I've been insulating hot water pipes in my boiler room and wanted to find
what the approved method is. I've done several hours of googling with
First off, it seems that the suggested type of insulation is an
elastomeric rubber - Rubatex, Insultube or Armaflex.
Now, I want to add a radiant barrier to the mix since it seems that quite
a bit of the loss is radiant. The information that I've seen indicates
that you need an air space to avoid conductance of heat through the
However, there's radiant barrier built on bubble wrap that you're supposed
to be able to wrap pipes, heaters, etc. without the air gap.
So a question is, would you add a radiant barrier under or on top of the
foam pipe insulation?
Intuitively, I would think that you'd foam it first, then wrap the foam in
radiant barrier. Since the foam would act (roughly) as a black-body
radiator, any heat that got through it could be reflected by the radiant
On the other hand, if you put the radiant barrier inside, you get the
radiant losses reflected directly back into the hot water pipes. Then the
foam insulation on the outside simply reduces further losses.
Does anybody understand the thermodynamics of this and have any sort of
quantitative handle on the savings that might be involved?
Let's try a real example. Suppose you've got 3/4" copper pipe with 180F
The radiant barrier is coated two sides and is double thick bubble wrap,
say 1/4". Barrier is spec'd at 97% reflective for "heat".
The insulative foam is rated at R4.6.
Assume a 1' section of pipe.
Air temperature is 70F.
Nick Pine - you out there?
The emissivity of coper is said to run from .023 to .6 depending on the
shinyness of the surface. Let's say that it's 0.1.
Using H = esAT^4, we get:
Pipe area = pi * .75 / 12 square feet = .1963
convert to square meters = .196 * 0.0929 = .018 sq m
0.1 x 5.68e-8 x .018 x 355^4
= 1.65 J/s (watts)
So radiant heat per hour = 0.77*3600 ~= 5900J = 5.6 BTU/hour
So my hot water is losing ~5.6 BTU/h per foot of pipe due to radiation.
The net heat flow is given by
Hnet = esA [T^4 - T0^4]
So net heat loss is 1.23 watts or 4.2 BTU/hour?
Is this right? That seems totally counter to experience. Hot pipes seem to
leak heat like crazy!
I suppose the emissivity could be 5x that estimate, then the pipes would
be leaking 20BTU/h per foot of pipe. If we figure 20' of pipe in the
boiler room, then we're up to 400BTU/h of losses or about the same as a
125 watt bulb.
Anybody? These numbers don't seem right.
Any more comments on insulating boiler room pipes?