Why a flat R-value is the wrong number for a pipe
Insulation is sold with an R-value that assumes a flat wall, where the heat enters and leaves through the same area. A pipe is not flat. Heat enters at the small inner surface of the sleeve and leaves at a much larger outer one, and on a small pipe with a thick sleeve those two areas differ by a factor of three or four. Using the flat figure overstates the benefit of thick insulation on small pipe, sometimes badly.
The correct form for a cylinder, referred to the outer surface, is the outer radius multiplied by the natural log of the radius ratio, divided by the conductivity. A half inch sleeve of typical foam on three quarter inch copper works out to about 2.86 rather than the 2.0 a flat calculation gives, but it is applied to an outer surface more than twice the area of the bare pipe, and the two effects partly cancel. That is the arithmetic this page runs, which is why the resistance it reports does not match the number printed on the packaging.
The surface film, and why bare pipe is not zero
Even with no insulation at all, a pipe does not lose heat infinitely fast. A thin layer of still air clings to the surface, and radiation from the surface is limited by temperature, so a bare pipe in still indoor air behaves as though it had a resistance somewhere around half a unit. That is what makes a bare-pipe figure finite and calculable: three quarter inch copper at a sixty-five degree difference sheds roughly twenty-seven BTU an hour per foot, not an unbounded amount.
It also explains the shape of the thickness table on this page. The first sleeve you add is competing against a resistance of about 0.55, so it multiplies the total resistance several times over and cuts the loss by roughly two thirds. The second half inch is competing against a resistance that is already three or four, so it buys much less. Insulation on pipe has sharply diminishing returns and they arrive at about the first half inch.
Hours hot is the input people get wrong
The heat loss per hour is a property of the pipe. The annual number is that figure multiplied by how long the pipe is actually hot, and that multiplier ranges over an order of magnitude between two runs that look identical.
| Run | Hours hot a day | What is happening |
|---|---|---|
| Continuously circulated loop | 24 | The pump keeps the whole circuit at temperature all night |
| Loop on a timer | 6 to 10 | Hot through the morning and evening windows, cold between |
| Short nipples at the heater | 24 | Convection keeps them hot whether or not anything circulates |
| Branch to a busy kitchen sink | 2 to 4 | Hot around draws and for a while after, cold the rest of the time |
| Branch to a guest bathroom | Under 1 | Cold nearly always; insulating it saves almost nothing in fuel |
A single figure for a whole house is therefore misleading. The useful way to use this page is one run at a time, starting with the ones that are hot around the clock, because that is where nearly all of a building's pipe standby loss lives.
What insulation is doing besides saving fuel
Fuel is the number this page calculates and it is often not the main reason to sleeve a pipe. Insulation keeps water warm between two close draws, so the second person into the shower may not have to purge the line at all. On cold lines it stops condensation dripping onto whatever is below, which in a finished ceiling is a real problem and has nothing to do with energy. In an unheated space it slows the approach to freezing, though it does not prevent it — a sleeve buys hours, and the pipe freeze risk calculator is the page that estimates how many.
Where insulation has to be chosen rather than merely added is around a flue, a vent connector or anything else hot enough to matter, and near equipment where a manufacturer specifies clearances. Foam sleeves have temperature limits and clearance requirements that come from the product and the appliance, not from a heat loss calculation. Check both before wrapping anything near a burner or a vent.
Questions people ask
How much does insulating hot water pipes actually save?
For a branch that is only hot around draws, very little in fuel — a run hot for two hours a day is losing about a twelfth of what the same run loses when held hot continuously, and the annual figure lands in single-digit dollars. For a circulated loop it is a different order of magnitude entirely. A hundred feet of bare three quarter inch copper at a sixty-five degree difference held hot all day loses roughly twenty-three million BTU a year, and a half inch sleeve removes about two thirds of it. Work your own run on this page rather than taking a general figure, because the hours-hot input moves the answer more than anything else on the form.
Is thicker pipe insulation worth it?
Up to a point that arrives earlier than most people expect. The first half inch of sleeve is competing against a bare surface resistance of roughly 0.55, so it multiplies the total resistance several times and cuts the loss by around two thirds. Going from half an inch to one inch is competing against a resistance already above three, and buys maybe another fifteen percent of the original loss. The thickness table on this page shows that curve for your specific pipe and temperatures. Thicker sleeve also has to physically fit, and the outside diameter figure in the results is there because a one inch sleeve on three quarter inch pipe will not go through the holes the pipe went through.
Does the pipe material change the heat loss?
Much less than you would think, and not in the direction people assume. Copper conducts heat hundreds of times better than PEX, but the pipe wall is a thin, low-resistance layer sitting in series with a surface film and possibly a sleeve that are far more resistive, so the wall barely features in the total. What the material does change is the outside diameter for a given nominal size, and heat loss follows the outer surface area, so the comparison that matters between two materials is their OD rather than their conductivity. That is why this page asks for the outside diameter and not the bore.
Should I insulate cold water pipes too?
Often yes, for a reason that has nothing to do with energy. A cold line running through humid air condenses water on its surface, and in a joist bay above a finished ceiling that shows up as a stain long before anyone finds the cause. Sleeving stops it, provided the sleeve is sealed at the seams and ends so humid air cannot reach the pipe surface underneath — a loosely fitted sleeve on a cold line can be worse than nothing, because it traps moisture against the pipe. The energy case for insulating cold lines is weak in most buildings; the condensation case is often the whole argument.
What conductivity figure should I put in?
The one on the product you are buying. Closed cell elastomeric and polyethylene pipe sleeves are commonly quoted somewhere in the region of 0.22 to 0.28 BTU-inch per hour per square foot per degree F, and the default here sits in the middle of that. Mineral wool and fibreglass pipe covering differ, and any figure moves with temperature. If the packaging quotes an R-value instead of a k-value, note that it is almost certainly a flat-wall figure for that thickness, so divide the thickness in inches by the quoted R to get k before entering it here.