‘Best’ exterior wall insulation technique?
1. Definitely want to use insulation between the brick and sheathing. Are the products that are attached to the sheathing in one sheet better at insulating and/or cheaper than the foam boards (or something similar) and regular sheathing?
2. If you use closed cell foam (CCF) on the inside, do you still need the wrap on the outside?
3. If you only use enough CCF for air and moisture barrier, can you apply open cell on top of the closed cell? Is there a particular CCF that is more eco-friendly regarding greenhouse gas emissions.
4. How do you determine the cost/value analysis of 2×4 with CCF, 2×6 with CCF and batting (denim, cellulose, or fiberglass?), or 2×6 with CCF and OCF if that is possible?
We are having trouble affording the house we want to build, so I have to take cost into account. I would like to do 2×4 walls with the continuous exterior panels, an inch or 2 of CCF and the rest with OCF. Or, would we get enough energy efficiency improvement of going to a 2×6 wall that it would make up the difference in construction cost?
Too many options – and this is before I even get to the cathedral ceiling or the attic!!!
Please advise
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Replies
Porcelain,
Q. "Are the products that are attached to the sheathing in one sheet better at insulating and/or cheaper than the foam boards (or something similar) and regular sheathing?"
A. I'm not sure what you mean by "the products that are attached to the sheathing in one sheet." Can you give an example?
Q. "If you use closed cell foam (CCF) on the inside, do you still need the wrap on the outside?"
A. I'm not sure what you mean by "the wrap," but I'm guessing that you are talking about housewrap. Housewrap is a type of water-resistive barrier (WRB), not insulation. For more information, see All About Water-Resistive Barriers.
If I guessed correctly, and you are referring to housewrap, the answer is: Even if you have closed-cell spray foam between the studs, you still need a WRB on the exterior side of your wall sheathing.
Q. "If you only use enough CCF for air and moisture barrier, can you apply open cell on top of the closed cell?"
A. Your question is confusing, because open-cell spray foam is an air barrier (just as closed-cell spray foam is an air barrier). What type of moisture barrier do you need between the studs? In any case, it is certainly possible to have a layer of closed-cell spray foam adjacent to a layer of open-cell spray foam.
Porcelain,
Q. "How do you determine the cost/value analysis of 2x4 with CCF, 2x6 with CCF and batting (denim, cellulose, or fiberglass?), or 2x6 with CCF and OCF if that is possible? We are having trouble affording the house we want to build."
A. Let's start at the beginning. Can you tell us your geographical location or climate zone?
1. https://www.thermalstar.com/Products/One.
Sorry, didn't know there was another type of wrap for a house besides housewrap. Also, I didn't mean to be confusing. I simply did not know if CCF and OCF would adhere to one another without any problems and if the cost savings would be significant enough to use both. Definitely want the benefits of CCF, but don't need to use more than we need or can afford.
2a.. We are in Alabama (33.459882, -86.328077) and the house is being built adjacent to a lake. Yes, we have humidity issues, extreme heat in the summer, and still have freezing temps in the winter.
Porcelain,
You described the Thermalstar product as something that is "attached to the sheathing." In fact, the Thermalstar product is sheathing. It is attached to the studs. For more information on this type of product, see this article: Nailbase Panels for Walls.
In Alabama, you don't really need an unusual, high-R wall, because you don't have cold winters. What you really need is a good designer and a conscientious builder who does a good job of installing whatever type of insulation you choose. Installing two types of spray foam between the studs is unnecessarily complicated in Alabama.
I'm guessing that you don't have enough construction experience to be choosing your wall details or wall specifications. It's time to find a designer and a contractor you trust.
“2. If you use closed cell foam (CCF) on the inside, do you still need the wrap on the outside?”
I think you will find it is best practice to have 2 water resent bearers in between masonry veneer and any wood.
“We are having trouble affording the house we want to build”
Brick is a very expensive way to cover your home, if you can reduce or eliminate it your budget change could be 10s of thousands of dollars. I have not seen a low cost system to support the cold brick exterior without a direct thermal to an interior.
“4. How do you determine the cost/value analysis of 2x4 with CCF, 2x6 with CCF and batting (denim, cellulose, or fiberglass?), or 2x6 with CCF and OCF if that is possible?”
There are 2 ways to answer this type of question 1 is a wild guess and 2 is to spend hours doing calculations. If you choose to do the calculations try using a computer program called BEopt. You already bought with your tax dollars. If you go this way start by watching the training videos found under the support tab. When you get your insulation bids BEopt will allow you to enter the R value and cost per Sqf and compare them against each other based on your fuel cost with your local weather considering your cost of capital.
https://beopt.nrel.gov/
Walt
GBA contributor Allison Bailes has a nice articles on questions you should ask when interviewing potential builders. http://www.energyvanguard.com/blog/10-questions-ask-your-prospective-home-builder?utm_source=EV+News+%2357+20170711&utm_campaign=NftF-%2357&utm_medium=email
Porcelain,
Steve's suggestion is a good one. Here is a link to Allison Bailes's useful article on GBA: Questions to Ask Your Prospective Builder.
And here is a link to a similar article that I wrote on the same topic: Green Building for Beginners.
In Alabama's climate there is no point to using closed cell foam anywhere in the above grade wall assembly. Even at 3.5" (2x4 framing) open cell foam is an air barrier, and your bigger leaks will be under the bottom plate and the seams of doubled-up framing such as top plates, jack studs etc.
The thermal performance improvement of going with closed cell between studs instead of half-pound open cell foam would be negligible (less than R1 whole-wall for 2x4 construction, less than R2 for 2x6 construction). Apply the closed cell foam budget to fatter continuous insulating sheathing, where the full benefit of the high R/inch isn't being robbed by the thermal bridging of the framing.
Open cell foam is greener, a LOT greener than closed cell foam. It uses only ~1/4 the amount of polymer per inch of depth, and it's blown with water, instead of HFC245fa, an extremely powerful greenhouse gas (~1000x CO2). There is now some closed cell foam on the market blown with low impact blowing agents, but they're still expensive, and pretty much a waste when installed in stud or rafter bays.
Taking an example case with 2x6 framing, in my area 5.5" of open cell costs about $1.50-1.75 per square foot (usually cheaper than dense packed cellulose, but more expensive than R20 fiberglass batts) and even 5.0" of closed cell foam runs $4.50-5.50 per square foot (and up), for a net performance gain of about R1.7. An additional 1/2" of rigid EPS or polyisocyanurate thickness would beat that R1.7 in performance, at an upcharge of less than 50 cents per square foot, often less than 25 cents. So open cell cavity fill + 1/2 uptick in exterior rigid foam hits a slightly higher performance point for half the money.
When I first heard of nail base panels I thought the advantage was to install the siding directly over the panel, with the OSB on the outside. If that isn't possible on most of these products, what's the advantage? You'll get a better system from two layers of rigid foam panels with OSB on top. Are the nail base panels cheaper?
Calum,
As my article ("Nailbase Panels for Walls") notes, some of these panels have OSB on the exterior side of the rigid foam, and some of these panels have OSB on the interior side of the rigid foam.
Zip R sheathing, for instance, is designed to be installed with the OSB facing out. So you're right -- with that type of panel, you can install siding directly to the panel if you want. (That said, a rainscreen gap is still worth considering.)
The answer to your next question -- whether nailbase panels are cheaper or more expensive than ordinary OSB plus rigid foam -- depends on the brand of nailbase you are considering, the local price for materials, and your assumptions concerning labor costs. You have to do the math.