Innovative, energy efficient, cost effective solar home in Montana

I recently got the chance to see a very innovative solar home being built near Bridger Montana by Andrew Ray of Rational Design/Build.

Andrew (and his frequent conspirator Clint Wicks of CW2 Construction) have been building homes for fifteen years, with Andrew getting his start with Steve Loken in Missoula, but this time its a really special home in that its for his own family. He is a very innovative builder and careful planner, and on this home he has taken out all the stops and included all of the best energy efficiency, solar, and material saving features he has used and studied over the years. Its a fascinating home.

The end result is a house that has about half the heat loss of a conventional construction home and that will get a large fraction of its space heating from solar. And, this home uses less material and took less labor to build than a conventional construction home. Andrew has tracked costs carefully, and feels that savings on material and labor (see below) offset the the costs of more insulation and the solar heating features.

The house is about a month from completion, and after completion, Andrew will document the house in a full article or possibly a book. Keep an eye out for this, as it will be well worth reading.

Table of Contents:


Floor Plan:

The house is about 1800 square feet . It has a great wide open feel with views of the surrounding ranch land and mountains in all directions.

Low Thermal Mass Sunspace:

I mention the Low Thermal Mass Sunspace (LTMS) first because I really like them and I think they are underused in the solar housing world. The LTMS concept is explained in detail here, but in a nutshell the advantages are:

The Low Thermal Mass Sunspace on Andrew's house is two stores high with 213 square feet of glazed area.

On Andrew's house the sunspace adds efficient solar heat collection with much better control over over the heating.

Compared to the passive solar technique of using a lot of south facing glazing, the LTMS 1) allows more control over the solar heat gain -- it can be turned down or shut off if the solar gain becomes more than the house can use for part of the day, and 2) It eliminates the large heat loss (especially at night) associated with the large amount of south glazing in conventional passive solar homes.

Compared to using conventional commercial solar thermal water or air heating collectors for space heating, the LTMS is just as efficient, cheaper, and looks nicer.

It has the additional benefit that the space inside the LTMS can be used for a wide variety of uses (lounging, clothes drying, wood drying, kids play area, climbing wall, shop, ...)

Andrew did a great job of visually integrating the sunspace with the home -- I think its fair to say that the house looks better with the LTMS than it would without it.

One of the challenges of using an LTMS is distributing the solar heat to the house in all of the places where its needed. Andrew is using the carefully sealed and insulated crawlspace to provide hot air distribution to all parts of the house. So, solar heated air from the top of the LTMS will be ducted to into the crawl space with the help of a blower. Grills in the floor of each room will be sized to deliver the appropriate proportion of heat to that room. A nice simple system.

Click the pictures to see full size versions



The left picture shows what the Low Thermal Mass Sunspace looks like from the outside.

The right picture is of the inside of the sunspace. Andrew decided to make the depth only about 2 ft, which works fine thermally, but I'd also consider making the space deeper if you want to use it for other purposes. The picture shows the full sized door that provides easy access to the space.


LTMS during framing

There are two operable windows in the top row of windows and two operable windows in the lowest row of windows. These will provide for good summer ventilation.

The windows used in the sunspace are double glazed, clear, U-0.46 SGC 0.54 VLT 0.58. The additional materials for the Low Thermal Mass Sunspace added about $6000 to the cost of the house.

The crawl space is a sealed plenum that will be used to distribute the air heated by the Low Thermal Mass Sunspace to all of the rooms of the house. Two ducts and blowers will duct hot air from the the top of the sunspace and into the crawl space. There will be a scheme for varying the volume of air depending on the solar input and the home's heating needs. About 700 cfm of actual flow will be needed under sunny conditions to efficiently remove the heat from the sunspace.

The testing I did on our experimental Low Thermal Mass Sunspace indicates that the sunspace on this house will deliver about 250,000 BTU to the house on a sunny, cold, mid-winter day. This is the equivalent of about 3.2 gallons of propane burned in an 85% efficient furnace. Some rough calculations using our Home Heat Loss Calculator indicate that this will meet the full heating requirements of the house for a cold mid-winter day.

Some heat storage in the system would likely be desirable as the LTMS produces the heat over the daylight hours, and the heat loss is spread out over the full day. The crawl space heat distribution system offers some possibilities for heat storage, and Andrew may look into this after some experience with the house.



The wall design incorporates a number of innovative features that make the wall very low in heat loss while also reducing material requirements and build labor over other high efficiency wall designs.

Mooney Wall:

The wall is an inside-out Mooney Wall. Think of a regular 2 by 6 vertical stud wall, then add horizontal 2 by 4 straps on edge to the outside of the 2 by 6 wall. This creates a wall that is 3.5 + 5.5 = 9 inches thick, which allows an R34 wall with very little thermal bridging. The inner 2 by 6 wall is insulated with 5.5 inch fiberglass batts that run vertically, while the outer 2 by 4 wall is insulated with 3.5 inch fiberglass batts that run horizontally.

The only thermal bridging that occurs in the main part of the wall is where the vertical 2 by 6's cross the horizontal 2 by 4's, and this is less than 0.5% of the wall area!


More pictures taken by Andrew during construction:

The left picture shows the wall after standing it up. Note the metal strap diagonal shear bracing and the continuous header on the top of wall.

The middle picture shows wall being built on the floor.

The right picture Shows the walls after stand-up from the outside.


Continuous Header:
Rather than having separate headers over each window and door in the wall, Andrew uses a continuous header along the top of the wall. This saves labor and eliminates the loss in R value associated with each door/window header. The continuous header is visible in the left two pictures below.


Note in the pictures how very simple the framing around the windows is. This is due to the continuous header at the top of the wall as well as ordering windows that fit within the 2 ft vertical and horizontal framing intervals. All of this saves labor and thermal bridging. It also allows windows and doors to be shifted off of the 2' grid, something that 2' on center framing is frequently criticized for (it's inflexibility).

The insulation for both the inner and outer wall is fiberglass batt. By keeping to the 2 ft spacing on both the inner (vertical stud) wall and outer (horizontal runners) the batts fit between the framing with no cutting along the length of the batts. While I'm more of a fan of cellulose than fiberglass, Andrew likes the fact that he and his partner can do the insulating job easily themselves, and feels that when carefully detailed the fiberglass is as good as cellulose. The total thickness of the wall is 3.5 + 5.5 = 9 inches, which, at R 3.8 per inch for the fiberglass and a bit more for the siding etc., gives a total wall R value of about 34. And, the low thermal bridging inherent in the Mooney Wall means the actual performance will be pretty close to this.

Careful detailing of the fiberglass insulation batts.

There is no sheathing used in this wall. The diagonal metal bracing provides the shear resistance in the wall that OSB or plywood sheathing would normally provide, and this is a code compliant alternative. This saves a great deal of material and labor. Andrew points out that only eight sheets of OSB are used for the entire house (in locations where the floor will be tiled).

One of the pictures above shows the board and batten rough sawn siding, which is Fir cut at a nearby sawmill. The siding is screwed to the horizontal 2 by 4's that make up the outer layer of the inverted Mooney wall. There is a layer of roofing felt behind the siding.

Laths measuring 1-1/2" by 1/4", are used to make a rainscreen and hold the felt on during construction.


Foundation and Crawlspace:

The crawlspace on Andrew's home has a number of unique features.


The walls are made with Insulated Concrete Forms (ICF’s) using the Fab-Form system. In this system, the footings consist of a poly bag that is integrated with the bottom ICF. This integral footing allows the footing and wall to be poured in a single shot. It also eliminates the need to setup and strip off the wood forms that would be a part of using a conventional footing.

The Fab-Form ICFs are supported a few inches off the ground with screw legs that allow quick leveling of the wall using an electric drill to extend or retract the screw pads.

The system appears to save time, labor, and results in a footing and wall that are one monolithic piece.

The reinforcing for the concrete was done using the Helix Micro-Rebar system. This system uses short twisted pieces of of high strength steel which is just added to the concrete at mixing time. The reinforcing is basically embedded in the concrete when the concrete arrives at the building site. For Andrew's crawl space pour, a total of 245 lbs of the micro-rebar was added. The crawlspace walls and footing were done in one 22 yard pour.

The combination of the Fab-Form and Helix Micro-Rebar saved a considerable amount of labor and cost and resulted in a high quality and well insulated foundation/crawlspace.

The ground under the crawlspace is covered with 6 mil poly to protect against ground moisture, and the poly is covered with a layer of pea gravel to protect it. The poly moisture barrier is sealed to the mud sill at the top of the ICF's. The sheet rock on the crawl space walls is required as a fire barrier since the crawl space serves as a plenum for the heating system. The top chord bearing floor trusses mean that no rim joist is needed.

Pictures below of the crawlspace show the sheetrock over the ICF's and the truss style floor beams. Note the 2 by 6 tongue and groove floor decking that that will become the finished floor and no plywood or OSB floor decking.




The windows are from Milgard with the following specs:

All except the sunspace windows are triple glazed.

The windows are ordered at custom sizes so that they fit exactly within the 2 ft spacing of the vertical wall studs and the 2 ft horizontal rail spacing. This makes framing for the windows very simple.



Full span trusses are used spaced at 2 ft.

There is no OSB or plywood subflooring used over these trusses. Instead 2 by 6 tongue and groove decking is laid directly over the trusses, and when the main part of the construction is done, the protective layer of paper will be removed and the 2 by 6 decking sanded and finished to make the finished floor for the entire house. Another major reduction in the use of OSB in the house.



Instead of conventional concrete footings, Diamond Piers are used to support the outer edge of the large decks. These consist of a precast concrete head into which four 50 inch long, schedule 40 galvanized pins are driven using an electric demolition hammer. Pin lengths vary depending on conditions and frost level. No footing holes to dig, no forms to set or strip, and no concrete to mix and pour! The cost of each deck pier is about $140.

Cofair Deck Flash is used on top of the deck joists. This keeps water from sitting on top of the joists, and seals around all of the deck screws. Andrew feels that this is a very good idea, and that they will be code required soon. He has been doing this for years using site-cut strips of roofing underlayment or window flashing tape, but the Cofair Flashing for decks is pre-cut, all black, and slightly textured for when you have to walk on it.


The metal roof has a very nice look to it, and will last a very long time.

Note how the roof is trimmed on the south exposure to form overhangs for the windows that reduce the unwanted heat gain from the summer sun, while still allowing passive solar gain in the winter.

The attic will be filled with loose fill fiberglass insulation to achieve about R60. The depth of the roof trusses allows for deep insulation right out to the walls.


Aesthetic Appeal

While the house is very energy efficient, solar heated, uses less material and labor, it also just looks really nice. A lot of energy efficient solar homes have a very plain, boxy look, and this is just fine for a lot of people, but most customers want a house that looks nice AND is efficient. The variations in roof lines, attractive decks, bump outs, and the contrast of the metal roofing with the natural board siding all combine to make this home very easy on the eyes.

Heating and AC:

In addition to the sunspace providing solar heat, there will be a wood burner in the living room, and a mini-split.

The mini-split is located in the crawl space and is a Fujitsu 15RLS3H 25.3 SEER 15,000 BTU 1.5 ton.

The wood burner will sit on the shelf just to the left of Andrew.

The wood burner, like the sunspace, will also include a system to collect heated air and duct it into the crawl space, which is used as a plenum to distribute the heat to the house. The wood burner is an Englander 17-VL 40,000 BTU/hr maximum.



A rough heat loss calculation using our Home Heat Loss Calculator indicates that the heat loss for the house is about 233 BTU per hour for each degree F of difference between the inside and outside temperatures. For the same house but using conventional construction, the loss would be about 433 BTU/F - Hr. So, a nearly one half reduction in heat loss! And, this is accomplished without a construction cost penalty thanks to all of the material and labor savings techniques incorporated in the house design.

The low heat loss of the house means that the solar heating features will be able to provide a much higher solar heating fraction than they would with conventional construction.



You can get hold of Andrew at, or leave a Comment or Question below.

And, stay tuned for Andrews full article on the home.


Comments and Questions:

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Gary January 12, 2017