Hi, My wife and I are building a new row house on an existing foundation of an old row house in Pittsburgh.  We hired an architect and subsequently a very reputable builder to renovate the row house.  The structure has only one shared wall (we own and adjacent city lot and plan to have a courtyard so the home won’t have a shared wall on both sides like most row houses.  As such, we only have one wall to run ductwork and plumbing up and very little wall space to insulate.  The HVAC sub has advised that we will need two furnaces and condensers with the 2nd set being on the third floor due to the lack of wall space (we have no interior walls that run up all three floors and we plan to have 6 zones in the house (lots of duct work). We told the HVAC guy we could reduce the number of zones by 1-2 easily to make ductowork fit better.

I started thinking about the $4k plus cost of the unnecessary hvac equipment and started to wonder if there was a way to better insulate my structure such that i can use two walls for ductwork thereby allowing for only one furnace and condenser.  The saved mech costs could help pay for the additional rough carpentry and insulation, giving me a much more efficient home for little extra cost. 

Here is the catch – the interior of the home is about 16’10” wide and 72′ deep.  So losing interior space on the two short walls – no big deal.  Losing it on the long wall – tough to swallow.  Further complicating things the area where we are building does not have the best expertise for green building, so we will run into expertise issues with complex solutions even if they are affordable.  Adding to that – while I am extremely happy with my builder and architect, neither of them are extremely knowledgeable in green solutions.  As such, I need to find a solution that not only improves the envelope of my home and gives me a second wall to run ductwork, but I need a solution that has “low risk” implementation such that the framers doing my house are not scared off by “complex” solutions or bid overly conservative bc it’s a new thing for them. 

I plan to hire a local energy consultant to help size our HVAC once we determine a more appropriate wall solution, but I currently need to determine what that best wall solution is.  As I said, my builder is good, but not green, so they haven’t proposed a solution that provides the best ROI while being more green, and they are reluctant to price out 4-5 solutions because of the time involved without charging me extra, which I understand.  I haven’t spoken with my architect yet, but I also flip houses on the side, and so i figured I would determine the solution that is easiest to implement. 

The way I see it, I have a few options, and the short exterior walls are the easy solution:

1. Short exterior walls – The easiest solution appears to be to lay 1-2″ of interior rigid insulation inside my 2×6 walls which will eliminate the thermal break associated with my framing.  I am in zone 5 in the US and need 2″ of exterior insulation to avoid moisture accumulation so using exterior sheathing would require verticle strapping, and the row house will be built up to the property line on the sidewalk, so i’m not sure what an additional 2″ of external sheathing would do to the look of the home.  I also don’t know if I would need my builder to install a drainage plane.  For those reasons it seems like interior rigid insulation with dense pack cellulose makes the most sense.  If I am wrong, please let me know.  That would give me R25-30 on my two short walls with no thermal break. 

 

2. Long Exterior Wall – Long wall is harder because I don’t want to lose much interior space and I need to be able to run ductwork up this side as previously mentioned.  The way I see it, I have four options –

a) double studded 2x4s with no spacing (or 1″) between walls.  I only lose 2-3″ of interior space and have no thermal breaks.  I can run 4″ ductwork up the interior wall and still have 3.5-4.5″ of dense pack cellulose behind it providing R-14-18 insulation, and almost R30 insulation where there is no ductwork or frame material.  This seems like the easiest solution to implement, but i lose 2-3″ of interior space across 72′ and 3 floors. 

b) same as above but I’ve read that there is a way for the outer 2×4 wall to extend out over the foundation, and not be a load bearing wall.  This would allow more spacing between walls for more insulation, but I don’t know the details around doing something like this.  If it’s an easy implementation, perhaps this is the way to go. 

 

c) 2″ (or more) of exterior foam sheathing with verticle strapping and a drainage plane.  This is easier as this long run side of the home isn’t adjacent to the public sidewalk.  The builder already pitched a 1.5″ closed cell + 4 inches of bat insulation package for the whole home, so they should be familiar with implementing an appropriate dry-inward wall associated with exterior vapor barrier insulation solutions. 

d) build a larson truss – This really accomplishes the same thing as B i think right?  I just don’t know which is more difficult to implement. 

The fourth long wall will remain a 2×6 wall with dense pack cellulose and a fair bid of ductwork.  The wall isn’t shared with the neighbor and there is a slight gap (1/2″ or so) between my home and the neighbors, but I am stuck with a 2×6 for this wall. 

Also, the foundation is a +150 year old stone foundation that is quite moist and only about 6′ high.  We have no plans to insulate this basement so far… not sure if there is a “good” solution to an old pittsburgh basement like this.  

 

So, am I missing some other easy/more cost efficient implementation for this long run wall?  Any help would be appreciated.