BadgerBoilerMN Thank you for your comments. I like your common sense approach. So a 2x4 wall with 2" of foam on the outside and cellulose in the cavity should give me the true R-19 value? Or does it need to be a 2 by 6? And I'm wondering if R-20 beneath the slab is too much? We're pretty committed to a minisplit with some sort of a back up. I was leaning in the direction of the HRVs made in Germany, Lunos e2 for sake of simplicity. And I have a HRS rater lined up to do all the heat calcs, blower door test etc. Thanks for everyones comments.

We build in climate zones 5 & 6. Nice to know this is a mild climate - I used to think that with temps down to -25 annually, it was a cold climate. Guess I've learned something. One of the most prominent sources for the recommendations for R40 walls, R60 roof and triple glazing is the research of the experts at Building Science Corp, so you should read what they have to say about these numbers rather than listen to different views on the topic.

One of the reasons we build this way is to wean ourselves off fossil fuels, and instead, learn to live with the cheapest fuel on the planet - solar energy. But since solar energy requires special equipment to turn those rays into fuel for my mini splits, the less equipment I need to buy the better. For years most of the houses built were drafty and took a lot of energy to condition the space; the newer methods have upended that and work great, and every blog I read from a homeowner who has just completed their new superinsualted house agrees and are delighted with the space. But for those of you who want to try doing less, do it & we'll compare notes after you move in and live there through a winter. Essentially, you'll find that less insulation = more heat, so it's your choice.

There is nothing wrong with double walls and lots of affordable insulation. I built decorative CMU exterior structural walls along with steel stud interior walls. I filled the cavity between the walls with rockwool for total wall r value close to 50. Also used steel trusses, metal roof and rockwool for ceiling r value close to 70. House is just a 1800 sf single story simple rectangle floor plan. Looks like old English Cotswolds cottage that my parents live in. Total exterior wall thickness is just shy of 24”. 8” CMU and steel 2x4 stud create 15” cavity filled with rockwool. The steel studs are just for fastening dry wall and retaining the rockwool. The attic above ceiling just has 24” of rockwool. Nothing complicated here just lots of rockwool in my little castle. Originally planned to use ICF but this approach was much cheaper since I did the work myself and got free rockwool by bartering my services. Still get the nice adobe buffered thermal mass benefit as predicted on Borst ICF calculator. I figure the summer thermal mass effect makes the walls have 30-40% more effective r value than the conventional 50 value.

Posted By agagent3 on 22 Mar 2017 12:42 PM
BadgerBoilerMN Thank you for your comments. I like your common sense approach. So a 2x4 wall with 2" of foam on the outside and cellulose in the cavity should give me the true R-19 value? Or does it need to be a 2 by 6? And I'm wondering if R-20 beneath the slab is too much? We're pretty committed to a minisplit with some sort of a back up. I was leaning in the direction of the HRVs made in Germany, Lunos e2 for sake of simplicity. And I have a HRS rater lined up to do all the heat calcs, blower door test etc. Thanks for everyones comments.

A 2x4/R13 wall with R8.2 of EPS on the exterior comes in just shy of R19 whole-wall at typical 25% framing fractions. You can also get there with 2" of rigid rock wool, no foam, but it's more expensive. With 2" of polyiso it'll be just a bit ahead of R20. Don't bother doing it with XPS, since the performance will eventually drop to that of Type-II EPS (R8.2), even if it's initial performance is higher, as it's HFC blowing agents leak out doing their damage.

Roofing polyiso is commonly reclaimed for re-use on commercial building demolition or re-roofing, and is 25-35% the cost of virgin stock, and nearly 0% the environmental cost.

You can use standard latex paint as the interior vapor retarder in your location with that much exterior-R. With a 2x4/R13 wall R8 sheathing has a hint of margin above the IRC prescriptive R7.5 for dew point control at the sheathing for zone 6, and quite a bit of margin for the prescriptive R5 for zone 5, so your fine from a dew point perspective. But IRC 2015 calls out R13+ R10 as the code minimum thermal performance for zone 6, (R13 + R5 in zone 5), so probably still going to be "worth it" to go with polyiso. The labeled-R would be R12 or R13 if foil-faced virgin-stock, but don't count on reclaimed fiber-faced foam to be more than R11 (though it sometimes is, especially if it was blown with CFHCs, prior to the Montreal Protocol.) Either way you'd have pretty reasonable moisture resilience with that much foam on the exterior, and standard latex paint on the interior, with cellulose cavity fill.

At max speed the Lunos E2 is only putting out ~20 cfm, and most houses would need multiple pairs. For an open floor plan house you might do fine with a single pair of Lunos Nexxt, which can put out quite a bit more volume. If it's all cut up and doored off (not a good idea if heating with mini-splits) you may need several pairs of E2s, which add up. Ducted HRV solutions can sometimes be cheaper than 3-4 pairs of Lunos.

If your area got ant/termites, don't use foam.

I must make a decision because spring is coming. I did a quick and dirty calculation on wall cost per R-value. The double stud wall came out on top. The foam component of the 2 by 6 wall gets expensive and is a pain to install; plus the screws are expensive as well. As noted ants could be problem. Frankly, the double stud scares me due the the possibility of moisture accumulation. In reviewing the literature I discovered the application of plywood sheeting on the inside of the outside wall as a vapor retarder and air barrier. What is the sure fire way to eliminate moisture problem in a double stud wall? Do I need something like MemBrain in the assembly? The wall cavity will be cellulose. And is an acoustical caulk a "best" practice for sealing studs to sheeting? Or is there another caulk that would work just as well? Thanks again for all the comments.

the best way to insure that moisture is not a problem is to make certain the house is airtight - under 1 ACH50 (which is not that difficult.) For added insurance, use a taped moisture barrier (NOT an air barrier) like Intello Plus on the interior, and use fir CDX PLYWOOD as sheathing.

Posted By agagent3 on 23 Mar 2017 11:48 AM
I must make a decision because spring is coming. I did a quick and dirty calculation on wall cost per R-value. The double stud wall came out on top. The foam component of the 2 by 6 wall gets expensive and is a pain to install; plus the screws are expensive as well. As noted ants could be problem. Frankly, the double stud scares me due the the possibility of moisture accumulation. In reviewing the literature I discovered the application of plywood sheeting on the inside of the outside wall as a vapor retarder and air barrier. What is the sure fire way to eliminate moisture problem in a double stud wall? Do I need something like MemBrain in the assembly? The wall cavity will be cellulose. And is an acoustical caulk a "best" practice for sealing studs to sheeting? Or is there another caulk that would work just as well? Thanks again for all the comments.

And ants are somehow not a problem with a double-studwall?

Using virgin stock exterior rigid foam is usually cheaper than double studwalls at performance levels up to ~R25-R30 whole-wall, but using reclaimed foam bumps that up to R40.

But it takes a crew who understands how and is comfortable with how to install the exterior foam. With untrained inexperienced crews it's lot easier to just go double-studwall.

If you haven't already, compare the construction discussions & performance levels of a variety of higher-performance assemblies in this document:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf

I did not realise double stud walls would have problems with moisture infiltration. Would tyvek on both inside and out help this?? I am in Western NY, its not required but would a 6 mill vapor barrier on the inside be wise or not?

A house that is airtight when newly built probably won't be airtight as it ages.

Posted By anon on 05 Jun 2017 06:48 PM
A house that is airtight when newly built probably won't be airtight as it ages.

True, but the amount of leakage that develops over time can be pretty small. A few years ago Marc Rosenbaum of Energy Smiths (http://www.energysmiths.com/index.php )retested some houses he had built & blower-door tested back in the 1980s, and they still came in at about the same cfm numbers some 25 or more years later.

Posted By Dana1 on 05 Jun 2017 09:50 PM

Posted By anon on 05 Jun 2017 06:48 PM
A house that is airtight when newly built probably won't be airtight as it ages.

True, but the amount of leakage that develops over time can be pretty small. A few years ago Marc Rosenbaum of Energy Smiths (http://www.energysmiths.com/index.php )retested some houses he had built & blower-door tested back in the 1980s, and they still came in at about the same cfm numbers some 25 or more years later.

Anon is correct, there is no conclusive data in AEC that test the life expectancy of seals & sealants in situ. It can be very low and very costly to replace depending on location, exterior I see you recommending often being the absolute worse!

Further, it's clear you have no experience creating empirical data, one test is FAR from cutting it. You should ask more questions, stop acting like you are always in the know.