radiator insulation?

Factual Questions
1

I have a radiator that is recessed into the internal-facing side of an external wall. The wall is composed entirely of some plaster and lath, air (2" maybe?), and then a single layer of brick that makes up the exterior of the house.

With the radiator recessed into the wall (kind of like this,) I think the portion of the wall directly behind the radiator doesn’t even have the plaster & lath, so I suspect that a lot of the heat from the radiator goes right out through the near-nothingness of the wall.

Is my suspicion correct and if so, would it help at all if I put some kind of reflective/insulating material in the recess behind the radiator? I’ve seen products like this but wonder about their effectiveness.

2

I’m sure you’ll get more authoritative responses, but that kind of product would help a little bit in terms of insulation and even more in terms of reflecting the radiant heat into the room. I can’t for the life of me figure out why a radiator would be installed in a recess like that. The recess would naturally result in less insulation behind the radiator and also reduce the directions the heat could radiate.

3

Not a very efficient installation, but it gives you 2 extra inches of interior space you wouldn’t have otherwise. If you go outside on a cold day, and feel the wall where the radiator is does it feel a lot warmer than the surrounding wall?

Are you a renter or an owner? If I owned the house I was add a reflector (already mentioned) and some insulation in the wall behind the radiator or just replace it with a low profile version and close up the niche permanently.

4

Reflective foil with maybe 1/2" of foam backing is probably the best you can hope for. Anything thicker will mess up the airflow around the radiator and make things worse. Also consider that solid madonry is such a poor insulator to begin with, as is plaster, that the difference in thickness isn’t such a big deal. I calculated the R-value for a 20" thick stone wall once and the 1" air gap from furring between the stone and plaster provided more insulation than the stone. That said, you’re still unlikely to feel any difference in temperature outside just by touch, though you’d certainly see it on an infrared camera.

5

Both the foil bubble sheet you link to or the foam foil jjakucyk suggests would be my first choices. I would be doing something like that right away - your analysis is going to be pretty close.

I would probably get as much insulation behind the radiator as I could. The key convective parts are the gaps between the radiator segments. I suspect the loss of circulation behind the radiator is less important than managing the heat loss. You could consider painting the radiator matt black as well. Although that is only useful if it isn’t covered, and thus might be considered unsightly.

6

Assuming the radiator is symmetric front-to-back, placing thick insulation behind the fins results in a significant reduction in the airflow, and therefore convective heat transfer. However, there will presumably be less heat loss to the outside wall. The hope is that the latter more than offsets the former and there is a net gain of energy into the room.

I’d suggest placing a fan to force convection across/around the fins to increase the heat transfer from the radiator. Of course, this will likely result in an increase of heat transfer to the exterior, but the hope is that this is more than offset by more energy into the room. You should be able to easily tell if this is effective.

7

ok, thanks for the replies everyone!

8

Well this insulation is not going to provide much in the way of preventing heat loss from the house because the area involved is so small. So I am curious as to what the OP can do cheaply to insulate the rest of the house. If it were wooden siding a traditional approach is to drill one inch holes in the siding and blow insulation into the empty spaces. But with brick siding? No. So what would Dopers suggest?

9

Well there’s definitely heat loss through the entire wall but that recess is especially problematic because of the radiator, no? Isn’t the increased temperature differential there just inviting the radiant heat to get absorbed right out the wall?

Regarding insulation, I’ve read that it can be a mistake to try to insulate these brick walls in older houses. Apparently, since there is no vapor barrier (assuming the method of insulation is blowing it in), the humidity of the house gets trapped and condenses in the insulation, and you eventually end up with permanently wet insulation inside your walls. This would worry me, which is why I haven’t considered doing it yet. If there was some proper method of insulating it relatively easily I would definitely explore it. But right now it seems the only real way to do it is to basically rebuild the whole interior side of the wall.

10

Yes - Insulating the area behind the rad absolutely helps. In fact you should do this for every radiator you have, not just for the recessed ones like you show. The radiators not only heat the air, but also “radiate” heat in every direction equally, including radiating into the exterior walls and out of your house, regardless of whether those walls are insulated. You want to do what you can to reflect that radiated heat back into the interior of your home where it can heat the air.

In my 90 year house in Toronto I installed sheets of reflective bubble wrap style insulation bought from Home Depot. I just screwed the four corners into the with small screws and big washers to add some surface holding area:

Where I had space behind the rads, before I added the reflective wrap, I added as much styrofoam as I could. You don’t want to be directly up against the rad, they need airflow around them to help the heating air part. I left at least 1" in the back. My rads vary between 2" to 8" from the wall:

About a year after I did the above myself, our local natural gas supplier Enbridge Gas, offered a program where they provided for free as many reflective panels as you needed to do all the rads in your house. These panels were very thin, so I also took advantage and added them too. I’ve never seen the panels at retail, but it may be worth contacting the company to see if you can get them. Their website offers some info on the benefits of reflective sheets for rads:

BTW - the negative with this is that the silver reflective material looks crappy when you can see it. Most of my rads have rad covers so they’re not so obvious.

Lastly, as another poster noted, adding a fan to increase air flow around the rad is supposed to help efficiency. I’ve seen on several TV reno shows where they talk about adding “whisper quiet” fans for this reason. I have yet to find one that is quiet enough to put under the rad and not notice while in the room. If you find one, please let me know. Also, I do question if the increase in rad efficiency would offset the cost of running the fan 24/7. Otherwise you’d have to find a fan that switched on only when the rad was hot (never heard of this) or manually turn the fan on and off, pain in the butt.

11

Some say insulating an old brick wall is a recipe for trouble, but I’ve not seen it in practice. The thing is, what you generally DON’T want in a situation like that is a vapor barrier. When water is trapped in a wall it’s usually BECAUSE of the vapor barrier. That’s the whole point, that it blocks vapor transmission, so humidity can’t get from where it’s higher to where it’s lower, so you get the condensation problems when it’s on the wrong side of the wall. When there’s no vapor barrier then the wall can breathe to both the exterior and interior, mitigating problems. Either way, a reputable subcontractor or consultant will do a dew point analysis to ensure that condensation doesn’t happen where it would cause a problem. Spray foam insulation also tends to work better for such retrofit situations too.

In fact in many of the mid-latitude climate zones in the US where there’s hot humid summers and cold winters the best practice now is not to have a vapor barrier at all, because it creates more problems than it solves. The exceptions are the very hot humid southern locations like the deep south, very cold locations near the Canadian border, and oddball situations like indoor pools and such where you know the vapor drive will nearly always be in one direction only.

12

Without doing the calcs, I believe using the fan will still result in a net gain, even when running 24/7. The increase in heat transfer coefficient will be significant. The electricity required to run the fans will be discharged into the room as heat, so its consumption is akin to that of a resistance heater (~2-3x the cost per BTU of natural gas). Still, the BTU output from natural gas >>> electricity, so the very small contributor is increasing in $/BTU while the very large contributor is decreasing. Lastly, the fans should move the heat away from the wall and towards the inside of the room, where I assume the occupants are, resulting in increased comfort.

I do agree though that if possible, it’s better to link fan and radiator operation.

13

Interesting take, jjakucyk - thanks. I will continue to research this topic.

It should be possible with today’s smart devices. For example you can plug the fan into a smart outlet, then program it to come on whenever your thermostat calls for heat.

GMANCANADA - I’ve heard those fancy Dyson bladeless fans are very quiet.

14

A fan wouldn’t really change the total amount of heat pumped into a room, it just does it faster, so the boiler would cycle off sooner. That could mean that the water in the system would cool more before the next call for heat. There’s a lot of variables in play, but it’s not so simple as fan–>more heat transfer–>lower gas bill.

15

The fans I’ve seen are always quite small and sit below or behind the rad and always quite noisy. I think the quiet ones are like unicorns and I’ve given up.

Based on the cost of the Dyson fans in Canada, I think the payback would be at least 300 years, they are laughably expensive. My background is marketing and the term we use for Dyson products is “solutions in search of problems”.

The only friends I know who like (love) Dyson products are all engineers. They seem to see the genius in creating complex solutions to things that really aren’t a problem. One friend’s wife laughs (and cries) that he bought the Dyson vacuum, then the fan and she expects to get the new hair dryer for Xmas.

I did extensive research (about 10 years ago) to insulate my double brick, lathe and plaster walls. Technology may have changed since but at the time, the only real option involved ripping out the paster on exterior walls and then either high expansion spray foam insulation (which also acts as a vapour barrier) or adding batts of insulation and plastic vapour barrier. Then drywall over the foam or insulation like new constriction.

The payback in energy savings was not even close then. My entire heating bill was only a couple thousand a year, the cost of a single room was pushing $10,000 all in.

Low expansion foam spray seemed promising: they drill small holes in the top of the wall between each stud and run in a hose and then pump in “low expansion” foam. However they were very clear that the foam will still expand slightly and has the potential to pop the plaster outward and crack all the walls. I assumed all responsibility for damages. I decided the risk wasn’t worth it (In fairness to the company, the rep said they no longer recommend it for very old houses because the plaster it too brittle).

In the end, in addition to trying to make the rads more “efficient”, I was told most of the heat leaks out through the attic / roof and if I had a dollar to spend, spend it on insulating the attic. I did that. It made a dramatic difference in my heating costs for less than $1,000.

16

@ jjakucyk - regarding the boiler cycling off sooner, it made me think of a question I’ve never seen a definitive answer for:

For old “boiler / radiator” heating systems: are programmable thermostats better or worse? [The idea being you reduce your temperature during the day when you’re out and during the night when you sleep, then cycle it back up when you’re active in the house.]

On the "pro"side, you’re not paying to heat your house when you don’t need it. On the “con” side, the water cools off and then takes longer to reheat, negating any energy saving?

Does anyone have an answer to this?

17

GMANCANADA: Yes you will save energy. As to the problem of it taking longer to reheat you need to set the time some time before you actually want it. 15 minutes? A half hour? Experiment.

18

It’s not a given that you will save energy. There’s so many variables to take into account, like how the building is or isn’t insulated, the amount of water in the piping and the size of the piping (thus how much it retains or loses heat and has to be heated up again), other internal heat sources like lights, computers, appliances, water heaters, solar and wind exposure, plus how the boiler functions, like whether it maintains a certain system water temperature based on the outside temperature and then distributes that water to the house via thermostatic valves or if it just runs like a furnace with a simple on/off control. These all have an impact on energy savings as they relate to temperature setback, the question is just what that impact is, which I can’t really answer since they all play on each other.

Here’s a couple anecdotes however. I grew up in a 1920s house with hot water heat (a mix of radiators and some heavy baseboards). Dad generally left the thermostat alone at about 70º except maybe dialing it down 2º or so at night more for comfort than anything. That boiler worked just like a furnace. When the thermostat called for heat, it switched on the burner and pump, and when it was satisfied, it turned them both off. Those round Honeywell thermostats had adjustments for the anticipator that would kick on or off sooner than a furnace or air conditioner to allow time for the water to heat and also to prevent overshooting the set point. As a set it and forget it system, it worked just fine. However, if we’d been away on vacation for a week, when Dad would set the thermostat down to like 57º or something, he’d only turn it up to maybe 65º when coming back because the boiler would just fire constantly until it hit its high temperature cutoff (about 180º I think) while keeping the pump going. If he set the thermostat to 70º when coming home, the system would be so hot after it finally hit that set point that all the residual heat would push the house to 75º or over, so starting lower and keeping an eye on it would prevent that. Programmable thermostats nowadays are smart enough to handle this sort of situation. They ramp up the temperature slowly over two or three hours. It’s the same with heat pumps, so they don’t go into high-power mode if they have multi-stage compressors, and so they don’t engage the very expensive to run auxiliary heating coils.

Other systems work differently though. In my church when I was a kid the boiler would always maintain itself at its high-temperature cutoff point around 200º. This was true no matter the outside temperature, and in fact the boiler didn’t even have any outdoor temperature controls so it could sit there burning up all summer if it wasn’t shut off manually. The boiler itself was insulated and had a damper in the flue so the heat wouldn’t just waft up the chimney. It basically acted as a hot water storage tank. The system was from the mid 1960s, and it used pneumatic Johnson Controls thermostats. Part of that system was a mixing valve that would send water through the boiler as necessary to maintain a system water supply temperature based on the outside temperature. Individual thermostats in each room controlled the radiator valves, and it could even do night setbacks by changing the air pressure of the system from 15PSI to 20PSI, which the thermostats could detect and switch to a separate bimetallic strip (innovative, but not too reliable in the long term). Anyway, being a newer system with thinner pipes and copper fin-tube convectors, the system wouldn’t retain heat like an older one with heavy cast iron radiators and thicker pipes, so there were less warmup and cool down issues. With the boiler always ready to go, and with the water circulating the building at the proper temperature, it made setting back the temperature much more effective in saving on gas.

Currently i’m working on a commercial remodeling project that uses very sophisticated variable refrigerant flow heat pumps with inverter compressors. The client was concerned about setback policies for unused spaces and recovery time. This is in southern Ohio so it does go below zero sometimes, and it’s a 100 year old building with little to no insulation. The mechanical engineer however said that these units can recover from 60º to 70º in 1-2 hours even at 0º outdoor temperature, and this is with no auxiliary heating coils. Bog standard residential heat pumps don’t stand a chance in conditions like that. However, what the engineer did say is that setting back more than 10º on a day/night cycle would use more electricity to recover than simply maintaining an even setting. If a room isn’t being used for many days or even weeks at a time, then yes it makes sense to set back more. Even so, throwing in things like demand metering (where you’re charged extra based on your maximum power draw in kilowatts) can also factor into the equation in unexpected ways.

19

Wow - thanks for the detailed answer. Bottom line - too complex a problem for a yes or no. I kind of suspected that.

I installed a Honeywell programmable several years ago. The model had “smart programming”. When you set the temp to be at 70º for 6:30AM, that did not mean turn on at 6:30AM, but to be at 70º by 6:30AM. The thermostat first turns on 30 minutes early (6:00AM), if it took 35 minutes to get to 70º, the next day it would turn on 35 minutes ahead of your target (5:55AM), if it took only 15 minutes to get to 70º, then the next day it would turn on at 6:15AM. It recalibrates like this daily to account for changes in the weather over the winter. Sounds like that was good choice. As it is, I only lower it 4 degrees at night, as you noted, mostly for sleeping comfort.

We have a Viessmann boiler that is getting near the end of it’s life, maybe 17 years old now. I’ve been holding off replacing it. I’ve also been considering a tankless hot water (on demand) unit. I mentioned this about 3 years ago to a friend who was a VP at our local natural gas company and he suggested I delay as long as I can. He felt I should consider the latest Euro tech.: combo boilers with tankless hot water. At that point he said the units performed only “OK”, but showed lots promise and they were seeing improvements in capacity and efficiency every year. One of those things where the tech will only get better and cheaper if you can delay. Anyone have an opinion on these?

20

My apartment building has two Lochinvar boilers that are about 5 or 6 years old, which I think are rated around 650,000 BTU/H each. What’s amazing is that these are barely half the size of a refrigerator, venting through 6" diameter PVC pipe, whereas the original coal steam boiler that took up a huge room and had a chimney about 8 feet across. Anyway, these replaced older boilers from the 1960s when the building was converted from steam to hot water. They also provide the building’s hot water via a couple of storage tanks. When they work, they work great (modulating output, efficient, clean), but they both break down frequently.

Boiler 1 is only for building heat, and boiler 2 is primarily for the domestic hot water, but it can also supplement boiler 1 on really cold days. Boiler 1 runs pretty much constantly all winter, modulating as necessary depending on the outdoor temperature. Boiler 2 fires up periodically to keep the domestic hot water up to temperature, but it does so all year. I assume the duty cycles of both are pretty similar. Boiler 2 goes down a lot unfortunately, and it’s the only one piped to the storage tanks, we lose hot water pretty quick. When Boiler 1 goes down, 2 doesn’t kick in as a failover, which is an unfortunate oversight in the programming. There’s been issues with bad control boards, and my understanding is that they’re going down because of unpredictable faults that are detected in the many systems the control boards monitor (intake air, voltage, temperature, water flow, you name it) and they just shut down, requiring manual resetting. That’s a problem when you don’t have on-site staff.

My thought is that a separate standalone boiler and tankless water heater might be a better bet. Nowadays I don’t think there’d be so much difference in efficiency or longevity, especially compared to the really crappy boilers with integrated tankless heaters from the 1970s and before, but at least if one goes down you don’t lose the other. I admit I can’t really come up with much compelling reasoning one way or the other beyond that. The technology is always progressing, but as in my apartment, more technology in this case can certainly mean there’s more to go wrong. It’s much like the check-engine light in a car, but at least the car doesn’t stop running if there’s a cracked vacuum hose or faulty oxygen sensor.