Solving a (self-created) Problem with Heat Distribution

First, some background…

Using a ductless mini-split to heat and cool a home (even a passive house) takes some thought.  One needs to think about where, exactly, the mini-split head, or cassette, will be placed, and how the warm (and cool, in the summer) air that it creates is likely to flow.  Take my home, for example.  We opted to place the “winter” mini-split head in the “living” area on the first floor (we also placed a second, separate, mini-split head at the top of the stairway to the second floor, which is essentially the second floor “hallway,” to cool the entire house in the summer).

That single mini-split head (on the first floor) heats the entire 2,000 square feet of living space (1,000 sqft on each floor) on all but the coldest days, and does the job amazingly well.  When the outdoor temperature is above 20 degrees, the 2nd floor stays within two degrees of the first floor (which, again, is at 70 degrees).  Frankly, that’s warmer than I prefer, since all we do is sleep on the 2nd floor).  But it makes my wife happy.  When it gets below 20 (and the sky is overcast during the day), the 2nd floor can drop to 66 degrees, and if the temperature drops to the single digits for prolonged periods it can eventually go a bit lower. In those instances, I’ll turn on the 2nd floor min-split, and it will periodically kick on, when necessary.

And now, the problem…

When the house plans were drawn, the architect anticipated that the entry to the first-floor office would be directly off of the living area.  He also anticipated that the first-floor mini-split would be mounted on the narrow wall between the living area and the kitchen area.  Both are shown in the drawing below:

(Ther drawing also shows an exterior door on the south wall that I decided against because it just wasn’t necessary.)

When I framed the interior of the house, I decided to move the office entry to the hallway because it unnecessarily ate up wall space in the living area, leaving no room for a T.V., for example.

I also had to find another place for the mini-split because the wall that the architect had intended to place it on was too narrow for the mini-split head; it wouldn’t fit.

Given those issues, I changed the first floor as follows:

Moving the office door to the hallway (and eliminating the unnecessary hall closet) freed up valuable wall space in the living area and widened the hall.

And as the drawing also shows, I moved the the mini-split to a wall that had suffficient room.

Here’s how it looks:

In this photo, you can see the location of the mini-split relative to the back hall:

And here is a similar view in which you can see the door to the office: 

So what’s the problem?  Somewhat ironically (at least in my view) the only problem that we’ve had is that, during the winter, the office, which is the room closest to the mini-split,  can be two to six degrees colder than the rest of the house, depending on the outdoor temperature. The only exception is during sunny days, when the office warms up perfectly.  But that leaves many winter mornings, days, and evenings when my wife isn’t happy (because the office is her place of work).  And it should be noted that she almost always works with the door fully open.

I pondered over this problem for two years.  Initially, I thought that the problem was the ERV.  More specifically, there is only one first-floor ERV supply, and it is in the office.  Why?  Because that’s where Zehnder said it should go.  And it’s consistent with the general rule; pull stale air from the kitchen/baths/laundry room and push fresh air into the bedrooms (the office could also be considered a bedroom).

But I eventually found out that the ERV wasn’t causing the problem, and had no detectable (at least by me) effect on the room temperature.  So while the ERV is constantly pushing cooler air into the room, the temperature (above 60 degrees) and flow ( about 24 cfm) of that air just isn’t enough to make a difference in the room temperature.  This should have been no surprise, as it is consistent with what the experts suggest.

So my current theory is a bit different.  First, after framing the office walls, I insulated them Roxul so my wife could work in relative peace if I chose to watch TV; admittedly a bad idea with regard to heating the room. But I also suspect that any warm air that makes it into the rear hall is probably more likely to rise up the stairs (which are to the right) than into the office (which is to the left).

The easy thing to do would have been to install some baseboard electric resistance heat in the office, which my wife (or a thermostat) could turn on as necessary.  But that clearly seemed like a wasteful overkill to me.  So what I decided to do instead was install a Tjernlund ASI AireShare Room-to-Room Ventilator fan. The idea came to me as I was painting the wall in the family area that separates the family area from the office.  Unsurprisingly, the air near the ceiling (which is almost 9′ high) was several degrees warmer than the general temperature of the house down among the living.  It therefore seemed reasonable to believe that, if I could push some of that air into the office, the problem might be solved and, since the fan only draws 25 watts at full speed and pushes the air at 75 cfm, I figured that its use would prove to be much more user-friendly and cost effective than baseboard heat.

If the intent is to move warm air, ideally, these ventilator fans would be installed with the intake high on one side of a wall and the diffuser low on the opposite side. In doing that, the bay in which the fan and diffuser are installed acts as the vent; air enters from the top and is pushed out through the bottom. However, I decided not to do that because, as I stated earlier, I had insulated the office walls with Roxul, and I didn’t want to completely remove the insulation from the bay in which the fan was to be installed.  Of course, this meant that the warm air would be brought into the room closer to the ceiling than the floor, which is less than ideal.  But the room isn’t that big (it’s about 120 square feet), so I was hoping that it wouldn’t matter.

With minimal effort, I was able to cut the hole on the intake side of the wall about a foot from the ceiling, clear out the insulation in a space of about 18 inches, and seal off the bottom of that space with a piece of 1/2″ foam board and caulk.  Prior to sealing off the bottom of the space, I fished Romex up from the basement and installed a variable speed wall switch. The intake vent and fan are located at the top of the space, and the diffuser is located at the bottom.

You can see the intake for the fan in the upper left-hand corner of this photo…


…and the exhaust above the chalk/cork boards in this photo (it’s a bit difficult to see because it is a narrow diffuser rather than a traditional vent)…

Here’s a better photo of the diffuser (my apologies for the temporary “flow-detector”) and the variable speed switch…

The fan has now been in service for about a month, and I am happy to say that it has performed better than I had anticipated.  So far, it has resolved the issue completely.  For example, last night the low was 23 degrees, and the entire house, including the office, maintained 70 degrees +/- one degree. While not annoying, the fan is detectable at full speed.  However, we run it at what I would estimate to be one-third to one-half speed, and it is essentially silent.

Exterior Foam Insulation

Just to backtrack a bit…For the exterior insulation, we used two two-inch sheets of Polyiso held to the sheathing with 5/4 by 4″ poplar furring strips and six-and-a-half-inch Fastenmaster Headlok screws.  The one-by-fours cost us about 25 cents per linear foot.   That seems like a decent price, particularly since they’re a full 1″ thick (actually the thickness varied a bit, but was generally about 1 1/8″, so we planed them down to 1″).


We’re using Polyiso because it has a higher R-value than EPS (which is what the ICFs are made of) or XPS (which is what we added to the outside of the ICF foundation).  Polyiso has an R-value of just over 13 for 2″.  Tuff-R and Super Tuff-R are Dow’s Polyiso panels.  Super Tuff-R has a more durable facing than does Tuff-R.  R-Max Thermasheath 3 is another brand of Polyiso that Home Depot stocks, and appears to be less expensive than the Dow products.  The R-value is the same for all three products (i.e. R-Max reports 13.1 and Tuff-R reports 13).

Home Depot in our area stocks R-Max.  It is $30.25/sheet off the shelf and on the Interweb.  I went to the local store and told them I needed 150 sheets.  They put it through their “bid room” and came back with a quote of $26.73/sheet, which is an 11.6% discount.  So the total cost ended up at just over $4,000.

Extending the Gables

As previously discussed, the exterior walls are covered with 4″ of polyiso foam.  On the gable sides of the house, that foam extends up about 18″ past the second floor ceiling.  This places the top of the polyiso in line with the top of the 18″ of blown-in cellulose that will be placed above the second floor ceiling.  Since the remainder of the gables will have no exterior foam, we had to build out the gable walls five inches to place it in line with the wall below it…actually, we built it out five-and-five-eights inches so the gable siding (fiber cement board and batton) would be a bit proud of the siding on the building below (fiber cement lap siding).  I’m sure that some (many) might think it over-kill, but we accomplished the build-out with 2x4s.  First, we “Timberloked” vertical 2x4s into the vertical truss members.  To do this, we placed the 2x4s on edge, attached a 2×4 to it on-the-flat to create a “nailer,” and attached a strip of 5/8″ plywood to the back.  This was then screwed through the sheathing and into the vertical studs in the wall truss.  That brought us out 4 1/8″.  Then we nailed horizontal 2x4s to the verticlals every 12 inches.  That brought us out to 5 5/8″.  We also used horizontal 1x4s to provide us with nailers over the 18″ of foam that extends into the gables.  To keep it in line with the walls above, that was also packed out with 5/8″ plywood strips, and the plywood was notched to allow air to circulate and travel up the wall (behind the siding), into and through the gables, and out the triangle gable vent that will be placed at the top.


Window Extension Jambs

Because of the four inches of exterior foam board and full one-inch-thick strapping to hold it on and provide a nailer for the siding, we needed to create extension jambs for the windows and doors.  As with most tasks, there are a number of ways in which that task could have been handled, and the solution is partially dependent upon the location of the window within the exterior wall.

In our case, the architect decided that the windows should be installed within the 2×6 framing (i.e. flush with the Zip sheathing), rather than even with the exterior plane of the wall/siding structure.  The architect did this to achieve better thermal performance, and also to provide a more appealing exterior by providing depth and shadow lines.

Given that position, we had to come up with a method for extending the window (and door) jambs five inches to place the exterior jamb surface to a place where it could be properly trimmed out; trimming the jambs out that far, would put them in the same plane as the surface of the 1×4 strapping.

Rather than building plywood extension jambs that extend into the rough opening and attach to the wide surface of the 2x6s that form the opening, we decided to use Azek, and build the boxes so they would attach to the outside of the building structure, with screws driven through the sheathing and into the 2x6s that form the rough window opening.

The jamb extension boxes are glued and tacked together.  We cut a 3/8″ wide by 7/16″ deep rabbet into the inside rear edge of the sides and top of the boxes.

IMG_0158 1

This allows the Intus windows (which don’t use a flange) to be installed 1/2″ proud of the wall sheathing and fit into the extension jamb and over the sill.  We also put a 5 degree pitch on the box sill, and made the sill a half inch wider than the sides and top to allow water to drain over the trim sill.

IMG_0157 1

Attachment flanges (1 x 1 3/4″ Azek strips) are tacked and glued to the sides of the boxes to allow the boxes to be screwed through the sheathing and into the studs.  On the smaller windows, we only put these flanges on the sides of the boxes (as shown above).  On the larger windows, we also put them on the top and bottom for added support.

Here are a couple of photos of the boxes installed (no nailers or trim yet):



Once the box is attached to the house and the 4″ of foam board is applied to the exterior of the walls, we glued and pocket screwed 1×8 Azek “nailers” to the sides and top of the boxes to serve as an attachment point for both the trim hoop and the siding. A 1 x 3 “nailer” is attached to the bottom.

Here’s what the nailers look like when attached:

IMG_0293 IMG_0292

And here’s what they look like on the house:

IMG_0373 IMG_0372

Once the nailers are attached, we’re ready to attach the trim.  The trim, which will be in the “Arts and Crafts” style, will look like this:

IMG_0284 IMG_0286

The sides are 5/4 x 4 1/2.  The top is 6/4 x 5 1/2, and the sill is 1 1/2 x 2 1/2, with a 15 degree pitch. The inside dimension of the sill is 1/8″ wider than the sill on the box.  When installed, the trim sill is pushed up against the bottom of the sill on the box.  The trim hoop is sized to result in a 3/8″ reveal all the way around.

All in all, it’s an extremely sturdy structure, and they’re very quick to build (much quicker than this explanation conveys).  However, care must be taken to ensure that they’re installed square (as we built them, there is about a quarter inch of play each way that can result in an out-if-square installation if you don’t pay attention).  Also, the Azek is flexible enough that variations in the sheathing (e.g. where the wall bows slightly) can result in a bowed front edge on the box.  If the bow is in the sides or the top, the 1 x 8 nailers will take it out when they’re attached.  But if the bow occurs in the sill, the 1 x 3 nailer may not provide enough rigidity to remove it.  The easier solution might be to shim between the attachment flange and the sheathing if necessary.


Attic Insulation

The house uses raised-heel roof trusses to allow for 18″ of blown-in cellulose above the second floor ceiling.  For maximum effectiveness, that insulation has to extend to the outside edge of the four-inches of R-Max foam board that will be attached to the exterior of the long sides of the house (the gable ends will be handled differently and discussed later). Regardless of what we used to contain the insulation at the outer edges, we knew that it would be easier to install before we installed the Zip sheathing on the second floor ceiling.

Our initially thought was to use insulation netting as a baffle to contain that insulation at those two outside edges.  But after giving it some thought, and discussing it with an insulation sub, that idea didn’t seem ideal.  We felt that the netting would be too unstructured; potentially allowing insulation to interfere with the soffit vents and the flow of air to the ridge vent.  That same quality would also make it difficult to install properly, as it would have to drop down between each of the 2×4 truss members. So we settled on an alternative using OSB and some 2×6 blocking.

The OSB, ripped into two-foot-wide strips makes up the exterior baffle.  The 2×6’s were stood on edge and placed between each of the 2×4 truss members to a) complete the baffle between the truss members and b) create a backer that the lower edge of the OSB baffle could be nailed to.  The top edge of the OSB was then tacked into the truss members that formed the roof line.  While it may be a bit difficult to picture after reading, it was fairly inexpensive and easy to execute.  It took five sheets of OSB and ten 2×6’s.

Here are some photos that show the different elements and perspectives:

Here you can see the 2×6’s that were attached between the truss members, above the 2nd floor ceiling:

IMG_0232 1

In this photo you can see the two-foot wide strips of OSB tacked in place to the 2×6’s (at the lower edge) and the roof members of the truss (at the upper edge):

IMG_0239 1

The gap that you see between the 2×6’s and the ceiling in the photos above and below is 3.5″ wide.  The bottom of that gap will be covered by the 4″ of foam board that will be attached to the outside wall.  So, in effect, the top edge of the foam boards, which will run along the bottom edge of the truss members, will complete the floor for the attic insulation.

IMG_0237 1