A colleague presented this one to me. 

Tech Pro says if all 3 components (floor, roof slope and knee wall)  behind the knee wall are uninsulated, than the knee wall is considered the thermal boundary.  However, what if all three are insulated?  In the file he is reviewing (done by an external EA) the knee wall was chosen as the thermal boundary.  I agreed with this in principle because the space is unheated.  You could treat the knee wall as buffered because the attic space behind it is not vented.  However, it could be argued that the roof slope is the better choice as it has a much higher R value (R28) making the space more likely to equalize to the internal portion of the house (knee wall is R10).  

I think it can be modeled either way, but an EA would have to defend their decision.

Any opinions... or pointers to clarifications in the Tech Procedures?

Thanks

Jack thanks for the feedback, I think we agree.  But I do have one point of contention.  Far be it from me to argue with EA of the Year, but here I go. ;)

Often in older half-story homes, when all three surfaces are insulated, there is no rafter space open for venting.  Occasionally there are gable end vents, but not often.  I would contest that this is then 'buffered' based on 3.5.4.3 in the TP, the last line of which specifically says that attics are not considered 'buffered' because of venting.  If there is no venting... ??? buffered just like an insulated garage, IMHO.  People should also keep this in mind when attached garages have unfinished interiors and open eaves... they technically are vented and are not buffered.  I've seen cases like that as well.

Anyway, moot point, I think the sloped roof is the better thermal barrier choice in this case, based on more than double the R value of the kneewall. But I just want anyone reading this thread to keep in mind that there are exceptions to every rule.

Cheers and congrats on the award.

Hi Jeff,

Thanks for referencing the NRCan document. That is always a handy document to have whenever we have a question. I agree with your thoughts in general.

I'm suggesting Roof as my first "rule of thumb" because I want the biggest volume possible for my net zero reno projects. The extra volume helps a lot. QA at NRCan can accept this explanation as long as the final insulation layer is at the roof and not the knee wall. Air sealing is way simpler as well in my opinion. 

Jack