Advice on tight building
I have my house rough framed, we are framing out the roof now. Climate Zone 2 ( Just north of Austin). 2×6 conventional framing 16 on center, and has the typical double headers around windows etc. I am nearing having to lock down my decisions on sheething, insulation etc. I have been considering using Zip R for a couple reasons.
1. breaks my thermal mass, that I have plenty of
2. Weather and air seal.
It is anything but cheap, I am looking probably 7000 for the 1inch.
However I am looking at other options
CDX / plywood (plywood at bottom / window areas) and sealing with Sega Wigluv, or prosoco, or a combination.
My roof will all be open cell 7.5 inches, the crawlspace will be 1in closed cell do to the moisture, even though it is encapsulated.
roof will be asphalt, as metal just isn’t within reach in the budget.
so doing OSB sheething, I would expect would do better against racking etc than ZIP R as the sheething is against the wood, not the insulation. If I did that option I would put EPS on the outside most likely. Seal at the wood, and then I have been told that I really need a rain screen behind my cemplank. So most likely would use Coravent, and firring.
Any and all advise would be appreciated. Zip seems like an easy / expensive option.
osb covered with wigluv, with exterior rigid foam, with an air gap seems cheapest I am thinking, but its not free either.
I also could just spray an inch of closed foam on the inside, and fill the rest with open cell. Then just air seal the outside and call it good.
Thoughts ? your help is greatly appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Sleaton,
Taped OSB or taped plywood, covered on the exterior with reclaimed (recycled) rigid foam, will be much less expensive than Zip R sheathing.
For GBA readers curious about Sleaton's decision-making process, I recommend this article: Plan Ahead For Insulation.
7.5" of open cell foam doesn't meet current IRC code minimums for zone 2- it's a full R10 shy of R38, which is a bit odd since even a 2x6/R20 *let alone ZIP-R) would be ~50% higher than code-minimum, as is the R6 on the crawlspace walls. See:
https://up.codes/viewer/general/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.2
Is the roof rafters, or trusses? If rafters, how deep?
I'm not sure what means "...breaks my thermal mass, that I have plenty of..." in 'merican??? Are you talking about a thermal break on the thermally conductive framing?
In Zone 2 it's possible to hit Net Zero Energy with an R15 "whole-wall" performance with a PV array that still fits on the house, but it takes R40-R50 "whole-assembly" on the roof, not R28. See Table 2 p.10 of this document, which is roughly what makes financial sense, and also is sufficient to get to Net Zero Energy, if that's a goal. (There are now MANY examples of 2x6 framed Net Zero houses in Austin, where Net Zero Ready is the local code minimum.)
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
A 2x6/R20 wall comes in at about R15 after factoring in the thermal bridging even without the ZIP-R.
An R40 roof could be 2x6 rafters with 5.5" of open cell foam (or R21-fiberglass or R23 rock wool batts), with 3" of rigid polyiso above the roof deck held down with an OSB nailer deck through-screwed to the structural roof deck with pancake-head timber screws. That's more money than 7.5" of open cell foam, but it's a heluva lot cheaper than ZIP-R sheathing that provides less benefit, and it's twice the performance to boot. If you find a local source of reclaimed 3" roofing polyiso ( ( https://austin.craigslist.org/search/sss?query=rigid+insulation ) it can be cheaper R than open cell foam if the roof lines are simple and labor costs are reasonable.
An inch of closed cell foam between the studs with the rest open cell foam is a waste of good foam- it barely moves the needle on whole-wall R due to the thermal bridging of the framing. Slightly better thermal performance can be gotten out of caulking the studs to the sheathing with polyurethane caulk and installing R23 rock wool, which would usually be cheaper than flash closed cell + open cell fill.
Thanks so much for the reply. You all don't disappoint, just what I was looking for. Sounds like I should focus more on the roof than the walls. The roof has 2x10 in the 12/12 pitch, it has 2x8 in most of the 4/12 pitch, and has 2x6 in maybe a 1/3 of the 4/12 pitch. It is kind of a tradeoff for me, I own a solar company, so power is cheap. So while I want a comfortable house that is built well, at the same time my solar ground array will more than cover my power needs. So open cell is 4.4, and closed is 7.3. It is cost prohibitive to spray closed cell everywhere. I guess I need to look into the 3 inches of foam on the deck option, that won't be cheap either, not to mention how much that complicates roofing. I guess the cheapest option is to do as I stated with the open cell, then put 15 inches of the messy blown in junk all over the roof of the rooms, thus meeting code. The other option is to put on more than there is rafters, not sure if that meets code.
Sleaton,
As I noted in the article I linked to, the time to design your roof insulation is before framing has begun.
You wrote that "we are framing out the roof now." At this point, your insulation options have been severely limited by your framing decisions. I know that sounds like a harsh assessment, but other GBA readers who haven't yet started to build need to understand the usual order of things: first, the house is designed; then, the framers start their work.
Combining open cell at the roof deck and blown fiber at the attic doesn't work for meeting code compliance on an R-value basis. The R-value of fluff on the floor gets discounted to zero.
Unless the open cell foam is free it'll be cheaper hit code-min with 2" of reclaimed polyiso above the roof deck, and install R38 "cathedral ceiling" batts in the 2x10 framing, and low density R38s in the 2x12 section. That doesn't really complicate the roofing a lot (unless you have really complex roof lines), and it doesn't need a fancy facia board at the roof edges- a 4" drip-edge (a common size in commercial construction) is enough to provide reasonable aesthetics for the now thicker roof. Gluing the roof deck to the rafters with a continuous bead of construction adhesive and tape any seams spanning rafters makes it pretty air tight even without foam, but you may still need an additional Froth-Pak kit or three to air seal all of the framing details depending on how complex it is.
With south (&/or west) facing pitches shaded by solar panels that should be enough insulation to keep peak attic temps reasonably bounded. The 2" polyiso as a thermal break roughly doubles the R-value of the rafter-framing fraction, and it'll be close to R50 at center cavity, and at a 7% framing fraction (typical, for 24" o.c. raftered roofs) it would have a whole-assembly R in the mid-40s, low 40s if 16" o.c..
[edited to add]
For the 2x8 raftered section going with R30 rock wool (or 0.7lb open cell foam) between the rafters and 2" of reclaimed polyiso above the roof deck still gets you to code-min performance, but comes up just a bit shy of R40 whole-assembly.
For the 2x6 raftered section R23 rock wool (or 0.7lb open cell foam) between the rafters and 2" of polyiso above the roof deck comes up at almost exactly code-minimum on a U-factor basis, and would need 3.5" foam to hit R40 whole-assembly. If it's not a large fraction of the total roof area you might still be able to get to Net Zero Energy.
I think there's one important point to make here in regard to meeting code minimum insulation levels, they may not apply in this case.
1) As Sleaton says, he will produce more power than he consumes due to the size of his array.
2) If he is located outside of Austin city limits and his construction is permitted thru Travis County, then the county code (chapter 80, https://www.traviscountytx.gov/images/commissioners_court/Doc/county-code/chapter-80.pdf) states:
(a) New residential construction of a single-family house or duplex shall conform
to the version of the International Residential Code published as of May 1,
2008, or the version of the International Residential Code that is applicable
inside the corporate limits of the City of Austin.
Since this is written "or", he may choose to use the earlier energy code adopted at the time given in the section, which in this case would be the 2006 IECC. If he did elect to use the earlier code, then 1) he could use the lower levels of insulation specified, and 2) he may get out of the envelope requirements completely by taking exception 1 to 101.5.2 which exempts low energy buildings (peak design rate of energy usage <3.4 Btu/h-ft^2 or <1.0 W/ft^2). The commentary to the 2006 IECC clarifies that solar and other renewable energy produced at the site is excluded from peak rate of energy use.
As long as his array proves out from a calculation standpoint as being "net +", then his peak rate of energy use is 0, and the envelope requirements do not apply. Similarly, he could use section 404, the simulated performance alternative against the baseline, and trade off the insulation against the solar array, since the site produced energy is subtracted from the energy cost in the model in the earlier code (I believe I read that was changed in the 2015 code).
This may not fly with every AHJ, and they are ultimately the final authority, but it is compliant (in a twisted way). However, I'm not actually recommending that he do so. The insulation is worth the money, and a solid thermal/air/moisture envelope has numerous benefits, its just that Sleaton's circumstance is not the norm due to the large PV array so he has additional options someone else would not.
Thanks everyone for the replies. And thanks Dan for the Solar point also. I live in Williamson county, in the country, so no inspections, no AHJ to deal with. I am just not someone that tries to dodge code. I didn't realize I had an issue as I had quotes from 5 spray foam joints, and they all bid 5.5 open cell in roofs, and 3.5 in the walls. Obviously no where near R38. I have since been scrambling and looking at more options. I found a place that I can get reclaimed POLYISO R12 2 inch for a pretty dang good price. So I am now trying to put all the pieces together.
1. On my outer walls, I am leaning towards not doing zip, going with plain old OSB, and then installing 2 inch polyiso over top. The question with that is how you hang the hardi plank, normally I nail the 8 in boards into the studs.
2. On the roof I have a few options. I could abandon the idea of the encapsulated envelope, blow in insulation, vent the ridge etc, and while I prefer a hot roof, this would pass code, and be far cheaper.
I will have 2x8 in my 12/12 pitch area. I could always fill that with foam, and put some polyiso under it. That is roughly r31 in foam alone. I will have 2x6 in a lot of the area of my roof. That is r38 if I did closed cell, which I am sure would be out of the budget. R23.65 if I did open cell foam, and I could put poly under that and be in pretty good shape without breaking the bank. I could also just fill the cavities with poly iso, and spray some foam to make sure there are no gaps either.
I think on here people would recommend putting the R12 on top of the roof, but I am shingling this sucker and have absolutely no idea how the heck that would work.