In the pursuit of energy efficiency through air tightness of a home, indoor air quality and the intake of fresh air become a priority. DIY'ers who overlook this part of the design open themselves up to adverse health effects from poor indoor air quality.

Not to put to fine a point on it,even an an R30 SIP roof would still meet code even in Omaha (Douglas County, Nebraska zone 13), since it's both "compact roof" with no/low thermal bridging to contend with. See:

http://www.neo.ne.gov/home_const/iecc/zones13-15.htm#13

(Read the note below the tables in regarding ceiling insulation R prescriptions.)

The ventilation rates on most Manual-J type tools are not ACH @ 50 pascals type numbers, but ACH-natural + mechanical. It would be under 1.5ACHnatural even in a typical 2x6 stick built, but well under 0.5 in a SIP building that was detailed for air-sealing during construction. There's no perfect model for conversion, since it depends on many factors, not the least of which is the height size & shape of the building, indoor & outdoor temperatures, wind exposure etc.. But in a 2-story building in MN the ACHnatural number will be something like 1/10th to 1/15th of the ACH/50 number according to tables developed by the Lawrence Berkeley Nat'l Labs. See the map & table on p.5:

http://www.energystar.gov/ia/home_improvement/home_sealing/ES_HS_Spec_v1_0b.pdf (I'm assuming you're in infiltration-zone 1)

If it tests as 1.5ACH/50, and isn't on a raging windswept ridge or shielded in a snug little gully, assume it'll be on the order of ~0.12 ACHnat, plus whatever active mechanical ventilation rates you operate at. If you're using HRV/ERV ventilation you can safely discount the mechanical ventilation factor by at least half.

Mind you, 1.5ACH/50 never happens by accident, but unintended air-leaks DO. Unless you actually test & rectify to that number I wouldn't assume anything less than 3ACH/50, with 4ACH/50 being more likely. Still, even at 5ACH/50 you'd beunder 0.5 ACHnatural (and that I WOULD assume.)

It has been a long time since I've had builders tell me they build leaky homes on purpose. It is humorous to hear similar sentiments on a green building site. The way to do it is "seal tight, ventilate right". It is impractical to build a very energy efficient home without purposeful attention to air sealing. Then bring in fresh air where you need it in the quantity you need. (If air sealing is not important there is no point in using SIPs.) ASHRAE IAQ standards are based on significant VOCs in the house (that would be much less for one building green). There are newer alternative requirements with monitoring that require far fewer air changes. There are even specific plants one could keep in the house that could eliminate the need for outside air (I would not try it without monitoring). I don't subscribe to the idea that a DIYer is more likely to have problems with their home than a professional. On the contrary the DIYer is more likely to be far more familiar with their home and give much more time and thought to how they build it. When the builder makes mistakes he often just moves on.

Posted By Torben on 22 Dec 2011 10:59 PM
It has been a long time since I've had builders tell me they build leaky homes on purpose. It is humorous to hear similar sentiments on a green building site. The way to do it is "seal tight, ventilate right". It is impractical to build a very energy efficient home without purposeful attention to air sealing. Then bring in fresh air where you need it in the quantity you need. (If air sealing is not important there is no point in using SIPs.) ASHRAE IAQ standards are based on significant VOCs in the house (that would be much less for one building green). There are newer alternative requirements with monitoring that require far fewer air changes. There are even specific plants one could keep in the house that could eliminate the need for outside air (I would not try it without monitoring). I don't subscribe to the idea that a DIYer is more likely to have problems with their home than a professional. On the contrary the DIYer is more likely to be far more familiar with their home and give much more time and thought to how they build it. When the builder makes mistakes he often just moves on.

Not to mention, a home being air sealed as a retrofit typically has lower VOCs  than typical new construction, having long since outgassed most of the nasty stuff. 

It's usually quite difficult to retrofit it so tight that it's a health hazard without mechanical ventilation. But while it's harder to retrofit to truly tight standards, there's no reason not to try.  The interior wintertime humidity levels are a good indicator of when you're making a real difference (and when to consider active ventilation.)

Posted By Torben on 22 Dec 2011 10:59 PM
I don't subscribe to the idea that a DIYer is more likely to have problems with their home than a professional. On the contrary the DIYer is more likely to be far more familiar with their home and give much more time and thought to how they build it. .

Torben; you are not a typical DIYer or a typical owner/builder, as you are also an engineer.

I'm not sure what a typical DIY-Owner/Builder is. It certainly is not a typical homeowner. The only ones I know have all had some kind of technical or trade background. I think you would have to agree you are not a typical contractor when it comes to green building. I would guess you would be very uncomfortable having a typical contractor build a SIP home for you. The biggest surprise to me in this process has been the lack of knowledge and excess of confidence of some contractors. I think there is a real advantage to having a homeowner extremely familiar with his house because he built it himself. I don't mean to imply that is typical.

Torben; most O-B's require a lot of hand holding thru the whole process, many quickly become overwhelmed and at the end of the project swear they would never take it on again. You are an exception to the rule................... I have helped many thru the process and sometimes feel like a therapist.

Chris,

Amen.  I feel the same way at times but I still love walking clients through the process of building their own home.  Since I retired from teaching I now donate a lot of time to helping the owner builder use new technology.  Some homes take up to a year and half to complete.  Banks are not happy about that sometimes.

Alton; The owner - builder realy needs to consider the risk and rewards, time = money. While you are building your home you will be paying the construction loan interst and your current home mortgage/rent. How much will you need to save on your construction costs in order to offset the additional construction loan costs, loan extention fees, money lost thru mistakes/ learning curve, strain on your marraige and the aggravation associated with your build.

Uh, I felt my ears burning, so I looked into this thread...

I can really see the need for some sort of liaison between homeowners and the construction industry. Alton and Chris you have both acted in that role, so you know what I mean. There are homeowners who feel like they need to resort to doing it themselves because the industry is so reluctant to do it right (I'm speaking mainly about energy efficiency but also material fitness for purpose). That homeowner ends up in over his/her head because there is so much more to the process that they didn't know they needed to know.

Back to the original topic, I found a free online version of Manual J and plugged everything in down to wall and window type and orientation. I then found a professional HVAC guy who uses Manual J and he got identical results as I did. I wouldn't have relied on my own Manual J numbers, but the fact that mine came out the same as his seemed to be a good check of his accuracy.