General questions re: building in Zone 7
Long time reader, first question…(I apologize in advance for length).
I will be building in the next year or so, this is my 5th home I have had built and have managed all my previous builds, so I know a LITTLE about building, but not even close to a professional. I live on the very Southern edge of Zone 7 in Central Minnesota (in fact, Zone 6 boundary is about 2 miles South of me). We have some hot, humid days in the summer, but obviously heating in winter is much more intense.
My present house that I had built about 7 years ago has 2×6 walls with dense packed wall insulation, about 16″ blown-in attic insulation, Thermo-Tech double hung windows low-e, 2″ pink foam insulation on outside of basement block walls and 1″ inside, and Geo-Thermal furnace (pump and dump, I live on 40 acres). Not satisfied with the performance of this house and want to do better next (and last) time I build.
My plan is to build a simpler house this time (about 1800 sq. ft. rambler with full walkout basement), 8/12 pitch roof, flat ceilings, get rid of all the recessed lighting, etc. Not a “Passive House”, but I want to get the biggest bang for the buck using smart construction methods.
1. Any thoughts on EZ Sips for wall insulation http://structuralinsulatedpanels.com/ ? I think code in this area is to poly the interior walls behind the sheetrock, so I’d have to poly even with foam on the walls. Any others thoughts on better options for wall construction are welcome!
2. I know double hung windows are not the best for energy efficiency, but my wife really loves them. Are there better options that are readily available for windows (that are not stupid-expensive)?
3. For heat this time, I am considering mini-splits in conjunction with a propane furnace (no natural gas available in this area). I HATE the look of the wall mounted units (they look like old window A/C units to me), but am I correct in thinking there are units that work with duct work? I don’t want to invest the $$ required to do Geo-Thermal again because I don’t think the return on investment is there. It will also have a gas fireplace because my wife loves a fire in the family room on a cold winter night.
4. Basement will be the same as before…2″ foam on outside and 1″ inside of block walls behind stud walls (ICF’s are not very popular in my area). Also, probably blown-in insulation in attic (flat ceilings).
I appreciate the collective knowledge shared on this site and please weigh-in on any and all thoughts to improve my final home build.
Thank you!
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Replies
Brad,
Lots of questions.
1. SIPs are more expensive than more conventional approaches to high-R walls or high-R roofs. Most builders choose to build a double-stud wall with dense-packed cellulose insulation or a 2x6 wall with insulation on the exterior side of the wall sheathing.
2. Try to minimize the number of double-hung windows in your home. In many locations, fixed windows work fine. Wherever possible, use casement windows or awning windows, since these window types aren't as leaky.
3. Yes, you can heat your home with ducted miniplits. I don't think you need a propane furnace for backup, however; instead, use electric resistance heaters for backup, or a wood stove.
Martin, Yes, I know lots of questions...sorry about that!!!
So, for a fairly high R value wall for Zone 7, would a logical approach be: (from inside out) sheetrock, poly, 2x6 wall w/ dense pack, house wrap, 1" or 2" pink foam and then siding?
If you click on my link to EZ Sips (http://structuralinsulatedpanels.com), they are not really sips, they are foam panels that you slide into a 2x6 framed wall. I just have no idea if they work as advertised, but they seem like a good idea to me.
I'm trying to talk my wife into casements.
Thank you for your help!!!
Brad,
Q. "So, for a fairly high R value wall for Zone 7, would a logical approach be: (from inside out) sheetrock, poly, 2x6 wall w/ dense pack, house wrap, 1" or 2" pink foam and then siding?"
A. No. In your climate zone, that type of wall needs rigid foam with a minimum R-value of R-15, and 2 inches of "pink foam" (XPS) isn't enough. (By the way, EPS is environmentally preferable to XPS.)
Another important point: once you install exterior rigid foam insulation, you don't want any interior polyethylene, because this type of wall assembly is designed to dry to the interior.
To learn more about these issues, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thank you for that link. Very informative.
I have been reading lots of information, but my brain starts to turn to mush...SO much information to absorb (and much of it different than "conventional" thinking).
If the sticking point with casements is aesthetics, consider whether a casement with an architecturally pleasing divided light pattern might accomplish the goals. For example, we went with the "cottage" pattern in this set of options http://www.lakeshorewindowsanddoors.com/images/windows/integrity/grilles/patterns-double-hung.jpg (from a different manufacturer). We like the look and get complements from the neighbors too.
I like the EZsips idea. Hadn't seen that before. The performance of the 2x6 version should be pretty similar to 2x6 with cellulose plus 2" of EPS on the outside, but you can't do the latter in your climate because the 2" isn't enough. You could argue whether or not the EZ SIPS would have a condensation problem at the outside edge of the 2x6, but I'm guessing it is OK.
However, you might want more insulation than that provides. You could put another 2" of EPS on the outside, but at that point it's probably pretty expensive and you might get the same performance with a double stud wall with cellulose between.
I think whether EZ sips makes sense depends a lot on what your builder thinks of it. If it would make the process easier, it might be worth it. If it would be a hassle, there are other ways to achieve the same R-value.
Charlie: No divided lights!!!
I'm sitting in a room with seven big 12 over 12 Windows. That's 168 panes. Needless to say, washing them would take forever, which is why they don't get washed very frequently. The rest of the house is just as bad.
In thinking about EZ Sips, the OSB (or whatever you use for sheeting) goes on the outside of the EPS, so the rules regarding the thickness of XPS on the outside of the sheeting wouldn't really apply, would they? The OSB would stay cold. As Charlie says above, maybe there would be some danger of moisture on the outside edge of the 2x6 studs since the 2" of EPS would be outside of them?
Maybe the R25 they advertise would not be adequate for my area anyway, but the "normal" premium contruction in this area right now is 2x6 walls with poly in the inside, loose fill blown in insulation, OSB and house wrap. When I talk to builders around here, there's not too much understanding of anything other than the normal, and it sure seems like these EZ Sips would be better than that...at least for the thermal break if nothing else.
Not trying to sell anyone on these, they just seem to be a neat idea to me (which doesn't mean much!!!) A little knowledge can be a dangerous thing...
Brad, I am a bit North of you and I plan to go with 2 x 6 walls with exterior foam. The cost of SIPS and having contractors not familiar working with them are my reasons for staying away from them. My wife is also set on double hung windows, but I finally found an article stating in new windows it is not as big a deal as it used to be, so I may go with that (true or not!).
Martin: I'm a bit confused by your answer to brad's second question/response. The 2012 IRC mandates an interior Class 1 or 2 vapor barrier on the inside of the wall unless the R15 standard against the sheathing is met.
I too am planning on building in Zone 7. Am I correct in thinking a wall of ( outside to in) house wrap, plywood, 3 1/2 closed cell foam, 7 inches of dense pack cellulose, a mem-brain vapor barrier under drywall be reasonable?
William,
I was just about to bring this back to the top to see if Martin (or anyone) could answer the question I had above in post 7?
The wall I am considering now would have: engineered wood siding, house wrap, plywood, 1" XPS, EZ Sips, 2x6 wall, mem-brain, drywall. . (Remember, these are not actual SIPS, they are rigid foam insulation of what I assume is EPS...check their website in post #1 above). Since the plywood is on the outside of the wall, the sheeting would stay cold.
Oh, and I think I have my wife talked into casement windows...probably triple glazed (Thermo-Tech?)
William Olmsted,
I'm not sure which aspect of my answer to Brad confused you.
You are proposing a 10 1/2-inch-thick wall insulated with the flash-and-fill method (a combination of spray foam and cellulose). That's unusual.
In Climate Zone 7, the code requirement for walls with exterior foam calls for the following ratio of foam insulation to fluffy insulation: 43% minimum foam to 57% maximum fluffy. (This calculation is based on the assumption that the code calls for a 2x6 wall with R-20 fluffy insulation to have an exterior foam layer of at least R-15. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
Your proposed assembly has about R-49 of insulation, consisting of R-23 foam and R-26 cellulose. That gives you a foam-to-fluffy ratio of 47% to 53% -- so I think you are good to go.
Brad,
Q. "In thinking about EZ Sips, the OSB (or whatever you use for sheathing) goes on the outside of the EPS, so the rules regarding the thickness of XPS on the outside of the sheathing wouldn't really apply, would they? The OSB would stay cold."
A. You are correct.
Martin,
Thanks for your answer. That it was positive and confirmed my thoughts is even better! I'm sure that you have sympathy for those of use trying to wade through the vast volume of information out ther. If only I was interested in retiring in zone 2 or 3.
If I may follow up. How is flash and fill unusual? Is it using the approach in Zone 7 or another reason?
My confusion was relative to Brad's proposed use of poly. While I understand it may be difficult to ensure a well-sealed barrier, it WOULD serve as both a vapor and an air barrier. Some of the research I've done seems to suggest that in Zone7, an effective AIR barrier on the inside may be more important than a vapor barrier.
Thanks for being a source of information for those of us that are trying to build in a reasoned, sustainable manner by understanding the how's and whys of the process.
Unusual how? I' under the impression
I also want to echo William's sentiment...Thank You everyone for weighing in on my questions . To the lay-person, the information overload comes with all the information available ...and much of it is different than the "conventional wisdom".
I will probably build a detached garage/workshop with pex heating in the floor and try the EZ Sips in the walls of that structure before using them on the house. That way, I will be able to see how they work.
If anyone has any actual experiance with them, please let me know your thoughts.
Thanks!
William,
The flash-and-fill method is almost never used for a 10 1/2-inch-thick wall, for the simple reason that 10 1/2-inch-thick walls are unusual. It sounds like this is a double-stud wall. Most double-stud walls are insulated with dense-packed cellulose. To insulate this type of wall with the flash-and-fill method takes an unusually thick foam layer. You have chosen to invest in thick foam, so the wall will work -- but it is unusual.
I am not aware of any U.S. codes that require interior polyethylene. U.S. requirements for an interior vapor retarder (not a vapor barrier) can be satisfied with the use of vapor-retarder paint.
Interior polyethylene can cause problems in any home that may be air conditioned during the summer. Moreover, any wall with exterior foam -- either rigid foam on the exterior side of the wall, or flash-and-fill insulation -- should be designed to dry to the interior, and should therefore never have any interior polyethylene.
Martin,
I' m sorry to respond yet again but working through this is how the uneducated among us acquire a headache.
Yes it is a double stud wall. The thought is that this technique is the easiest and most economical way to get to a R40+ structure. Going with a single 2x6 wall would require 3-4 layers of 1" XPS or Polyiso board outside of the plywood and then dense-packed cellulose in the bay's. Including the drywall, it seems you are still at 10 1/2 inches. Air conditioning isn't on the board for us as it isn't overly necessary in Zone 7.
Am I missing something?
William,
As I said, you are good to go -- as long as you don't install interior polyethylene.
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