How tall are your ceilings?

9 feet.

That sounds great. I don't know the vapor barriercautions/requirements in your area, but everything else sounds good. I assume you're putting an hrv or erv. Next step is airsealing as you go and a blower door test before you insulate so you can find and fix any leaks.

Posted By robert.thompson on 26 Feb 2013 07:03 PM
9 feet.

So for your interior walls are you going to have drywall installed?  If so, are you aware that most interior drywall is installed with the long edge horizontally and that 54" wide drywall is available if you have 9' walls so that you only have 1 joint to finish in the middle of the wall?  Unless you are doing all of the drywall installation and finishing yourself, it will generally cost you more if you deviate from standard practice such as installing the boards vertically because there will be more joints that need finishing.  And if you install it vertically, you will have to cut 12' drywall down to 9' tall unless you can find 9' drywall locally.

The reason I brought that up is that your wall internal 2x4 horizontal strapping needs to land on the joint between the sheets of horizontal drywall.  The problem with 9' walls and 54" wide sheets of drywall is that the spacing of the strapping won't match the available width of the mineral wool insulating batts you have proposed.  With blown cellulose it wouldn't be a problem but with pre-cut batts it will.

Thanks arkie6. :)

I appreciate you input - I had not thought of these drywall issues.

I'll look into the 54" drywall - just another one of the million things that I did not know!

Rob.

Rob,
As I understand your wall plan is curently:
"From outside to inside:

Horizontal siding
Vertical 1"x3" strapping
2" Rigid foam - R-10
Zip Wall Panels
2"x8" wall framing @ 16" OC filled with 7 1/4" Roxul ComfortBatts R-28
2"x4" horizontal strapping @ 24" OC filled with 3 1/2" Roxul ComfortBatts R-14
6 mil vapour barrier
1/2" Drywall. "

Comments!
The rigid foam is a vapor barrier as is the 6 mil poly, you have a classic moisture trap! There is no way for moisture between the 2 vapor barriers to dry! This WILL result in problems!!!

Using the 2x8 16" OC framing and r28 Roxul is at best r3.5 better than 2x6 framing and r22 Roxul for a lot more cost!

Moving the ZIP sheathing to the inside of the 2x6 wall and retaining the horizontal strapping but eliminating the interior poly. Gives a service cavity, better air sealing and puts the sheathing in a warmer location and allows drying to the interior. Using EPS for the exterior foam allows at least some limited drying to the outside. Going to 3" exterior foam (EPS) regains the r value lost by dropping to 2x6 framing.

Hi Liebler:

How about using Roxul 2" ComfortBoard IS rigid insulation (R-8) under the ZIP Panels, so:

1. Siding
2. 1"x3" strapping
3. 2" Roxul ComfortBoard IS rigid R-8
   
4. 2"x8" or 2"x6" studs @ 16" OC with either R-28 or R-24 Roxul ComfortBatts
5. 6 mil Vapor Barrier
6. 1"x3" stapping
7. Sheetrock

This would eliminate the moisture issue, right?

The mutineers are griping about the 2"x4" strapping (with insulation) on the inside, and, I am beginning to go over to their side because:

1. a lot of work to place 2"x4"s on edge.
2. The additional wall insulation cost, between the 2"x4"s, has a 10 year payback, not including the cost of the 2"x4"s
3. It appears that spending more money on foundation & bedrock floor insulation has a shorter payback.

I will be meeting the mutineers this weekend to finalize the wall construction so any input would be greatly appreciated.

I could be wrong, but it seems that, aside from air-sealing, the  foundation & bedrock insulation is more important for conserving energy than are the main walls and even the attic insulation.

Also, it would seem that installing an air-to-air heat pump would have the biggest impact on energy costs so the money saved on "long-term payback insulation" might be better spent on the heating unit.

Rob.

Rob,
   Your proposed alternative is r10 less than the previous wall AND puts the sheathing (which is a "smart" vapor retarder at about 1 perm dry) back out in the cold where it'll be wetter.   Attaching the horizontal 2x4s requires a good power screw driver and appropriate long screws but it is not at all difficultt though it will take some time & it can wait till the roof is on.     Regardless of the exterior foam or Roxul it'll be a much less "moisture fragile" wall if the sheathing is on the inside of the studs and you get rid of the poly.    EPS is lower cost and easier to use than the Roxul comfort board.  The moisture permeability of EPS will allow adequete drying to the outside.  If you go with the r38 ish wall I've described you should be easily able to heat with a Mitsubshi hyper heat minisplit but dropping to r25 as you are proposing, I'm not so sure.
With the sheathing on the inside caulking and sealing can get the attention it deserves.
Of course you could just build a 24" OC vertical stud wall on the inside after the outer wall is up and the roof on.
Unfortunately a ruthless economic analysis of insulation cost never justifies adding any that is a big part of why north American houses use so much CHEAP energy.

Hi Liebler:

I think that I am confused, are you suggesting, from out to in:

1. Siding
2. Strapping
3. Exterior insulation
4. 2x6 studs with insulation
   
5. Zip Wall Panels
6. 2x4 strapping with insulation
7. Sheetrock.

My problem with vapor barrier, here in Quebec, is that the BI always want it on the warm side of the inner-most studs.

(The wall I was proposing was either R-30 or R-36, depending on the studs.)

Thanks, again, for your time.

Rob.

Rob,
YES, that is what I've suggested. I'd add Tyvec or other high perm WRB either side of item 3. This puts the Zip sheathing which is a wonderful air barrier and smart vapor retarder near the ideal location for heating climates. The ideal location has more than 2/3 of the r value toward the cold side. With 3 " EPS you'd have r13.8 outside and about r 14 in the 2x6 wall for a total of r27.8 outside the sheathing and r 10 inside. even with 2" of EPS you would still have more than 2/3 of total r value on the cold side (23.2/33.2).
When you say you are proposing an r30 wall that has r8 exterior insulation and r22 between r 5.5 studs that are about 20 % of the area (an optimistic framing fraction for 16" OC)  (1/( 0.8/22+0.2/5.5)=r13.75).      My guess is you are ignoring totally the effect of the studs which is rather substantial and reduces the clear wall r to about 21.75 (r8 + r13.75) In a similar way the 2x4 strapping (r3.5) reduces the r14 between them to an effective average of r 10+ when we consider that the insulation will cover about 90 % of the area with the wood covering 10%.

Rob,
FWIW In my quick calculations of wall r values above I've used r1/" for wood which is a bit pessimistic for most dry softwoods which tend to be closer to r 1.25/". I've also ignored the r values of the sheathing (about r0.5) and drywall (about r 0.4) and air films which results in a sort of worst case r value. I've assumed r 4.6/" for EPS and r4/" for Roxu;l both are good values but the r value of EPS increases with lower temperature so it is also kind of worst case. Also it is convenient but not the most accurate to compute the r value for each layer then add them. Having done many composite wall calculations both the analytically correct way and using the layer by layer technique I know the error is well under 5%, again the simplification tends to result in a lower average r value.

I'm also curious why you are using 16" OC framing rather than 24" "advanced framing" which reduces the amount of wood hence reducing both cost and heat loss. 

Posted By arkie6 on 27 Feb 2013 07:49 AM

Posted By robert.thompson on 26 Feb 2013 07:03 PM
9 feet.

So for your interior walls are you going to have drywall installed?  If so, are you aware that most interior drywall is installed with the long edge horizontally and that 54" wide drywall is available if you have 9' walls so that you only have 1 joint to finish in the middle of the wall?  Unless you are doing all of the drywall installation and finishing yourself, it will generally cost you more if you deviate from standard practice such as installing the boards vertically because there will be more joints that need finishing.  And if you install it vertically, you will have to cut 12' drywall down to 9' tall unless you can find 9' drywall locally.

The reason I brought that up is that your wall internal 2x4 horizontal strapping needs to land on the joint between the sheets of horizontal drywall.  The problem with 9' walls and 54" wide sheets of drywall is that the spacing of the strapping won't match the available width of the mineral wool insulating batts you have proposed.  With blown cellulose it wouldn't be a problem but with pre-cut batts it will.

9' drywall should be readily available as it is used all the time in commercial applications.

Just to throw a monkey wrench into the works, have you thought of making a traditional concrete block wall, filling it with sand, and putting 4-5" or more of foam on the outside?

Thanks Liebler, MikeSolar & FBBP. :)

I did not consider a concrete block wall and will discuss this option with the 'mutineers'.

The drywall will be 54" wide and the inside strapping studs will be placed properly, whatever that is - 27" OC plus one in between for a total of 5 horizontal studs?

My proposed, and much dreamed of, double-wall is toast!

I will be building a 'compromise-wall' instead:

1. Compromise #1: it won't be all that 'super-insulated' as the center-of-cavity R-value will be only about R-36. The clear-wall & whole-wall R-values will be way south of super-insulated.
2. Compromise #2: it won't be a true 'double-wall' as I don't want to build the house single-handedly.

I believe that the 'compromise-wall' reasonably addresses:

1. Thermal bridging.
2. Sill plate insulation.
3. Foundation insulation and water-proofing.
   
4. Water vapor issues.
5. Vapor barrier integrity.
6. Air-leakage.
   
7. The construction skill level of the crew.

Aside from what can be seen in the drawing below, gaskets, caulking, tape will be liberally used to reduce air-leakage.

The vapor barrier must be placed as shown, for this wall, here in Quebec.

I do have these questions:

1. Should some sort of vapor permeable fabric or rain guard be placed over the Roxul ComfortBoard IS? If so, which product would you recommend.
   
2. How do you attach the 4" of foam to the concrete foundation and then attach the 1/2" cement board over the foam - use 6" Tapcon screws through the cement board?
   

If you see any big problems, i would appreciate it if you would let me know.

A slightly larger drawing can be found at: https://docs.google.com/file/d/0B7b9_F7Z3_0pSVZ3QXRqWHhLeGM/edit?usp=sharing

Rob.

Rob,
Your compromises are giving up a tremendous amount of heat loss primarily in the thermal bridging of the studs and exterior furring not covered by the exterior Roxul. The uninsulated "service cavity" on the inside offers essentially no insulating value for a lot of added work! If you are not going to insulate the service cavity put ordinary OSB on the inside and cover it with poly. before adding the strapping. The exterior insulation needs to continue over the studs or the thermal bridge is severe. As shown with 25% framing fraction, typical for 16"OC 1/ (0.75/36 + 0.25/9.25)=r21 Does this even meet code?
If you use Zip sheathing on the outside ( which I strongly discourage) you don't need any other WRB even with exterior Roxul. But exterior Roxul's effectiveness is reduced by "wind washing" so covering it with Tyvec is good practice and probably worth while. However if you move the sheathing to the warm side you'll definitely need an exterior WRB, such as Tyvec either over or under the exterior insulation which I'd rather see as EPS, Again why not 2x6 24" OC advanced framing!
If I were building it I'd put the sheathing on the inside, use 2x6 24" OC wall framing, use 24" OC for the strapping and put Roxul bats in the service cavity as well (I might add an extra strap mid wall for drywall support), use EPS exterior sheathing covered with Tyvec. With 2" EPS I'd have an r35 wall or with 3" close to r 40.
You might save real $ long term by hiring a less mutinous crew! At least for the framing and insulation, which I recently read is only about 15% of a typical house's cost.

2x8 studs on 16" centers just seems wrong to me. I seriously doubt you will find pre-cut 9' (104-5/8") 2x8 studs, so you will have to buy 10' lumber and cut every one to length. Seems like such a waste of material and labor. And what is the quality of your 2x8 lumber? Is it straight enough to make decent studs? Down here, 2x8 are rarely if ever (I've never seen it) used for studs. In my area, 2x8 are generally not as straight as pre-cut 2x4 or 2x6 studs.

Why are you stuck on Roxul batts? Is wet sprayed cellulose not available in your area? It has slightly lower R value per inch but is better at air sealing and moisture management.

For the exterior 2" rigid insulation, you would get more R value if you used rigid polyiso foam board at ~R6.5/inch aged R value. That 2" on the exterior would then go from R8 to R13. But you would need to find a polyiso foam board that doesn't have near zero perm facers like aluminum since you have a vapor barrier on the interior. Another option would be to go with 3" of EPS foam which has an R value of ~4/inch @ 75F (and the R value goes up as the temperature goes down). That would give you at least R12 under all conditions you would likely experience. And the vapor permeability should be high enough so as to not create a moisture trap with your interior vapor barrier. If you used multiple layers of EPS exterior foam with staggered seams, I don't really see the need for installing ZIP sheathing and tape. Plus the Zip panels and tape cost about twice as much per square foot as standard 7/16" OSB.

Regarding your interior horizontal 2x3 strapping. Are you aware that interior drywall does not set on the floor but is pushed tight at the top against the ceiling board and this generally leaves the bottom of the sheetrock ~1/2" off the floor. The weight of this sheetrock hanging on your horizontal strapping will tend to want to roll it downward and result in cracks at the ceiling joint. You will have a horizontal strap at the subfloor, but you will only have a small amount of contact and only 1 screw or nail at this point. Over time I would be concerned with sag and cracking of the drywall.

With your 2x3 horizontal strapping, you have 2.5" of wall depth there that has no insulation. Seems like a waste of wall space to me.

Here is an idea. Install typical 2x6 exterior studs at 24" on center, then run 2x3 or ripped 2x4 horizontal strapping at 27" on center for your 9' walls, then strap over that with vertical 2x3 or ripped 2x4 at 24" on center. Around here, 2x3 are only marginally less expensive than 2x4, so you can buy 2x4 and rip them in half yourself and save some $.

You could fill the 2x6 with Roxul batts prior to installing the interior strapping or wet sprayed cellulose after everything is in place. Run your pex and electrical through the chase created by the horizontal 2x3 or ripped 2x4. Then after the electrical and plumbing is installed, have wet sprayed cellulose installed to fill the entire cavity or the last 3" created by the horizontal and vertical strapping if you elect to fill the 2x6 with Roxul. Around here it costs ~$1/sq ft for wet sprayed cellulose.

The advantage to this approach is that you get your crew to help with building the standard 2x6 exterior walls and getting the home in the dry. 2x6 @ 24" on center uses less lumber that is already pre-cut to length and reduces R losses compared to 16" on center. Then you can come back and easily install the horizontal and vertical strapping by yourself or with help if it is available. The horizontal and vertical interior strapping creates a thermal break with the 2x6 studs except for small contact areas where they cross. And your entire wall cavity of 8.5" is filled with insulation compared to your 9.75" wall cavity with only 7.25" filled with insulation.

Hi Liebler:

Here is a better drawing:



Rob.

Hi Liebler & arkie6:

Thank you for your thoughts. :)

I am 'digesting' them!

The idea for the 2"x3" was to protect the vapor barrier when the electrical boxes were installed and to eliminate the drilling of the 2"x8" and passing wires through the vapor barrier - maybe it's not too bright.

No, I have not been able to locate a 'wet sprayed cellulose' contractor in my area but, based on what you two are saying, I will look harder.

Like I said, I am 'digesting' all that you have said and am rethinking my 'compromise-wall'...

Thanks,

Rob.

I don't think the 2x3 is a dumb idea. we are always trying to find ways to keep the vapour barrier intact and this one works especially well if there is a radiant barrier reflecting energy back to the drywall.

BTW, my next house will be masonry construction, using Ytong, with foam on the outside. The mutineers won't like it though. haha.

Thanks Mike, I'll look into reflective VB as well.

Here is the sub super-insulated wall that I will go with, unless, (aside from low insulation values) someone can spot any flaws:



Notes:

1. Gaskets, caulking & tape will be used everywhere.
2. The Tyvec, EPS & OSB will run over the top plate & up the heel of the roof trusses.
3. The inside strapping will be as arkie6 suggested.
4. If budget allows, the inside strapping spaces will be sprayed with wet cellulose and then VB applied. (or reflective VB will be used - I don't know the cost)

As pictured above, the wall insulation (R-30.8) will cost about $1,650, the foundation wall insulation (R-20) about $1,600. Add to this bedrock (R-12) & rim joist spray foam of about $2,800 and attic cellulose (R-60) at about $2,500. Total: $8,550 for insulation.

My little little heating cost calculator says the it will cost $1,300 in baseboard electricity, or, $540 in heat-pump electricity - in a perfect world. As long as it stays around $1,500, I'll be OK. One of those 'super-duper' fireplaces or stoves is in the budget for about $2,500 and that should help.

Thanks again to all of you for your help, I really appreciate your time.

Rob.