Cornfused - Exterior wall insulation/construction

harp24February 17, 2012

Hi all - First time poster, long time reader.

My question relates to insulation of a new home we will be building in Central Illinois. I've researched this high-and-low, and the more I read the more I am confused.

I'll be acting as the GC and partial builder on this house, and I'm interested in hearing your thoughts on this exterior wall insulation plan.

Starting from the inside wall out:

1. Drywall

2. 2x4 studs with blown cellulose

3. 7/16 OSB

4. Tyvek or equivalent

5. 1" XPS

6. Exterior siding currently undecided. We have to use some brick or stone and the rest will be vinyl, fiber cement, or cedar lap.

Although I think this would make for a fairly tight home with low thermal bridging, I'm worried about flashing of windows to the foam and also where the condensation point could occur inside the wall.

Any thoughts on this plan would be appreciated!

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renovator8

If possible the cellulose should be the Dense-Pak kind.

Building science is a good reference

Here is a link that might be useful: building science wall section

    Bookmark   February 18, 2012 at 6:29AM
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renovator8

Building Science window installation

Here is a link that might be useful: Building Science window install

    Bookmark   February 18, 2012 at 6:30AM
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david_cary

I am surprised that wall would be code since that is r-17 and I would think r-18 would be code (although with foam you would be better than 2x6 with batts but rules are rules). I used to live in central Il (as a kid) and in less it has gotten a lot warmer, you should consider some more insulation.

I think I would shoot for at least mid teens and you are really only at r-13. 2 inches of foam or a double wall would get you there.

Generally a better envelope will pay for itself over time (particularly in your climate).

What are you heating with? Say you are using NG which is super cheap - what are the possibilities that fracking gets shut down because of some catastrophe? What do you think would happen to NG prices?

    Bookmark   February 18, 2012 at 8:24AM
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renovator8

The IECC minimum requirement for central IL is R-20 for a conventionally sheathed wall and R-13 + R-5 for cavity insulation + insulated sheathing which dense-pack cellulose and the additional structural sheathing should provide.

2x6 studs would be the easiest way to increase the R value of the wall to where it ought to be but the windows, roof, and the air-barrier are important too. The roof/ceiling insulation should be R-38.

How will the exterior cladding be installed?

    Bookmark   February 18, 2012 at 11:40AM
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harp24

Thanks for the comments!

The reason that I thought cellulose(R13 in 2x4 wall) and 1" of XPS(R5) would be adequate is because our current house, which is 5 years old, has only R13 Fiberglass batts. It seems to do OK, but are you saying that it wasnt built to code? Or maybe code has changed recently?

I really want to do the XPS to handle the thermal bridging. Would it be preferable to do 2x6 wall with cellulose and forget about the 1" XPS?

I do plan to be very vigilant with the caulk and air sealing.

We are doing geothermal heat.

If we go with vinyl, I'd just attach through foam into osb with nails. If we go fiber cement or wood, I might consider furring it out. I honestly havent looked into that yet...

Craig

    Bookmark   February 18, 2012 at 12:34PM
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worthy

If we go with vinyl, I'd just attach through foam into osb with nails. If we go fiber cement or wood, I might consider furring it out. I honestly havent looked into that yet...


"Water management, smanagement!"
Here's what it looks like a few years down the road,
as the OSB rots away. Photo: http://buildingenvelope.wordpress.com

Water management systems are essential for all the claddings you're considering. That's even if all the trades you see around your neighbourhood are just banging in the nails and caulking it tight.

Here is a link that might be useful: Water Managed Wall Systems

    Bookmark   February 18, 2012 at 1:33PM
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brickeyee

"The reason that I thought cellulose(R13 in 2x4 wall) and 1" of XPS(R5) would be adequate is because our current house, which is 5 years old, has only R13 Fiberglass batts. It seems to do OK, but are you saying that it wasnt built to code? Or maybe code has changed recently? "

Code may have changed, or your AHJ operates under local alterations to the model codes.
A lot of folks on this site seem to forget that many states allow changes by localities to the various model codes.

In any case, new work must comply with the code in effect when the work is performed or permuted (with allowances for work that spans a code change).

    Bookmark   February 18, 2012 at 3:07PM
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flgargoyle

It won't cost much more to go 2X6 in the walls, keeping the foam outside as well. If you go advanced framing technique, there are less studs, so the cost difference would be negligible.

In short, building codes in most areas have changed a LOT in recent years, and will continue to do so. For example, some states now require residential sprinkler systems, and blower door testing for air leaks. Better read up on your local codes before you make any decisions.

    Bookmark   February 18, 2012 at 4:04PM
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robin0919

Have you looked at ICF?

    Bookmark   February 18, 2012 at 7:17PM
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energy_rater_la

I wouldn't give up the foam sheathing boards on the exterior. see buildingscience.com factsheet titled
building a perfect wall.
putting the foam sheathing to exterior will stop thermal bridging. as long as seams are taped and any holes are repaired before the cladding is installed ..this is
your first step to an air tight wall.
use seal sill or caulk sole plate to slab
insulate conventionally
air tight drywall to interior.

window/door flashing details are done with pan flashings
and flashing tapes.

what insulation for attic?

best of luck.

    Bookmark   February 18, 2012 at 7:50PM
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lzerarc

What is the point in BARELY meeting code? Why not spend $1-3k more on contruction and get a house that will save you money over the entire life?
Going 2x6 is a must in my opinion in any heating climate. However if you go with exterior foam, you need to increase the exterior foam from 1" to 1.5" to keep your dew points at the sheathing plane safe. Either using 1" or 1.5", I would still recommend a strapped siding/cladding, especially using it over foam to get a drainage plane.
Spend the few extra k up front to get a better, tighter shell and it will pay you back forever.

    Bookmark   February 18, 2012 at 9:08PM
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harp24

Flgargoyle, thanks for bringing that up about the 2x6 walls. I should give that more thought. It would be nice to have thicker walls with less lumber.

Robin, I have a neighbor/custom home builder who has been using ICF ARXX for many years. I do plan on talking to him about doing the basement. Don't think I'll go that route with the 1st and 2nd floor walls, though.

energy rater la, YES! I have looked at the 'perfect wall' on building science. This is what I think will work best for us.

However, I still see some folks advocating for putting Tyvek on the outside of the XPS for flashing purposes. This actually seems to make more sense to me, but maybe the downside would be possible condensation between XPS and OSB?

Attic will be blown in, minimum r-38. Again, plan to caulk and foam wherever I can up there.

Craig

    Bookmark   February 18, 2012 at 9:29PM
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robin0919

Why would you not want the 'whole' house in ICF?

    Bookmark   February 18, 2012 at 10:18PM
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Alex House

Have you looked into double wall construction?

Pros:
More insulation
No penetrations of vapor barrier
Break in thermal bridging between the two walls.
Added depth allows use of cheaper forms of insulation

Cons:
More wood needed for interior wall.
Greater use of floor space.

    Bookmark   February 18, 2012 at 11:43PM
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david_cary

Brickeye - I really can't imagine Il allowing local jurisdictions to downgrade their wall requirement to r-13. I would suspect most states allow local to more stringent but not less so. Il is a big state with big state regs.

A 5 year old house with R-13 in Il really surprises me so what do I know?

Generally with geo, shell increases to a point are cost neutral or save money because you can decrease the size of the geo. Now if you are cooling dominated then changes to reduce size are mostly focused on windows and require compromises that many don't want to do. But in a heating dominated area, you can spend $3k increasing the walls and save $5k on the geo unit (as an example). All that and your electric bill will be less. Win-win.

Of course how far to go depends on your geo install costs. So just speaking upfront money probably dictates 1.5 inch of foam on a 2x6 wall if I had to guess. And then long term costs probably favor a double wall or 2 inches of foam on a 2x6.

Money is cheap now and future utilities cost are probably not.

    Bookmark   February 19, 2012 at 6:30AM
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renovator8

The IECC minimum requirement for central IL is R-20 for a conventionally sheathed wall and R-13 + R-5 for cavity insulation + insulated sheathing which dense-pack cellulose and the additional structural sheathing should provide.

2x6 studs would be the easiest way to increase the R value of the wall to where it ought to be but the windows, roof, and the air-barrier are important too. The roof/ceiling insulation should be R-38.

How will the exterior cladding be installed?

    Bookmark   February 19, 2012 at 8:55AM
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flgargoyle

Not to hijack the thread, but I'm having a problem answering Renovator8's question- How will the cladding be installed? I see references all over about putting 1X4 furring over rigid foam insulation, and then fiber cement siding over that. The problem is- the manufacturers all call for 1-1/4 nail penetration into solid wood- pretty hard to do in wood that is actually only 3/4" thick. No one- not Building Science, not JLC, not building codes- no one seems to specifically answer this question. Building Science hinted that it would have to be 'engineered'.

Since this seems to be a rapidly expanding construction technique, it seems like there should be a prescriptive answer. Anyone got any ideas?

    Bookmark   February 19, 2012 at 9:39AM
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lzerarc

I use Hardi board on 1x3 on my exterior foam projects. This has been approved by Hardi as an acceptable attachement method.
I prefer the double stud wall as it is a wash (sometimes cheaper depending on desired r value of wall) compared to exterior foams. The details and attachement of windows and siding is no different then a typical sheathed 2x4/2x6 wall. One can avoid strapping costs and time, foam flashing at window details, etc. I usually always use ZIP sheathing as the exterior air barrier and never mess with house wraps.

    Bookmark   February 19, 2012 at 11:36AM
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worthy

The problem is- the manufacturers all call for 1-1/4 nail penetration into solid wood- pretty hard to do in wood that is actually only 3/4" thick.

You nail through the furring into the wall framing.

    Bookmark   February 19, 2012 at 12:09PM
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flgargoyle

Then you would need a really long nail- there's foam to go through, too. I've seen houses with 4" foam on the outside! I don't plan to go that thick, of course. With one inch foam, it adds up to a 3+" nail. I'd have to use framing nails.

I'll have to run the numbers of exterior foam versus double stud walls. I'm doing the labor, so it will boil down to material costs. I'm building in upstate SC- a fairly mild climate, so I don't need to go crazy on insulation. I plan to use Zip sheathing as well.

    Bookmark   February 19, 2012 at 12:27PM
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renovator8

Installation instructions:

⢠HardiePlank lap siding can be installed over braced wood or steel studs spaced a maximum of 24" o.c. or directly to minimum 7/16" thick OSB sheathing. Irregularities in framing and sheathing can mirror through the finished application.

⢠HardiePlank lap siding can also be installed over foam insulation/sheathing up to 1" thick. When using foam insulation/sheathing, avoid over-driving nails (fasteners), which can result in dimpling of the siding due to the compressible nature of the foam insulation/sheathing. Extra caution is necessary if power-driven nails (fasteners) are used for attaching siding over foam insulation/sheathing.

FACE NAILING

Nails - Wood Framing
⢠6d (0.113" shank x 0.267" HD x 2" long)
⢠Siding nail (0.09" shank x 0.221" HD x 2" long)

OSB minimum 7/16"
⢠Siding nail (0.09" shank x 0.221" HD x 1-1/2" long)

BLIND NAILING

Nails - Wood Framing
⢠Siding nail (0.09" shank x 0.221" HD x 2" long)
⢠11ga. roofing nail (0.121" shank x 0.371" HD x 1.25" long)

OSB minimum 7/16"
⢠11ga. roofing nail (0.121" shank x 0.371" HD x 1.75" long)
⢠Ribbed Wafer-head or equivalent (No. 8 x 1 5/8" long x 0.375" HD).

    Bookmark   February 19, 2012 at 1:54PM
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renovator8

⢠is the Garden Web's version of a bullet point

    Bookmark   February 19, 2012 at 1:56PM
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SpringtimeHomes

Welcome to the confusing and controversial world of building envelopes. To further simplify and confuse: Airtight is most important, and thermal bridging is more important than R value (to a point).

Fine Homebuilding has a new Lstibureck article out on foam sheathing that is a must read for those interested in this topic. A couple takeaways: choose XPS over EPS. Strongly consider a rain screen.

Upgrading to 2x6 gets you another 2" of CAVITY insulation. If youre considering foam sheathing then hopefully you know the many weaknesses of cavity insulation. I would prefer thicker foam sheathing staying with 2x4. I would also put the housewrap on the outside of the foam. Its not doing you much good between the sheathing and foam. If you get the right thickness of foam sheathing(as jlizer suggests is 1.5") then youre sheathing will generally be kept warmer than condensation temps. The real challenge of course is preventing what happens in worthy's pic. This is possible to do with tapes (ZIP system, others) and sealants but having a house wrap to integrate with flashing is a nice detail that doesnt rely on a particular chemical companies formulation.

Havent done the double stud wall yet but to me it seems more trouble than its worth. It doesnt reduce thermal bridges at plates or your floor bands and eats a tremendous amount of interior floor space while reducing the amount of light (and heat for passive solar) that your windows let in.

I would consider the ICFs for basement.
@Robin; not everywhere else because they are expensive and dont take to changes very well.
Better yet, look at pre-cast concrete walls (Superior is one brand)

The complications listed above with wall thicknesses and furring for siding is one of the many reasons we have been using SIPS.

Its great you are aware of the importance of air sealing. Strongly recommend you have a blower door test done at the framing stage. What you dont measure you cant improve.

Here is a link that might be useful: Blower Door Test Explanation w pictures and diagrams

    Bookmark   February 19, 2012 at 3:08PM
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lzerarc

Spring- its all in the details with double stud walls. It can be easier, it just has to be detailed as such. It can also be 100% thermally broken as well. There are a a couple of ways to do it. The details and water shedding in thicker foam walls is more complex then double stud, even though I have done more foam wall buildings then double stud.
With that being said, my go to mid to high r20s wall is 2x6 w/ blown fiberglass, ZIP sheathing as my air barrier, 1.5" XPS foam, taped, and 1x3 vertical furring. R35+ I change to double stud with ZIP sheathing and blown fiberglass. The interesting fact is, around here, it costs about the same to use the r25 foam wall as it does an r40 double stud wall. Admittingly, the blower doors tests are producing slightly better numbers with the 2x6, ZIP and foam due to the belt and suspenders exterior shell.

    Bookmark   February 19, 2012 at 8:38PM
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robin0919

Spring., I know ICF runs approx $13/sf turn key. What would a 2x4 wall or 2x6 wall run per sf with OSB sheathing and insulation installed?

    Bookmark   February 19, 2012 at 10:11PM
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lzerarc

Rob- around here, about half that.

    Bookmark   February 19, 2012 at 10:31PM
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SpringtimeHomes

Probably a little more than half here depending on the details. The Pre-cast concrete runs about 9-12 and is erected in hours instead of weeks.

Jesse, know any good sites showing good double wall sections of the details mentioned? Still dont like the loss of floor space and light..

    Bookmark   February 19, 2012 at 10:50PM
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lzerarc

Yes, you do lose a few inches of floor space depending on the thickness of the wall. However depending on the r you are after, it is the most economical and easiest way to hit that vs thick foams.
For light, a lot of people splay the window opening. This helps spread the light really well.
Check out my blog for an example of a double stud wall detail.
There are other ways of doing it (such as bearing the floor structure on an inner wall and the roof structure on the outer wall), but the detail I show is my preferred method when using ICF foundation.

    Bookmark   February 20, 2012 at 8:50AM
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SpringtimeHomes

Very sweet detail. I guess you're using foam to cover the floor band.

As for the few inches of thickness, consider this: I can get a 5.5" R40 SIP wall. Thats 5" less than your 10" double stud. Consider a two level 30x38' home. 272'(Perimeter) X .42(5"/12")= 113 square feet. Thats alot of space. There's an arguement for multiplying 113 by your cost per square foot and spending it on a thinner wall.

Its possible this still wont cover the difference to upgrade to such a nice urethane SIP panel but by the time you figure in the labor of your air sealing details, I bet its pretty close. Maybe Iam still too chicken to try..

Also to the OP, if you do cedar clapboard, stick with traditional ashpalt building paper.

    Bookmark   February 21, 2012 at 10:46PM
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Alex House

lzerarc - Check out my blog for an example of a double stud wall detail.

I've looked and searched and can't uncover any link that you may have posted in the past. Would you please post the link because I'd love to see what you're doing.

    Bookmark   February 24, 2012 at 9:03PM
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worthy

Hardie Plank can't seem to make up their mind on installation procedures.

Contra the instructions posted by Renovator8, this set of instructions says the NBC requires a "water resistive barrier" between the Hardie and sheathing.

    Bookmark   February 24, 2012 at 9:32PM
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lzerarc

Worthy
that is correct. however that barrier can be a house wrap (Tyvek), XPS foam (over stapping), or an all in one product such as ZIP sheathing.

Alex
Link is here

Here is a link that might be useful: home blog

    Bookmark   February 25, 2012 at 11:53AM
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