Monday, July 25, 2016

Construction - DIY Concrete Wall Insulation

recent post delt in part with the construction of the concrete east wall of the future
garage pictured at the left with the French drain at its bottom.  It needed to be insulated exteriorly then backfilled to almost half of its height.  Here I am using that wall to demonstrate a DIY custom insulation method that I will be using subsequently for those parts of the north and west walls that lie above the horizontal insulation/watershed umbrella. (There is no need for insulation below the umbrella so as to maintain as much uninhibited earth contact as possible.) The method I am using was somewhat described in another post in conjunction with the first retaining wall west of the house. What follows is a more detailed account. Remember that a click on any photo enlarges it for better viewing.

The DIY design described here is a bastardization of the proprietary product Insofast.  Perusal of the Insofast website was encouraging -- the product would seem to be perfect for insulating our concrete walls inside and out. However, for our  requirements, their quote was at least $3,000 more than the following DIY method.  And Insofast maxes out at an insulation thickness of 2.5", which would have worked for the garage, but not very well for the house. Nevertheless, I am indebted to Isofast for stimulating improvisation.

Essentially, the insulation is expanded polystyrene (EPS) (styrofoam) supported by metal drywall track (ordinarily used as the bottom member of a metal stud wall, i.e., the part that is fastened to the floor).  Even though the track is galvanized, there would be a limit as to how much moisture it could endure before rusting so I am wrapping the entire assembly in plastic sheeting to isolate it from the concrete on one side and from the outside environment on the other side.  The secondary function of the track is to support the cement board cladding that serves as a base for the stucco finish.

From my drywall supply company, the track comes in two widths -- 2 1/2" and 3 1/2" -- and two gauges -- 20 and 24.  For the garage where maximizing the R-factor is not critical, I used 2 1/2".  For the house walls, I plan to step up to 3 1/2" track for a higher R-value.  As for thickness of the track, the thicker 20 ga is more suitable for supporting the heavy stucco wall.  It then takes two thicknesses of  foam board --
1 1/2" and 1" -- to fit the 2 1/2" track. The plastic sheeting is good quality 6 mil that I buy from a farm supply store in 24' widths -- the kind sold to farmers probably for covering hay. 

My original intention was to use one thickness of 1/2" cement board but, after finding that it was not rigid enough to support a thin layer of stucco, I used two thicknesses. Even two thicknesses would be too compressible and would have to be backed up by something more rigid for long term durability in high traffic areas.  In our case, most of the insulated concrete walls will be below grade after backfilling and most of those above grade are away from high traffic areas.  The lone exception is the east wall discussed here; it will be in a high traffic area but less than half of it will be exposed.

Therefore, I covered the top of the first layer of cement board with a pressure treated 1 x 6 and butted the second layer of cement board up against it.  Not only does the 2 x 6 make the top of the insulation more rigid but it also widens the wall by 1 1/2" which will work better with the stick-built truss wall above it.

Shop-Made Tracks
The drywall tracks that support the insulation and the cement board were shop-made
Drilling holes for the Tapcon screws 
ahead of time.  All but the first track at the south end of the wall and the last track at the north end have to be back-to-back.  Before screwing them together, one of them was
Pre-made tracks already cut to length for the north garage wall
perforated on one of its short sides with holes large enough to accomodate the 3/16" Tapcon screws that fasten them to the concrete wall.  The drill press and a long wood block cut from a 2 x 4 to fit inside a track and marked to guide hole placement made short work of holes drilled 8" apart, which was more frequent than necessary but it gave more options for screw placement during installation.  Next, the pre-drilled track was clamped  back-to-back with another track and the two of them screwed together with metal screws in pre-drilled starter holes.

Screwed-together double track showing holes for Tapcon screws in the bottom piece
Jumping ahead to the time when the tracks would be fitted to the wall, let me share what I found to be the best way to cut them to length.  Use straight-cutting metal shears to cut all four short sides at the cut mark.  Then, instead of trying to cut across the long dimension of the back-to-back tracks, simply to bend the tracks back and forth a few times to "worry" the metal into separating.

Pre-cut Foam Board
Purchasing ESP is cheapest in 4 x 8 sheets but, to fit the present situation, the sheets need to be split lengthwise into 24" pieces.  I have sawed them by two methods and prefer one over the other for the sake of accuracy. The less desirable method is to use a straight edge guide and a circular saw.  Accurate cuts are difficult for several reasons that I will leave up to the reader to find out for him/herself.  The other method is to get someone to help run the sheets through a stationary saw against a proper rip fence. For this, I have used both my table saw and my radial arm saw.  (The latter works for me because the saw table is quite long -- 16' -- which is atypical for arm saws.) Either saw does a splendid job of making a mess of the shop, so I recommend planning ahead and cutting as many sheets as possible in one session to minimize the number of times the mess has to be cleaned up.

The advantage of using a stationary saw is that the sawn edges are straight and each piece is a standard width.  The straighter they are the better they fit the tracks and the less air space exists between the foam and the track.  Air spaces are also minimized by having consistent and matching widths of the two pieces of foam that go together to fit the track.  I had some 1 1/2" foam pieces left over that were cut with the circular saw for the west wall behind the retaining wall.  When they were matched with the 1" that were cut with the radial arm saw, there were air spaces up to 1/2" in some cases -- which was acceptable for a garage wall but would not be for the house walls.  And a stationary saw is mandatory for accurate cutting of thin sheets like the 1" due to the foam's flexibility (even when they are doubled up to increase rigidity).

The pre-cut 1" and 1 1/2" foam panels are easier to cut to length and install between the tracks when they are glued together.  Accordingly, a spray adhesive specific for EPS did the trick.  Just be careful that the edges are flush when gluing. 

Installation of the Track and Insulation Panels
In order to protect the track from moisture, the plastic sheeting must be in place before the
Plastic sheeting covering the near side of the wall;
remainder thrown over the top of the wall temporarily;
 four tracks installed supporting four courses of insulation
first track is installed.  
The 24' width of the 6 mil plastic sheeting was a little more than the length of the wall so all I had to do was cut a piece from the roll that was a couple of feet longer than twice the height of the wall.  It was draped over the wall so that it reached the ground and turned outward a foot or so on the near side of the wall.  The balance of the sheet went over the top of the wall and hung down on the far side temporarily. The plastic was left long towards the south (left) end of the wall so that later it could be folded under the cement board in such a way as to protect the left side of the first track.
Tracks and insulation installed, ready for cladding

The south corner track, a single member rather than back-to-back, was cut to length and screwed to the wall through the plastic using a hammer drill, drill/driver and 
1 1/4"  x  3/8" Phillips head Tapcon screws. The first section of foam was cut to length and fitted into the single track, in this case, to the left.  The next (double) track was then slipped over the right side of the foam and pressed by hand or even tapped with a block of wood and a hammer so as to eliminate as much as possible any air spaces between the foam and the left and right tracks.  It was installed with the track having pre-drilled holes to the right. While holding the track firmly towards the left, the right track was screwed to place through the plastic sheeting.  The remainder of the sections were installed in a like manner.  At the north end, the plastic sheeting was folded back over the last track to protect it from the right side.

Installing the Cement Board
Sheet plastic folded back over the wall so as to protect
the tracks and insulation from the exterior environment;
first layer of cement board partially installed 
The cladding comprised the cementitious (cement) board and a top coat of stucco both of which will always absorb some moisture from the environment. So the plastic sheet was brought back over the wall to cover the top of the insulation, hang down in front of it and extend over the French drain at the bottom so as to separate completely the insulation and tracks from the cladding and direct any running moisture to the drain.

Unfortunately, the cement board comes in 3' x 5' panels which doesn't equate well with the tracks on 24" centers.  So they had to be cut to width as well as length which is best done with a nibbler rather than a circular saw in order to avoid the noxious and health-impairing dust.  My nibbler was originally purchased for $80 on Craigslist at a time when I thought I would be gladding the exterior of the house with fibercement board (which later morphed into preference for steel siding which will require a different nibbler). It wasn't until later that I found out that I was lucky to have the fibercement nibbler because it cuts cement board as well.

The installation of the board was time-consuming.  In order to hang it with the same
One-by-six in place; second layer of cement board
butted up against it and ready for parging with stucco
cement board screws that are used for fastening the board to the bathroom floors and shower wall studs, pilot holes had to be drilled in the metal track.  And in order to keep the twist drill sharp for the metal, holes had to be drilled through the cement board first.  This meant three tool changes -- a cordless drill with a 3/16" masonary bit, a cordless drill with a 1/8" twist drill and a drill/driver for the screws.  

The mismatch between the cement board sizes and the 24" OC of the tracks created multiple left-over pieces.  Since I knew that there would be two layers of cement board, I didn't hesitate to work the small pieces into the wall for the first layer even though they were not as rigid as large pieces.  The 3' x 4' second layer pieces covered up the multiple seams of the first layer and added rigidity.  

Adding the Stucco
Fortunately, parging the cement board with stucco was easy after my experience with fiber bonded cement for the dry-stacked block walls of the solar connector.  The material that I
Wall after stuccoing; French drain covered with a layer of
 clean rock; 
about half of the wall will be buried in
backfill starting with +/- 4' deep at the north corner and
ending with +/- 2' deep at the south corner 
used was Quickcrete Professional One Coat Fiberglass-Reinforced Stucco.  And, as with the fiber bonded cement, getting the consistency just right is critical for easy handling. If it is too dry, it falls off instead of sticking to the wall and, where it does stick, tends to be too thick.  If too wet, it falls off and, where it does stick, tends to be too thin.  With a little experience, the right consistency goes on easily a trowel-full at a time (painfully slow) and can be troweled to a rather smooth finish. The manufacturer recommends limiting the size of each mix to that which can be applied in one hour.  An 80 lb bag mix was about right for meeting the time requirements during the hot summer after I waited to start the project in the afternoon when the wall was in the shade.

It is mandatory to keep a hose or sprayer handy.  Whether the substrate is concrete, other stucco layers or, as is the case here, cement board,  it must be kept moist for the stucco to adhere properly.  And the stucco should be kept moist for a couple of days afterwards to control crazing due to shrinkage as it cures.

In the present situation, both layers of the plastic sheet were trimmed so as to overlay about half of the top of the French drain so that any moisture between the inner sheet and the wall or between the outer sheet and the cladding will be directed to the drain but without completely covering the drain with plastic.

Interior Surface of the Concrete Walls
The east wall of the garage used here as an example and all of the concrete house walls that are insulated on the exterior will be insulated on the interior in a similar way using plastic sheeting, metal track and ESP foam board.  The only difference will be drywall for cladding instead of parged cement board.   

Insulating both sides of the wall will give a nominal R-20 for the garage where 2 1/2" tracks were used and R-28 for the house walls where 3 1/2" tracks were used.  An R-20 for the garage is probably overkill but an R-28 for the house is marginal compared to the 
R-50-ish stick-built walls and ceilings.  Fortunately, I can live with it since the amount of concrete that will be exposed above the insulation-watershed umbrella will be minimal. 

Backfilling Against the Wall
Provisional backfill to ward off the runoff from behind the wall;
the carpentry  work can now resume  in earnest
The reason for interrupting the carpentry phase and concentrating on the garage wall was to insulate and clad it so it could be partially backfilled to force the runoff from the the backfill behind the house away from the east garage wall and to cover the French drain.  Accordingly, I dropped a few track loader buckets of dirt against the wall to suffice until the entire slope could be properly contoured eventually.


  1. Two questions:
    1) I am looking at doing something similar but with two layers. Only 10" will be exposed and I only really need the first two inches of the subterranean foam covered. You had listed two layers of fiber cement board but I'm wondering if you went with narrower spacing for the "furring", or your steel strips, if you think one layer would suffice.
    2) Why foam on the inside of the wall as well? You have a large thermal mass that could be used to your advantage. While the foam will make it easy to get the room up/down to temperature, it will also make it more susceptible to night/day swings.

  2. Thanks, Paul; your questions are insightful.

    I used two layers of cement board only on the one wall discussed here because a lot of the wall will be exposed above the backfill and it will be on the side of the house seeing the most traffic.

    With one exception, the house walls were insulated on the inside only above the insulation/watershed umbrella component of the Annualized GeoSolar system (click on "Featured Post", left column, for details on AGS) but extending a foot or so below the umbrella as a finagle factor. The foam on the inside of the north garage wall, discussed here, insulates the earth lying under the umbrella behind the garage. Similarly, the floor of the garage is insulated to serve as the umbrella on the east side of the house rather than to make the garage warmer. But the foam on both sides of the east wall of garage is indeed to make the garage warmer. The temperature in the garage will swing as you suggest but the temperature in the house will vary only a few degrees year-round.


As a do-it-selfer-in-training, I welcome your comments.