Installation of a GREE FreeMatch R32 two-head heat pump in Calixa-Lavallée (Marguerite-D'Youville RCM)

A wall without studs: what insulated concrete forms change about everything we want to attach to it

The parish was called Sainte-Théodosie when it was established in 1878, having been separated from Saint-François-de-Verchères. In 1946, postal confusion with Saint-Théodore led to its double name, Sainte-Théodosie-Calixa-Lavallée, and since 1974 only the second half has remained.

The musician it honours was born in 1842 in a territory that then belonged to the parish of Verchères and is now part of Calixa-Lavallée — civil registers therefore record Verchères, and both statements are accurate for reasons of chronology rather than geography. Calixa Lavallée composed the music for “O Canada” in 1880, to lyrics by Adolphe-Basile Routhier; the work did not become the national anthem until 1980, eighty-nine years after its author’s death in Boston.

The municipality now has 527 residents according to the population decree in effect since January 2026 — 509 in the 2021 census — across approximately 32 km², of which, according to its own planning bylaw, more than 99% is protected agricultural land. Its population has barely changed since 1951. The Church of Sainte-Théodosie, built from 1888 to 1890 according to plans by architect Victor Roy, was designated a heritage building by the Municipality on June 10, 2025, along with its 1945 Odilon Jacques organ.

This is the project where we installed a 2-head GREE FreeMatch R32 wall-mounted heat pump — and the job presented a problem we are seeing more and more often in recent rural construction: the house is built with insulated concrete forms, and an insulated concrete form wall has no studs.

What was installed

Values taken directly from data sheet X610068B. This combination of capacity and distribution has already been used elsewhere in our series, and we prefer to state it. What follows about choosing the heads, however, has not.

A wall without studs

What is actually inside an insulated concrete form

A wall made of insulated concrete forms—ICFs in industry jargon—is a sandwich: two expanded polystyrene panels, a poured-concrete core between them, and foam that remains permanently in place instead of being stripped from the form. The two panels are held together by polypropylene ties whose furring strips sit flush beneath the foam surface: these are what receive the screws for the drywall and exterior cladding.

Two orders of magnitude, taken from the manufacturers' manuals:

• The foam is generally 63 to 82 mm thick on each side, depending on the target insulation value, and the concrete core is 4 to 12 inches thick.
• The furring strips are spaced approximately 6 to 8 inches on center, depending on the system—one manufacturer places them 6 inches apart on its standard block and 8 inches apart on its high-performance block, while another specifies 8 inches. There is no single value, and that is the first trap.

In a wood-framed wall, the contractor looks for studs spaced 16 or 24 inches apart, and the detector finds them. In an insulated concrete form, the detector finds plastic strips every 6 to 8 inches—more numerous, more evenly spaced, and much less resistant than a stud.

Three ways to attach something to it—and they are not equivalent

Manufacturers publish three. We apply them in this order.

1. Into the fins of the ties. This is the fastening intended for drywall and siding. One manufacturer publishes a value of approximately 90 kg of drywall-screw pull-out strength per fin—and that value must be read for what it is: a screw pull-out test, not a design load for equipment.

2. Through the foam, anchored in the concrete core. This is the method manufacturers explicitly reserve for significant loads: the fastener passes through the drywall and foam and reaches the concrete with a concrete anchor. One manufacturer states it in a single sentence: for most structural or heavy applications, fasteners must penetrate the foam and anchor directly into the concrete.

3. A recessed load-distribution plate. The published method for heavy objects consists of shaving the face of the foam with a hot knife to clear the fins, then screwing in plywood of the same thickness, which distributes the load across several support points instead of concentrating it on two screws.

What this looks like on a heat-pump job site

The indoor-unit wall plate is a rigid rectangle whose hole pattern was designed for studs spaced 16 inches apart. On an insulated concrete form, there is no reason for its holes to line up with the fins, and forcing a screw in beside a fin amounts to screwing into foam. The answer is the third method: recessed plywood that becomes the support, allowing the plate to be attached freely.

The outdoor-unit brackets, on the other hand, support a 51 kg cabinet in cantilever, plus the vibration of a compressor, for fifteen years. This is not a fin load. It is a concrete load, and it must be decided before the quote, not in front of the wall.

The sentence we will not write

We will not publish any allowable load value for a heat-pump bracket on an insulated concrete form wall, for a reason worth publishing exactly as stated: none of the manufacturers we consulted publishes one. They all refer to their system-specific technical documentation and the Code. A contractor who quotes you a pull-out value for an equipment bracket is quoting a drywall-screw test value applied to a problem that is not the same.

Our position does not change from one jobsite to another: we do not improvise structural calculations. We identify the assembly before drilling, locate the concrete when the load requires it, and reseal every penetration as a wall that must continue to drain.

And the prefabricated wall, which raises the opposite question.

The same problem arises differently in a house with factory-built prefabricated walls, which are also common in recent rural construction.

Here, the difficulty is not the absence of studs—there are some. It is that what lies behind the drywall is an ordering option, not a standard. Quebec panel manufacturers offer, at the buyer's choice: the exterior cladding, insulation, the vapor barrier—polyethylene or membrane—, interior or exterior furring, the membrane around openings, and sealing at panel joints. Two neighboring houses from the same supplier may therefore have neither the same vapor barrier, nor the same service cavity, nor the same sealing.

We will not say that a prefabricated wall “normally” contains one layer or another: we found no authoritative Quebec source on this point, and making it up would be exactly the kind of approximation that punctures a vapor barrier. The practice itself is simple: we characterize the assembly before drilling, and request the panel plans from the manufacturer when they exist.

The smallest head you can install depends on the large unit, not the small room.

Here is the technical point of this page, and it follows directly from the type of house.

A house built with insulated concrete forms has a very airtight, highly insulated envelope. Its heating and cooling load is low—that is the point. The natural instinct is therefore to choose small heads. And that is where the equipment's mechanics impose its limit.

The modulation floor does not come from the head; it comes from the compressor.

A multi-zone system cannot go down indefinitely. It has a modulation floor: the smallest output it can produce continuously. Below that, it no longer modulates—it stops and starts again.

This minimum output is a property of the compressor, not the indoor unit. And it increases with capacity. As stated directly in the manufacturer’s data sheet:

Now take a 9,000 BTU/h indoor unit—a perfectly ordinary capacity for a bedroom—and look at the headroom it retains when it calls on its own, at night, depending on the system to which it is connected:

• on the 18,000: approximately 23% above the minimum output;
• on the 24,000: approximately 20% above;
• on the 30,000: approximately 10% above;
• on the 36,000 and 42,000: approximately 1.5% above.

The same 9,000 unit, in the same bedroom, goes from comfortable to borderline depending on the compressor selected. It has not changed. The minimum output has risen beneath it.

Two practical consequences

First consequence: at the same total capacity, the distribution is not neutral. Two indoor units totaling 21,000 BTU/h can be divided into 12,000 + 9,000 or 15,000 + 6,000. The second combination gives exactly the same total—and it is a poor choice, because a 6,000 unit falls below the minimum output of every system in the range, without exception. A system cannot satisfy it on its own. That is why we chose 12,000 + 9,000: on this system, the 12,000 unit retains 60% of headroom, while the 9,000 unit retains 20%.

Second consequence, and it’s counterintuitive: on a highly efficient home, going up in capacity costs more than elsewhere. In an ordinary home, a slightly oversized compressor is offset because the rooms frequently call for heating or cooling. In an envelope with low demand, the system operates near its minimum output for much of the year—and a higher minimum output turns modulation into short cycling. Our Saint-Charles-sur-Richelieu page published the opposite argument, the one that permits stepping up a capacity when the minimum output does not change; here, the reverse applies, for the same reason.

Why the number read at the grille will disappoint you—and why that’s a good sign

One last point—and it’s one that a homeowner can check themselves with an infrared thermometer bought at a hardware store.

In a low-demand home, a correctly sized appliance spends most of its time at the low end of its range. The air coming out of the grille is therefore less cold in summer and less hot in winter than expected — not despite proper sizing, but because of it. An appliance that blows very cold air continuously is running at full power, which, in a home built with insulated concrete forms, means it is oversized.

Two warnings before taking out the instrument, and we owe the underlying explanation to our Sainte-Clotilde page, which published it in detail: an infrared thermometer reads a surface, not air, and its result depends on emissivity, distance, and angle. And the difference between return air and supply air means nothing without knowing the modulation rate — the twenty-degree rule found everywhere comes from fixed-speed appliances, which have only one operating condition.

A number read at the grille is therefore a snapshot of an operating condition at a given moment. It proves neither capacity nor installation quality. This is the only check on this page that we advise you not to perform alone — not because it is difficult, but because it is misleading in both the reassuring and alarming sense.

What this page adds to our others

Saint-Charles-sur-Richelieu raised a metering question: who pays for what the appliance consumes. This one raises a support and flooring question: what will the screw bite into, and how far down can the appliance go? A wall without studs and an envelope that requires little are two forms of the same problem — an energy-efficient home does not make the work easier; it shifts it elsewhere.

At AirGreen, we install, maintain, and document HVAC systems in Calixa-Lavallée, Verchères, Contrecœur, Saint-Amable, Sainte-Julie, Varennes, and throughout Marguerite-D'Youville, as well as in Montréal, Laval, Longueuil, on the North Shore, and on the South Shore. If your home is built with insulated concrete forms or prefabricated walls, mention it when you call: it is information that changes the quote, and it is better to provide it before rather than after.