How to Run a Line Set or Cable Through an Exterior Wall

Why this matters

A ductless head, a heat pump, a generator inlet, a satellite or camera cable: every one of them ends with a hole through the exterior wall, and that hole is usually drilled in the last twenty minutes of the install by whoever has the longest bit. It passes through siding, sheathing, the water-resistive barrier (WRB), insulation and drywall, and each of those was doing a job. Done carelessly, the hole leaks behind the siding, lets cold air into the bay, gives mice a highway into the house and shows up a year later as a stain under the indoor unit. Done right it takes perhaps half an hour longer and nobody ever thinks about it again.

Step 1: Pick the location from both sides before you drill anything

Stand inside and outside and agree on one stud bay. The location has to satisfy the inside constraint (where the equipment's back plate or the inlet box needs the hole) and the outside constraint (where the siding will take a seal) at the same time.

• Outside: land in the middle of a siding course or board, never on a lap edge, a butt joint, a corner board or a trim piece. A mounting block cannot sit flat across a lap, and a block that rocks cannot seal. Keep the hole and its drip loop out of the splash and snow zone, a foot above grade as a common minimum, and away from the path of a downspout or an inside corner that concentrates water.
• Inside: stay out of the dedicated space above an electrical panel and the clear space in front of it (see related: Where Ducts, Pipes and Wires May Not Go), and do not go through a wall that separates the house from an attached garage without treating it as a fire separation (see related: The Garage-to-House Fire Separation).
• In the cavity: a bay with a receptacle or switch below it carries a cable, and an exterior wall in a kitchen or bath may carry supply or drain piping.

Why first: every later step depends on this one. A hole in the wrong course cannot be sealed well, and moving it means a second hole to patch.

Step 2: Scan, then pilot from the inside out

Scan the bay for studs, cables and pipe (see related: How to Scan a Wall Before You Cut or Drill). Then drill a pilot of about 1/4 in from the inside, at the angle you intend for the finished hole, and go outside to see where it landed.

Why inside out: the inside location is the fussy one, fixed by a back plate or a box. Piloting from inside lets you check the outside landing with a hole small enough to fill with sealant if it is wrong.

Hazard: a hole saw does not stop at a cable. If the scan is not clean, open the pilot just enough to look into the bay with a light or an inspection camera before the hole saw goes in. If a cable sits in the path and you cannot see which way it runs, shut off the circuit that feeds the devices in that wall and prove those devices dead with a tester checked on a known live source before and after, before cutting.

Step 3: Size the hole for the sleeve, and size the sleeve for the worst part of the bundle

Measure what has to pass, at its widest point. For a pre-charged ductless line set that is usually the flare nut on the suction line with insulation, control cable and condensate hose bundled beside it. Add roughly 1/2 in of clearance and choose a sleeve with at least that inside diameter, then a hole saw sized to the sleeve's outside diameter.

Why it matters: a hole sized to the bundle with no sleeve scrapes the insulation off the suction line on the sheathing edge, and bare suction line in a wall cavity sweats into the insulation.

Step 4: Drill with the outside end lower

Hold the hole saw so the outside end of the hole is lower than the inside. Ductless manufacturers commonly call for a downward slope of about 5 to 10 degrees toward the outside, which across a 6 in wall is roughly 1/2 to 1 in of drop. For a plain cable the slope matters just as much: it is what keeps water that runs along the cable outside from following it in.

 INSIDE                               OUTSIDE
        drywall   stud bay   OSB  siding
           |                  |   | + block
 high end  ======sleeve=====  |   |
           |             ============ low end
           |                  |   |   |
           |                  |   |   | bundle
           |                  |   |   +--+ dips to a
           |                  |   |      | drip loop

What breaks if skipped: a level or inward-sloping hole runs condensate backward into the bundle, and rain wicking along the line set runs straight into the cavity.

Step 5: Set a sleeve through the full wall

Use a rigid sleeve, cut so it runs from the face of the interior drywall to the face of the exterior mounting block. The sleeve protects the line set and cable from the rough edges of sheathing and siding, gives the sealants a smooth surface to bond to on both ends, and closes the stud bay off from the hole so insulation does not fall into it and rodents do not reach the cavity.

What breaks if skipped: the sealant bonds to ragged OSB fibers and vinyl, the joint cracks with the first temperature swing, and the open cavity becomes an air path from outside into the wall.

Step 6: Mount a block or flange that is integrated with the WRB

The WRB, not the siding, is the wall's real water line. A flanged mounting block or a wall-penetration flange is lapped into it shingle fashion: the top edge goes under the WRB and siding above, the bottom edge lies over the WRB below, and the sides are taped. The laps and tape sequence are their own procedure; How to Flash an Exterior Wall Penetration into the WRB.

On vinyl siding, use a split mounting block made for the siding profile. Vinyl moves with temperature, and a sealant bead run straight from a sleeve to the vinyl tears free as the panel slides. Seal the sleeve to the block, and let the siding float behind the block's trim ring. On stucco or EIFS, the rules change; see related: Stucco and EIFS and the Hole You Cut. Hazard: this is the step done on a ladder, often at second-floor height; foot the ladder on firm ground, keep it clear of the service drop and any overhead conductor, and do not reach past the rails to tape a flange.

Step 7: Seal the outside annular space and form the drip loop

Fill between the bundle and the sleeve at the outside end with a sealant suited to exterior use, over a backer if the gap is deep, so water running down the bundle cannot enter. Then form a drip loop: the line set or cable dips below the penetration before it rises or runs away, so water running along it drips off the low point rather than traveling to the seal. Finish with a wall cap or line-set cover that sheds water onto the face of the wall below.

What breaks if skipped: a cable that runs straight down the wall from the hole has nothing to stop water walking along it into the opening.

Step 8: Seal the inside, and keep the fill removable where service needs it

Inside, seal between the sleeve and the drywall and between the bundle and the sleeve so the stud bay is not ventilated with outdoor air. A non-hardening putty or a low-expansion foam that a later tech can cut out works well; hard-setting foam packed into a sleeve around a line set means the next technician cuts the line set to replace a head. If the wall is a fire separation, the fill is no longer your choice: it must be the approved or listed material for that assembly (see related: How to Firestop a Penetration So the Rating Survives).

Step 9: Close the pest path

House mice pass through gaps of about 1/4 in, and they chew through foam and soft sealant. Where the sleeve's annular gap is larger than that before sealing, pack it with copper or stainless mesh first, then seal over the mesh. Run the exterior line-set cover down to the condenser pad or close to grade with a closed end cap rather than leaving the bundle as a ladder. Do not use steel wool outdoors; it rusts away in a season and stains the siding.

Worked example: a ductless head on a 1985 house with vinyl siding

A two-person HVAC crew is installing a wall-mounted ductless head in a second-floor bedroom. The wall is 1/2 in drywall, 2x4 studs, 7/16 in OSB with house wrap and vinyl lap siding. With a split mounting block the hole runs about 5-1/2 in from drywall face to block face.

Location. The head's back plate allows the hole at its lower left or lower right corner. The lower left lands in a bay with a receptacle 12 in below, so the crew picks lower right, in the next bay. Outside, the pilot comes through 1 in above the bottom lap of a vinyl course, too close to the lap edge for the block. The back plate has a second right-side position about 2 in higher, so the crew fills the first pilot and drills again from there; the new pilot lands near mid-course.

Size. The suction flare nut with insulation, the control cable and the drain hose bundle to about 2-1/4 in at the widest point. Adding 1/2 in gives 2-3/4 in, so a 3 in sleeve with an inside diameter just under 3 in passes it, on a hole saw matched to the sleeve's outside diameter.

Slope. The crew aims for 7 degrees. Across 5-1/2 in that is 5.5 times 0.12, about 0.7 in of drop, so the outside end of the hole sits roughly 5/8 to 3/4 in lower than the inside.

Integration. They cut the vinyl back to the split block's opening, slip the block's top flange up under the house wrap above (slitting the wrap horizontally and taping the slit after), lay the bottom flange over the wrap below, tape the sides, and reinstall the vinyl into the block's trim ring.

Seal and loop. Outside, mesh packs a 3/8 in gap at the bottom of the sleeve, then sealant over it. The bundle exits, drops 4 in into a drip loop, and enters a line-set cover running down to the condenser. Inside, a removable putty seals the sleeve to the drywall and the bundle to the sleeve.

What would have changed the answer: if the back plate had no second position, the right move was a slimmer block profile or relocating the head a few inches, not drilling through the lap and caulking it. If the house had been built before 1978 with painted wood siding, the cut releases lead dust; a single 3 in hole is under the 20 sq ft exterior threshold for minor repair in the EPA renovation rule at 40 CFR 745 and so outside its work-practice requirements, but the crew still mists the cut, collects the dust with a HEPA vacuum and keeps it off the soil.

How to verify you got it right

• From outside, the block sits flat with no gap at its edges, its top flange is under the wrap or siding above, and its bottom edge is not sealed shut.
• The bundle shows a visible low point below the hole.
• After 5 minutes of a garden hose run gently down the wall above the block (keeping the spray off the outdoor unit and its disconnect), the inside face of the sleeve and the drywall around it are dry to the touch and to a moisture meter.
• The ductless drain test flows outside freely, confirming the slope survived installation.
• Nothing larger than a pencil tip passes anywhere around the sleeve on either side.

References

• Ductless equipment manufacturers' installation instructions for wall sleeve size and line-set slope.
• 40 CFR Part 745, Subpart E (EPA Renovation, Repair and Painting rule), for minor-repair thresholds on pre-1978 housing.
• See related: How to Flash an Exterior Wall Penetration into the WRB; How to Scan a Wall Before You Cut or Drill; The Garage-to-House Fire Separation; How to Firestop a Penetration So the Rating Survives.