I'm building a home and I have several questions around the exterior walls that I'm wondering if anyone can help with.  I live in Zone 6 so I'm in a cold climate area.  I'm trying to make choices in building the home that allow for the best energy efficiency at a reasonable cost.

We're considering building:
R19 insulation in the walls
2x6 walls, 16 OC
OSB
1" XPS on outside
Vinyl siding

A couple of questions:
1 - Do we need a housewrap somewhere in here if we have the XPS?  Where would be the right place to locate this?
2 - Is 1" of XPS enough?  Do we need to worry about moisture issues? I've seen the article listed below that concerns me.
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

According to this article we need a minimum of R11.25 foam sheathing on 2x6 walls in order to not have condensation in our sheathing or framing.

3 -  If I do the 1" of xps and then attach the vinyl siding is this going to cause issues with the siding?


I'd love to hear your thoughts on what we're looking to do here.

Thanks for your time.

You should attach strapping to nail the siding to and to create an air gap for drying.

There's lots of good free info at http://www.buildingscience.com/doctypes/designs-that-work/dtw-case-studies

Definitely check out the building science website, and also the GreenBuildingAdvisor.com website. Both have articles on wall insulation. Yes you do need a houserap to proivide an air barrier, and it should be put up well and taped. It should be over the OSB and under the XPS. Consider using 2" XPS; a little more expense, but no exttra labor and it will save you money on heat. The strapping, while it isn't necessary to provide venting for vinyl siding, is a good idea for keeping the insulation from becoming swiss cheese. Why put extra holes in it when you're trying to tighten the house? You might also consider using cedar shingle siding instead of vinyl. Eastern white cedar, green ( not kiln dried), either B or C grade works well on walls, and is inexpensive. You will need horizontal strapping, 5" on center for the cedar. I've used knotty cedar for trim, also fairly inexpensive. Both the cedar shingles and cedar trim need only minimal maintenence and will last for decades.

have a look at this document:

http://energetechs.com/wp-content/uploads/2009/12/Wall-assemblies-cost-analysis.pdf

for zone 6 I think you could use a better wall that the one you described.

good luck
Adi

Posted By adi43d on 27 Oct 2010 09:54 AM
have a look at this document:

http://energetechs.com/wp-content/uploads/2009/12/Wall-assemblies-cost-analysis.pdf

for zone 6 I think you could use a better wall that the one you described.

good luck
Adi

interesting doc - don't think I've seen that one before.  I don't think that the cost / r-value / sq ft gives the whole picture, though.

The cheapest wall was the 12" thick double stud wall, but the cost is only for the wall assembly.  Other things that need to be considered are window sizes (the deeper the opening, the less light a window will let in, so you may need bigger windows with thicker walls) and things like extension jams for window sills.  Those things add up & will make the real cost higher than if you only look at the wall assemblies..

If we are to look at the whole picture I think there are upsides for the thicker wall too. Because of the better insulation smaller, simpler and cheaper mechanical sistems are needed, lower energy bills every month, probably higher resale value are few things that come to mind.

Unfortunatelly I don't speak from experience yet. I'll break ground next year but some other people have aready done that and their stories are compelling. Here is one example:

http://www.solartoday-digital.org/solartoday/20081112/#pg26

Adi

Posted By adi43d on 27 Oct 2010 11:28 AM
If we are to look at the whole picture I think there are upsides for the thicker wall too. Because of the better insulation smaller, simpler and cheaper mechanical sistems are needed, lower energy bills every month, probably higher resale value are few things that come to mind.

Unfortunatelly I don't speak from experience yet. I'll break ground next year but some other people have aready done that and their stories are compelling. Here is one example:

http://www.solartoday-digital.org/solartoday/20081112/#pg26

Adi

I'm by no means an expert (that's Dana!!) but I think you're confusing thickness with r-value.  I completely agree that a better insulated house will use smaller systems, have cheaper utilities, etc.  I'm saying that it's possible to get the same r-value in a wall that's not 12" thick.  I posted something similar in a different thread recently.  the PDF you attached earlier referenced the double stud wall, 12" thick, and listed an r-value of R36.   If you built a single 2x4 wall, sprayfoamed the cavities, and did 2" of xps on the outside, that would give you about R33, air sealing, and a thermal break - very comparable to the double stud wall.   Spray foam is going to be more expensive, but you're using less lumber & you have a 6" thick wall, which eliminates some of the other things I mentioned (possibly needing bigger windows, extension jams, etc.)  If you compare the costs of only the wall assemblies, the 12" thick wall will probably be cheaper, but once you factor in some of those other things, I would guess that it's going to even out.

The good thing is that you have more options than ever, and no matter which route you go, it's going to be much better than a minimum code/"builder grade" wall.

the last document I posted describes with enough detail budgeting and details like windows and mechanical systems (which are reasonable an fairly cheap). I tried to say that for the same money the original poster can have a better house. foams are for sure one of the solutions but not the most budget oriented one.

I don't think your solution is optimal from several points of view:

sigle 2x4 will have to be build 16" oc while double 2x4 can be build 24" oc rendering the difference in used wood almost to nothing. there is the difference in labor
R33? - maybe clear wall but you have about 20-25% framing factor where your wall will be R13 while the double wall will have a clear wall value of 40.8 (12" of cellulose at R 3.4 per inch). put something like JM spider inthere and you're close to R48 - R50 (clear wall) for less money than your proposed wall
I can do a WUFI analysis on it to see how if behaves from the humidity point of view but a osb or even plywood sheating sanwitched between 2 foams .....not good enough for me. if you get water inthere it is not going to dry easy.

I dont expect anyone to take my word for it that is why I'll post few more links where people can find information about double wall systems and their viability and cost competitiveness.

http://chandlerdesignbuild.blogspot.com/2009/12/increasing-attention-is-being-drawn-to.html
http://www.100khouse.com/2010/07/16/most-affordable-effective-super-insulated-wall-assembly/
http://greenedmonton.ca/mcnzh-insulate-and-seal

some of the guys from the links above are well seasoned builders and I think people have a lot to learn from them.

just to complete my previous post - here is a very interesting analysis of window placement within the wall cavities (if there is a wall cavity). apparently the window performace can be improved by more than 20% just by placing them closer to the cente or the cavity. (I'll need a second opinion on this one though from someone above my pay grade :-))

http://phnw.org/PHnw-2010-04-26_presentations/PHnw-2010-04-26_Barry.pdf

some might say it is a lot more work but with products like this:

http://www.weatheroutflashing.com/products-double-buck-window-and-door-corner-sill-pan-flashing-with-back-rib-black.htm

I think it becomes very doable within a reasonable budget.

Adi

Posted By jlbartley on 26 Oct 2010 01:02 PM
I'm building a home and I have several questions around the exterior walls that I'm wondering if anyone can help with.  I live in Zone 6 so I'm in a cold climate area.  I'm trying to make choices in building the home that allow for the best energy efficiency at a reasonable cost.

We're considering building:
R19 insulation in the walls
2x6 walls, 16 OC
OSB
1" XPS on outside
Vinyl siding

A couple of questions:
1 - Do we need a housewrap somewhere in here if we have the XPS?  Where would be the right place to locate this?
2 - Is 1" of XPS enough?  Do we need to worry about moisture issues? I've seen the article listed below that concerns me.
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

According to this article we need a minimum of R11.25 foam sheathing on 2x6 walls in order to not have condensation in our sheathing or framing.

3 -  If I do the 1" of xps and then attach the vinyl siding is this going to cause issues with the siding?


I'd love to hear your thoughts on what we're looking to do here.

Thanks for your time.

 The short answers, in order:

1:  Housewrap can be used as an air-barrier, drain-plane or both.  Tape the seams of the XPS, but don't count on it serving as your primary air-barrier  (they SAY they've solved the shrinkage issues over time, but... ) it can go on either side of the XPS, but there are fewer issues if it's not against wooden sheathing, so put it on the exterior of the XPS (they say they've fixed the tannin-degradation issues too, but...)

2: You definitely have to worry about moisture issues in ANY house that uses wood. 

1" of XPS IS enough if...

A: You have a 10mm of rainscreen gap behind the siding, between the top with both top & bottom venting (6mm would be enough with vinyl)

B: You use air tight methods on the interior sheet rock (and VERIFY it's integrity with blower door testing.)

C: Use vapor-retardent primer or kraft-faced batts as your interior vapor retarder.


3: Vinyl siding is used on the exterior of foam sheathing all the time, but part A on question 2.  Vinyl siding is inherently back-ventilated, but you need to maximize the capacity of the wall to dry toward the exterior if only using 1" of XPS sheathing in your climate zone.


The longer discussion:

What others said about the inadequacy of an ~R23-ish clear wall R-value in zone 6 for new construction.  R19 batts typically deliver only R16-17 in the real-world in cold climates, due to the inevitable compressions & voids during installation, and they allow severe amounts of inside-the-insulation convection losses during extreme cold.   Blown & sprayed goods do a much better job, since they fill in the available space to near-perfection.  At higher density they do even better, since the convection & infiltration losses will drop, and the R-values per inch also go up (to about R3.7-ish for dense-packed cellulose.)  Cellulose has the lowest convection & infiltration issues, but noo-skool superfine fiberglass like Certainteed Optima or JM Spider are comparable, but only IF DENSE-PACKED to 1.8lbs/ft^3 or higher.  They deliver ~10% more R value than dense-packed cellulose, but unlike cellulose, have no capacity for buffering wintertime moisture accumulation.  With cellulose (at any density) using standard-latex and rainscreened siding allows that buffering capacity to work by maximizing drying in both directions, making the assembly more resilient.  5.5" of cellulose is sufficient buffering capacity for zone-6 as long as air-sealing of the interior gypsum is fully up to snuff.

Bear in mind, air tightness (particularly on the interior wall into the wall cavity) is 1000x more important than vapor diffusion when looking at moisture issues.  Blower-door testing the shell and fixing all the air-leaks to the exterior prior to insulating the cavities is a worthwhile endeavor, as is blower-door testing the whole space again once the sheet rock & plumbing are all in places.  (You may have to sheet-rock the attic prior to the shell test, if it's going to be a vented attic.)

To have enough foam on the exterior to make it a "don't care" situtation for air-leakage condensation in the cavites for zone-6 you'd need roughly half the R to be in foam. with a 2x6 construction that would mean 3" of exterior foil-faced rigid polyisocynaurate aka "iso" instead of XPS (2 layers of 1.5" goods seams FSK taped/caulked), which would yield about R19-R21, then either UNFACED batts (or better, wet-sprayed cellulose) and NO interior vapor barriers, no foil or vinyl wallpapers, only standard latex on the interior wall board.  Furring for mounting the vinyl siding would have to be long-screwed to the studs, but you'd have about an R38-R40 clear-wall R-value for a ~10-11" thick wall.  This is about the simplest-easiest way of achieving a relatively high-R wall, and it'll have less than half the heat loss of your proposed stackup if you make it air-tight every step of the way.

If you stuck to the R11.25 number as-per the article you'd still have some condensation potential on the sheathing, but it wouldn't be at a damaging level unless there was significant interior-side air leakage.  Using cellulose for cavity insulation would take care of that.  You can get to R11 with 1-3/4" iso, but use only fiber-faced goods(sold as roofing insulation), not foil faced or you won't have sufficient exterior drying capacity.  Alternatively, 3" of unfaced EPS bead board would deliver ~R12, with even better exterior drying characteristics.  Combined with wet-sprayed cellulose you'd be in the low R30s range, and highly resilient to wintertime moisture. Here too you'd end up with a ~10-11" wall thickness (just as in the iso case.)

No matter what you do, unless you live in a very high-wind zone 16"o.c. 2x6 is overkill- 24" o.c. will have the same structural capacity of 2x4s 16" o.c. .  Reducing the number of studs reduces the thermal bridging, increasing the clear-wall R by about R1.5.

Thank you all for the responses. 

I'm studying what you've replied with to dig through my options. 

One thing that my builder just mentioned to me tonight (he is not up on any sort of green techniques).  He said that if we would put foam on the outside of the house like this we would need to modify our floor trusses to accommodate for the additional thickness.  Is this correct?  I know we need to account for additional thickness in the windows and doors but do the floor trusses typically need to be modified in order to add foam to the outside of the house?

The floor joists and rim joist would be pulled in whatever the thickness the foam is... no big deal. This will allow the outside face of the foam to be in-line and flush with the outside face of the foundation.

Or you make the foundation slightly bigger by the thickness of the foam

Better make sure your builder is doing all the details correctly. If not you could easily be losing a lot of heat, or even worse having moisture damage from warm moist air leaking into the wrong places.

Posted By Jere on 28 Oct 2010 02:51 PM
The floor joists and rim joist would be pulled in whatever the thickness the foam is... no big deal. This will allow the outside face of the foam to be in-line and flush with the outside face of the foundation.

Or you make the foundation slightly bigger by the thickness of the foam

OR...

...you can use insulated concrete forms for the foundation, or add rigid EPS/XPS to the exterior of the foundation to match the thickness of the exterior foam, eliminating the thermal bridging of the foundation.

Lack of (or too little) foundation insulation seems to be rampant around New England, even amongst those who should know better.  In most places there are no code requirements.  High-R houses really need to be high R on all 6 sizes of the cube, not just the walls and attic.