Insulating an Attic

What this calculation does not cover

• Layers are summed straight through the insulated cavity. Studs, joists, plates, headers and rim areas conduct several times faster than the insulation between them and are not deducted here, so the real whole-wall or whole-ceiling figure is lower than this total. Run the Thermal Bridging Effective R-Value Calculator on the same wall to see by how much.
• The figures are imperial R-values in hr·ft²·°F/BTU, and so is the fixed R-0.85 air film allowance. Metric RSI values in m²K/W are about 5.68 times smaller, so entering those makes the film allowance alone roughly six times too generous. Convert before you type.
• The air film allowance is one fixed number written for a wall: still indoor air with sideways heat flow, and an outside face exposed to wind. It does not change for heat flowing up or down, for a surface facing a vented attic or an enclosed crawl space, or for a reflective low-emissivity face, all of which shift the film values.
• Product R-values are lab ratings for material at full thickness with no gaps. Batts compressed under wiring, voids at plates and corners, and loose fill that has settled all deliver less than the printed number, and nothing here downgrades the total for installation quality.
• R-value covers conduction only. It says nothing about air leakage, wind washing through the insulation, or moisture in the assembly, and this total is not a code compliance check: the required figure depends on climate zone and on which element you are building, and codes are frequently verified against a whole-assembly U-factor that includes the framing this sum leaves out.

What this calculation does not cover

• Perimeter multiplied by length gives the duct's own outer surface, but the blanket has to pass around the outside of its own thickness, so the circumference it actually spans is larger than the perimeter entered. A rectangular duct gains roughly eight times the wrap thickness and a round duct gains 2π times it, so a run needs appreciably more blanket than its bare perimeter suggests — on a typical rectangular duct about a fifth more than the figure returned here at 10%. Enter the perimeter measured over the finished insulation if you want the ordering figure.
• The result is an area only, and says nothing about the wrap thickness or the installed R-value that area has to deliver. Duct wrap loses a substantial part of its out-of-the-roll R-value where it is compressed at corners and under the wire or straps holding it on, and the R-value required for duct in an attic, in an unconditioned space or buried is set by the energy code in force. Buying the right area of the wrong thickness still fails that check.
• The waste allowance is a flat percentage of the straight-run area, so it scales with length rather than with the number of fittings. Elbows, transitions, takeoffs and damper access panels are cut from rectangles larger than the surface they finish up covering, and hangers, trapeze supports and flanged joints interrupt the blanket, so a fitting-heavy run can outrun even the 20% this field allows.
• Only the blanket itself is quantified. Outward-clinch staples, FSK or foil tape over every longitudinal and circumferential seam and staple line, and mastic where the wrap meets a flange are all separate items, and on duct carrying cooled air it is the continuity of that vapour barrier, not the area of blanket, that decides whether moisture condenses inside the insulation.
• This is an external wrap take-off and does not estimate the alternatives. Internal acoustic liner is measured on the duct's inside faces and also cuts the free cross-section the duct delivers; rigid board and duct board are ordered as boards on a mechanical fastener grid rather than as a continuous roll.

What this calculation does not cover

• Putting a baffle in every rafter bay is not the same as ventilating the roof. The count says nothing about whether the soffit intake and the ridge or high-level outlets together provide enough net free vent area for the insulated ceiling area below, a ratio fixed by your local code, nor whether the soffit vents behind the baffles are actually open. A baffle keeps a path clear; it cannot create intake.
• The count assumes a baffle physically fits at the wall plate. It does not check that the rafter depth there leaves room for the full insulation thickness plus the clear air channel required above it, which is the pinch point on a standard-heel truss. Where it does not fit, the fix is a raised heel, a deeper rafter or a tapered insulation edge, not more baffles.
• The result is a count of bays, not of packs or of running length. Baffles are sold as fixed pieces, commonly around 1.2 m long and pre-sized for 400 mm or 600 mm bays, so any bay baffled further up the slope, such as deep loft insulation or a vaulted section vented to the ridge, takes two or more pieces. No allowance is added here for cut-offs, staples or the edge sealing that stops blown insulation being driven behind the baffle.
• The figure covers one straight eave run. A gable roof has two eaves and has to be worked out and added run by run, and every bay in the run entered is counted whether or not it can be ventilated, including bays closed off by a chimney, a party or fire wall, or a stretch of eave with no soffit vent beneath it.
• Every bay is assumed identical and the spacing box stops at 0.61 m (2 ft), so roofs framed at wider centres cannot be entered, and the trimmed short bays around a rooflight or a hip are not represented — split such a roof into uniform runs and count the odd bays by hand.

What this calculation does not cover

• This counts area against the coverage printed on your package. It does not check that the batt width suits your framing spacing or that the batt thickness fits the cavity depth — a 16 in (406 mm) batt in bays framed at 24 in (610 mm) centres leaves gaps that no package count corrects.
• No R-value enters the calculation. Nothing here tests the finished assembly against the R-value your climate zone and local code call for, and no allowance is made for heat flow through the framing members themselves, which conduct more than the cavity beside them.
• The area is the plain rectangle you enter. Nothing is deducted for windows, doors or the attic hatch, and nothing is set aside for the areas that must stay clear — recessed lights, flues and chimneys, and the eave gap soffit ventilation needs. Those clearances come from the manufacturer's instructions and local code, not from this count.
• The waste factor is a flat percentage of the whole area, not a cutting plan. Irregular bays, hips and valleys, and heavily obstructed framing produce short offcuts that waste far more than a percentage of the total suggests.
• This is a purchase quantity, not an energy or compliance calculation. It says nothing about air sealing — batts do not stop air leakage — nor about which way a faced batt's vapour retarder must face, which depends on your climate.

What this calculation does not cover

• Building volume here is nothing more than floor area multiplied by average ceiling height. That ignores the thickness of intermediate floor assemblies, flattens stairwells and double-height spaces into an average, and omits any conditioned basement or attic whose area you did not enter. The target moves in direct proportion to the volume, so a volume measured ten per cent differently from the tester's gives a CFM50 limit ten per cent different — agree the volume basis with whoever will run the test before you build to this number.
• Nothing here defines the tested envelope: whether an attached garage, an unheated basement, a conditioned crawl space or a plant room sits inside or outside it. That single decision changes both the volume in the denominator and the leakage the fan has to move. Two people can test the same building to two different ACH50 figures purely by drawing the boundary in two different places.
• ACH50 normalises leakage against volume. UK and much European testing, and some North American programmes, instead specify air permeability or leakage per unit of envelope area. This page never asks for envelope area, so a target handed to you in those terms cannot be met by hitting this CFM50 figure.
• The number assumes a properly conducted test at 50 Pa. It carries no correction for wind, stack effect or baseline pressure, no averaging of a depressurisation result against a pressurisation one, and no allowance for how the building is prepared — which trickle vents, flues, extract fans and drain traps get sealed and which are left as found. Those are the usual reasons two tests of the same building disagree.
• This is a target airflow, not a ventilation or combustion-air check. A building tightened towards a low ACH50 needs mechanical ventilation sized and commissioned for it, and any open-flued or naturally aspirated appliance inside the envelope needs its own combustion air and spillage assessment. Hitting this figure says nothing about either, and those are the parts that carry a safety consequence.