a quick and straightforward way to build. Sheet metal clad SIPs have thermal bridging issues that can lead to condensation in truly cold climates, due to the higher thermal conductivity of metal compared to OSB. A serious gotcha with OSB clad SIPs in cold climates has to do with adequate long term air sealing of the seams- you have to take it VERY seriously in US climate zones 5 & up.

There's actually a thread going on right now just about that topic you touched on.

Interestingly, the house across the street from my ICF build was built using traditional 2X6 construction where convenient (around doors, windows and short walls) and using 5-1/2" SIPs (staggered 2X4 studs, 2X6 top/bottom plates. They built the walls on the ground, leaving huge gaps where they could stick in 4' wide SIP panels. In some cases there are 3 SIP panels together, totaling 12'. When they put the sips into the spaces, there were huge gaps. So on my occasional tours of the project I watched how carefully they did or didn't fill gaps and air seal the exterior OSB. Really poor job in the end. Its a shame, as the owners we have met since are great folks who really care about efficiency. They paid a bunch to have a tight house, and I don't think they got one. They were across the country during construction, and they totally missed any opportunity to have things done correctly. Its a shame that a supposed hit-end builder can't be trusted to do the job properly. But, it was the final motivation for me to build my own house.

SIPS are not practical here. I priced them and they range from 5.72 to 6.00+ a square foot while a 2x8 with mineral wool is 3.22 and a 2x6 w/ 2" of foam is 3.86 per square foot. A SIP home is too non-conventional here. And the reason being is this is not a dense populous area. I think I would consider a double stud wall either sharing the same 2x8 plates or only spaced 1/2" apart between each 2x4 plate so I can still use the mineral wool insulation. If a reputable cellulose is found, I'd maybe consider that but all I see around here and even Menards is just the blow in cellulose and borrow the blower hopper when you purchase. How is a DIYer to get the dense pack or wet blown type?

Posted By CTSNic on 23 Dec 2016 09:32 AM
SIPS are not practical here. I priced them and they range from 5.72 to 6.00+ a square foot while a 2x8 with mineral wool is 3.22 and a 2x6 w/ 2" of foam is 3.86 per square foot. A SIP home is too non-conventional here. And the reason being is this is not a dense populous area. I think I would consider a double stud wall either sharing the same 2x8 plates or only spaced 1/2" apart between each 2x4 plate so I can still use the mineral wool insulation. If a reputable cellulose is found, I'd maybe consider that but all I see around here and even Menards is just the blow in cellulose and borrow the blower hopper when you purchase. How is a DIYer to get the dense pack or wet blown type?

..... so the insulation is installed for free?

Some sources suggest that damp spray cellulose with adhesive isn't too hard to DIY if you can rent the equipment.

Posted By cmkavala on 23 Dec 2016 09:40 AM

Posted By CTSNic on 23 Dec 2016 09:32 AM
SIPS are not practical here. I priced them and they range from 5.72 to 6.00+ a square foot while a 2x8 with mineral wool is 3.22 and a 2x6 w/ 2" of foam is 3.86 per square foot. A SIP home is too non-conventional here. And the reason being is this is not a dense populous area. I think I would consider a double stud wall either sharing the same 2x8 plates or only spaced 1/2" apart between each 2x4 plate so I can still use the mineral wool insulation. If a reputable cellulose is found, I'd maybe consider that but all I see around here and even Menards is just the blow in cellulose and borrow the blower hopper when you purchase. How is a DIYer to get the dense pack or wet blown type?

..... so the insulation is installed for free?

Look, I know you are a pro sips person.  But metal sips have condensation issues and make popping noises in my extreme temperature range.  And the wooden sips are more labor intensive to install than framing a wall and lifting it in pieces I work with as a single person.  Installing insulation is sweat equity I'd rather have than pay more for the panels and then fasten them together and hire someone to unload them then have help moving them in place on the decking and lifting them.  I don't mind framing time.  I'm about saving and practicality. So what does your statement mean?

Posted By CTSNic on 23 Dec 2016 11:59 AM

Posted By cmkavala on 23 Dec 2016 09:40 AM

Posted By CTSNic on 23 Dec 2016 09:32 AM
SIPS are not practical here. I priced them and they range from 5.72 to 6.00+ a square foot while a 2x8 with mineral wool is 3.22 and a 2x6 w/ 2" of foam is 3.86 per square foot. A SIP home is too non-conventional here. And the reason being is this is not a dense populous area. I think I would consider a double stud wall either sharing the same 2x8 plates or only spaced 1/2" apart between each 2x4 plate so I can still use the mineral wool insulation. If a reputable cellulose is found, I'd maybe consider that but all I see around here and even Menards is just the blow in cellulose and borrow the blower hopper when you purchase. How is a DIYer to get the dense pack or wet blown type?

..... so the insulation is installed for free?

Look, I know you are a pro sips person.  But metal sips have condensation issues and make popping noises in my extreme temperature range.  And the wooden sips are more labor intensive to install than framing a wall and lifting it in pieces I work with as a single person.  Installing insulation is sweat equity I'd rather have than pay more for the panels and then fasten them together and hire someone to unload them then have help moving them in place on the decking and lifting them.  I don't mind framing time.  I'm about saving and practicality. So what does your statement mean?

CTSNic,
So the point is .....you put no value on your own labor when you do an analysis of a total wall system cost, I am not trying to push metal SIPs on you but, before I would be framing a wall with double studs, staggered studs , foam sheathing or mineral wool, I would certainly be taking a hard look at OSB SIPs instead.
your not going to be assembling and lifting 12 to 16' stud walls with out help anyway, that being said, it only takes a couple of guys to move a SIP wall into place , it seems like a no brain'er to me?
they are absolutely less labor intensive to install than wood framing, you can't get a better wall system than a SIP wall
Have you ever installed mineral wool, if not your in for a real treat.

Posted By cmkavala on 23 Dec 2016 01:34 PM

Posted By CTSNic on 23 Dec 2016 11:59 AM

Posted By cmkavala on 23 Dec 2016 09:40 AM

Posted By CTSNic on 23 Dec 2016 09:32 AM
SIPS are not practical here. I priced them and they range from 5.72 to 6.00+ a square foot while a 2x8 with mineral wool is 3.22 and a 2x6 w/ 2" of foam is 3.86 per square foot. A SIP home is too non-conventional here. And the reason being is this is not a dense populous area. I think I would consider a double stud wall either sharing the same 2x8 plates or only spaced 1/2" apart between each 2x4 plate so I can still use the mineral wool insulation. If a reputable cellulose is found, I'd maybe consider that but all I see around here and even Menards is just the blow in cellulose and borrow the blower hopper when you purchase. How is a DIYer to get the dense pack or wet blown type?

..... so the insulation is installed for free?

Look, I know you are a pro sips person.  But metal sips have condensation issues and make popping noises in my extreme temperature range.  And the wooden sips are more labor intensive to install than framing a wall and lifting it in pieces I work with as a single person.  Installing insulation is sweat equity I'd rather have than pay more for the panels and then fasten them together and hire someone to unload them then have help moving them in place on the decking and lifting them.  I don't mind framing time.  I'm about saving and practicality. So what does your statement mean?

CTSNic,
So the point is .....you put no value on your own labor when you do an analysis of a total wall system cost, I am not trying to push metal SIPs on you but, before I would be framing a wall with double studs, staggered studs , foam sheathing or mineral wool, I would certainly be taking a hard look at OSB SIPs instead.
your not going to be assembling and lifting 12 to 16' stud walls with out help anyway, that being said, it only takes a couple of guys to move a SIP wall into place , it seems like a no brain'er to me?
they are absolutely less labor intensive to install than wood framing, you can't get a better wall system than a SIP wall
Have you ever installed mineral wool, if not your in for a real treat.

Considering you are a "SteelSipMan" you either sell or build with them so your views will be biased.  I don't appreciate the negative connotation towards the subject.  I put value in my time.  I enjoy building more than I enjoy a day job - so that being said, I would much rather build a house than work another job.  I put value on it, but I am looking at material cost.  I have worked with mineral wool.   How is working with steel sips less labor intensive than framing a standard 2x6 wall?  I don't see it. Not in my region.  What about the expanding/contracting noises? Metal working tools? Condensation from below 0 temps and warm interior? Unloading equipment? Splices? Thermal bridging? Wiring and plumbing?  ALL AT A HIGHER cost?