Insulation Calculations for Geeks

December 28, 2009 | Filed under: Uncategorized

Now and then I search google for an answer to a question and don’t get a  good answer, and this time I’m doing my bit for the greater good to answer these:

“Is it worth putting foam insulation lagging on hot water pipes?”

“Is it worth putting reflective foam insulation around lagged hot water pipes”

“What is the difference in insulation between no pipe-lagging, foam pipe-lagging and foam plus foil lagging?”

Yup, this isn’t about games :D

background: i have a cold old house with a  freezing cellar, and the last occupant did NO insulation. The hot water pipes for the radiators go under the floor in the cellar ceiling, exposed to the cold and damp draughty cellar. A lot of my heating costs were being wasted heating up a damp cellar we never use. So this needed fixing.

Now there is ‘reading books and believing them” and then there is “getting real data”, so I went and got one of these (an infared thermometer £20)

Then I lagged some pipes using that foam insulation stuff, tied on with cable ties. Then as an experiment I doubled up on them by also wrapping that foil stuff you get for the back of radiators around them too, and took measurements. Heres a picture showing all 3 scenarios, where I haven’t finished yet (I ran out of tape…)

With NO lagging, and the radiators on, the temperature of the exposed copper pipes was 57 degrees.

With just tied-on foam insulation, the temperature of the foam was 21 degrees. ( a reduction of 63%ish)

With tied on foam AND a layer of reflective radiator foil, the temperature of the external foil was 12 degrees. (a further reduction of 15%ish)

Obviously the lower the temperature the better, I want all that heat kept in the pipes going to my radiators, not leaking out into the cold air of the cellar.

Conclusion: it is HUGELY worth you lagging any pipes in unused unheated rooms with foam insulation. It is also very worthwhile wrapping the lagged pipes with a  layer of reflective radiator foil. I tape it up with special reflective tape and both the tape and the foil is dirt cheap. The foam tubes are about 50p each.

If you pay heating bills where you live, you would be mad not to do this :D

And if you don’t know where the heat goes, get an infared thermometer. They double up as laser pointers to entertain cats.

10 Responses to “Insulation Calculations for Geeks”

  1. Ben says:

    I hate to muddy the waters (and to potentially make more work for you), but wouldn’t it be better to put the foil on first and then cover it with the foam? Since the foil is actively reflecting heat, I’d think it would be best used at the point of highest heat.

    Of course then there’s the argument for what might be a silly level of thoroughness: Foil, then foam, then more foil. At that point though, you’re looking at some diminishing returns.

    You have my sympathy. We’re in a rowhouse that’s somewhere around a century old. Thankfully we’re not bound by any historical record issues, so I’ve been able to fight drafts using aerosol cans of expanding foam. Wonderful stuff for filling cracks. Horrible horrible stuff to get on your hands.

  2. cliffski says:

    Yes you are right, i’m sure it would have been better that way round, but I had no real plan :D I had assumed that putting the foam stuff on would be enough, and it was an afterthought to start adding the reflective stuff after the event. If I had any totally exposed pipe left that would make a good experiment :D

  3. bbot says:

    Aluminum foil will reflect infrared light. When you point a non-contact thermometer at it, you’re not measuring the temperature of the foil, but whatever the foil’s reflecting. Here, it’s probably the cold cellar.

    It’s still worthwhile, since the foil’s going to have a lousy albedo, and thus radiate little heat.

    (And, of course, infrared thermometers give wildly differing readings depending on the albedo of the target, since they’re measuring radiated light. They’re really supposed to be used for wild ass guesses, or for measuring things you can’t touch, like running engines, or molten sodium in vacuum chambers, which you’ll have to calibrate the thermometer for the particular material. Use contact thermometers for numbers you can trust.)

  4. cliffski says:

    Ah interesting, I did suspect it might be harder to get an accurate reading from the foil. Dang. I need to find myself a better thermometer now :(

  5. cliffski says:

    I got a better thermometer, a contact one, but its readings are all over the place, with very little consistency, even after leaving it taped to pipes for prolonged tests.
    However it also detects a massive drop from the exposed pipe to the simple foam covering, so I’m still 100% sure thats hyper-effective.

    If anyone ever finds a definitive answer with statistics to whether it’s worth covering the foam with foil, I want to know :D

  6. DBB says:

    On a tangential note, I wanted to mention that I really enjoy your updates about the new Cliffski Residence. It’s really fascinating to see what’s going on. I love old buildings and restoration stuff, maybe it’s watching too many Grand Design programmes.

    Good luck with all the work. I hope 2010 is, above all, warm in your new house!

  7. BlckKnght says:

    This comment thread is clearly not geeky enough yet.

    I suspect that the values you were getting from your IR thermometer, while not really accurate temperature values, still were telling you what you wanted to know: How much heat were your pipes losing to the cold environment. The actual temperature on their surface is not really all that important in the long run.

    If you did have accurate temperature data, you’d be able to calculate the heat loss using the Stefan-Boltzmann law: j* = 5.67e-8 * T^4

    That assumes that you’re radiating pipes are blackbodies, which they certainly are not, with our without a foil covering. This is where the IR thermometer readings come back into the picture. What the thermometer directly measured was the IR radiation coming from the pipes. It then applies the formula above in reverse to give an estimate of the temperature, but unless it is also analyzing the material for albedo and other properties it’s not going to be able to do that precisely. But that’s doesn’t really matter. What it’s results tell you is that the pipes without insulation were radiating quite a bit more heat than the insulated and foil wrapped versions. It even gives us a way of estimating how much things have improved.

    If you raise the provided temperature readings to the fourth power again to get back to something more or less proportional to the heat flow rate, you’ll find that the foam insulation reduced your pipes’ heat losses by about 37%. Foam plus foil was good for a 44% reduction. These should be relatively honest numbers, since they’re just using the thermometer measurements against one another. I’d hesitate to estimate any absolute benefit though, unless you have another way to calibrate the results (you’re saving 44%, but 44% of what?). Perhaps you can report back if you notice a significant change in your heating bill?

    Anyway, stay warm and have a happy new year!

  8. ChemEng says:

    And now we get double geeky.

    There are 3 ways that heat can be transferred – radiation, conduction and convection. BlckKnght’s analysis is correct for radiation but at the temperatures involved radiation is likely to be the least important mode of transfer.

    Conduction is where there is direct contact to something that can take the heat away. If your hot water pipes were attached to large steel beams then that would really suck the heat out of them. But in a domestic environment the pipes are probably mounted on brickwork or wood, both of which are good insulators and your losses will not be great. If they are mounted on steel anywhere a bit of teflon sheet should be inserted to break the heat transfer circuit.

    The most important factor is likely to be heat loss by convection. This is the passage of air over the pipes. It can occur by forced convection using a fan (which I guess you are not doing!) or by natural convection such as draughts or even the movement of air caused by the heating itself. The foam and tape you have used is extremely effective against convection and without doing any detailed calculations I would not be surprised if the insulated portions are losing less than 10% of what they were before.

    In conduction and convection it is not the reduction in temperature itself that is important. The important factor is the temperature *difference* between the pipe and the surrounding air. If the bare pipes are at 57 degrees and the air in the basement is 10 degrees then your temperature difference is 47 degrees. This is called the “driving force”. If the insulated pipes are at 12 degrees then the driving force is only 2 degrees i.e. only 4% of what it was. Unfortunately some of this good work is undone by the increased area for heat transfer with the insulation having a larger circumference than the original pipe, but the effect of the insulation will be significant.

  9. cliffski says:

    Thanks for that, this is extremely interesting stuff. We have bought books on the theory and practice behind modern insulation but most of them do not apply to an existing, old style house such as ours. I hadn’t even considered the effect of a larger surface area with the foam and insulation, which is a good point. I just wish I could get easily at *all* the pipes, but in many cases that would mean lifting floorboards which I’m not aiming to do *yet*

  10. Drew says:

    Not my cat. He is terrified of laser pointers.