What the gap is for
Cladding is not waterproof and was never asked to be. It sheds most of the rain and lets a small fraction past, at butt joints, at fastener heads, at the bottom of every course and anywhere wind drives water sideways. What happens to that fraction is the difference between a wall that lasts and one that quietly rots.
Fixing the cladding tight to the sheathing gives that water nowhere to go except into the materials on either side of it, and gives the back of the cladding no way to dry. Holding the cladding off by a small distance gives the water a drainage path down and out at the bottom, and gives both faces of the gap air to dry into. The strips that hold it off are furring, or strapping, and the assembly is generally called a rainscreen. Three quarters of an inch is the usual figure people work with; less than that still helps and more brings its own trim problems.
This page counts the strips, reports how much of the cavity they block, and works out how far the outside of the house has moved, which is the consequence nobody plans for.
Strip direction is not a free choice
Vertical strips are the straightforward case. They run with gravity, so water draining down the back of the cladding has an uninterrupted path to the bottom of the wall, and air moving up the cavity has the same. Horizontal lap siding fastens across them happily. This is the default for a reason.
Horizontal strips exist because vertical cladding needs something horizontal to fasten to. The difficulty is that every strip is then a small dam lying across the drainage path. Draining and venting past them has to be handled deliberately, whether by leaving gaps in the strips, by running a second layer of cross strapping, or by using a purpose-made drainage product instead. All of those are detail decisions that belong with the assembly and the products, and none of them is settled by a quantity page.
| Cladding | Strips run | What it costs |
|---|---|---|
| Horizontal lap siding | Vertical | Simplest case, clear drainage |
| Vertical boards or panels | Horizontal | Drainage has to be provided past the strips |
| Shingles or shakes on a nailbase | Vertical, sheathed over | Extra sheet material, more depth |
| Panels with an open joint | Both, cross strapped | Twice the strips and the most depth |
Vent area, and why it is smaller than the gap
The cavity needs to be open at the bottom to drain and open at the top to vent, and the area that is actually open is a good deal less than the depth of the gap suggests. Three quarters of an inch across a foot of wall is nine square inches of gross opening. Put a two and a half inch strip every sixteen inches and about sixteen percent of that is blocked. Add insect screen, which typically passes around half its gross area, and you are down to somewhere near four square inches per linear foot.
That is still a great deal of opening compared with what the cavity needs to dry, which is why rainscreens work at all. It is worth computing because it is the number that gets destroyed by accident. A continuous bead of sealant along the bottom edge, a flashing lapped the wrong way, or a trim board run tight to the base closes the drainage path entirely, and a cavity that cannot drain is worse than no cavity at all, because it holds water against the sheathing instead of shedding it.
The four inches your trim just lost
Furring moves the outside of the house out by the strip thickness plus the cladding thickness. Three quarters plus five sixteenths is just over an inch. Every consequence of that lands on the trim and the openings.
Corner boards have to stand proud of the new cladding face, not the old one, so nominal one-inch stock that would have worked before may now finish flush or behind. Window and door casings have the same problem. Jambs gain depth, so windows may need jamb extensions or a deeper trim return. Where the wall meets a roof, a deck ledger or an adjoining wall, the geometry of every one of those junctions changes. The build-out check at the bottom of the result exists because this is the item that gets discovered after the trim has been delivered.
The exterior trim and corner board calculator counts the trim once you know how thick it has to be. For the cladding itself, the lap siding course layout calculator and the board and batten calculator take it from there.
Is it worth doing at all?
That is a genuine question rather than a rhetorical one, and the honest answer is that it depends on the wall, the climate, the cladding and what the rest of the assembly is doing. A vented cavity buys drying capacity, and drying capacity is most valuable where a wall gets wet and where it dries slowly: driving rain, deep overhangs absent, an absorbent cladding, a wall with limited drying to the inside. It buys less on a sheltered elevation with a cladding that does not hold water.
Against that, it costs strips, fasteners, a deeper trim package, more complex flashings at every interruption and a base detail that has to drain while keeping insects out. Whether it is required at all where you are building, and what form it has to take, is decided by adopted code and by your building department rather than by a preference. The materials behind it are quantified by the house wrap calculator, and whether the wall as a whole handles moisture sensibly is the subject of the wall condensation risk calculator and the assembly R-value calculator, since furring also interrupts continuous insulation and changes the thermal picture.
Questions people ask
How deep does the cavity need to be?
Deeper drains and dries better, and the returns fall off quickly. Most of the drainage benefit is present at a small fraction of an inch, because water runs down a surface rather than needing a channel, while most of the drying benefit needs enough depth for air to actually move, which is where three quarters of an inch comes from. Beyond about an inch and a half you are adding trim and flashing complexity for very little more performance. What depth is acceptable or required in your situation is a code and assembly question, and the answer differs by jurisdiction.
What spacing should the strips be at?
The spacing is usually set by the framing behind rather than by the cladding, because the fasteners need to reach structure and there is no point putting a strip where there is no stud. Sixteen inches on centre is the common answer for that reason. The cladding has its own requirement for fastener spacing, which comes from its instructions and may be closer than the framing, in which case the strips have to follow the framing and the cladding schedule has to be met some other way. This calculator takes the spacing you give it.
Can I use plywood rips instead of solid strips?
People do, and the practical differences are dimensional stability and how the strip takes a fastener. Ripped plywood is stable across its width in a way solid wood is not, which matters because a strip that cups moves the cladding with it. Solid wood is easier to fasten into and cheaper by the foot in most places. What neither of them changes is the arithmetic on this page, which cares only about thickness, width and length. Whether a given material is acceptable in the cavity depends on the assembly and on what your building department accepts.
Do the strips have to stop at windows?
They have to get out of the way of the flashing, which usually means they are interrupted and the opening gets its own blocking around the perimeter to carry the trim and close the cavity to insects. That perimeter blocking is what the opening blocking field counts. How the cavity is terminated and drained at each opening, and how that interacts with the sill flashing, is a detail that has to be resolved before anything is fastened, because a cavity that drains onto the back of a sill flashing lapped the wrong way is a leak that looks like finished work from the outside.
Does furring make the wall warmer?
On its own, barely. An air cavity has a small thermal resistance, and a vented one has less than a sealed one because the air is deliberately moving. The reason furring comes up in insulation conversations is that it is the usual way of holding cladding over exterior continuous insulation, where the strips pass through the insulation layer and become thermal bridges of their own. That is a different calculation from this one, and the assembly R-value calculator handles it, including the effect of the strips themselves on the whole-wall figure.