Author Topic: Anyone super-insulate a hot tub?  (Read 500 times)

agoraphone

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Anyone super-insulate a hot tub?
« on: December 16, 2018, 09:30:09 PM »
I'm looking to get a hot tub in Maine and wondering about adding insulation due to the cold climate. The tub I get would probably be well insulated to begin with, but I want to make it as efficient as possible. I was thinking I could just make a box of rigid foam insulation all around the tub. Anyone try this? I'd love to know your experiences and thoughts. Thanks!

davisgang90

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Re: Anyone super-insulate a hot tub?
« Reply #1 on: December 17, 2018, 04:27:03 AM »
Quality modern hot tubs have significant insulation.  I can't see surrounding the outside with foam would do much other than make it look like an eye sore.

agoraphone

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Re: Anyone super-insulate a hot tub?
« Reply #2 on: December 17, 2018, 06:12:01 AM »
Quality modern hot tubs have significant insulation.  I can't see surrounding the outside with foam would do much other than make it look like an eye sore.

Well you could say the same thing about homes - that the typical R-38 is enough in your walls and R-60 in the ceiling. But if you want to be super efficient and use as little energy as possible (especially in cold climates), you could make it R-60 in the walls and R-90 or even R-140 in the ceiling. Maine gets COLD.

nereo

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Re: Anyone super-insulate a hot tub?
« Reply #3 on: December 17, 2018, 07:58:39 AM »
Quality modern hot tubs have significant insulation.  I can't see surrounding the outside with foam would do much other than make it look like an eye sore.

Well you could say the same thing about homes - that the typical R-38 is enough in your walls and R-60 in the ceiling. But if you want to be super efficient and use as little energy as possible (especially in cold climates), you could make it R-60 in the walls and R-90 or even R-140 in the ceiling. Maine gets COLD.

Except that the volume of a hot tub and the thermal capacity of water is orders-of-magnitude different between a home and a hot tub.  modern hot tubs already use closed-cell foam inside the shell as well as throughout the cabinet, and when the hot tub remains closed the heat loss is fairly minimal given its surface-area:volume.

When you are using the hot tub, heat-loss is going to be very high due to the fact that the top will be off and the water will be in motion (convection).
 
When its not in use, the majority of the heat loss is through the cover (analogous to the roof in your example), so having a high quality cover is key.  Next up is adding a bubble-mat to the water's surface (which slows down thermal loss from the water to the air space above) and (if you really want) placing an additional level of rigid foam on the top. 

Ultimately further insulation efforts won't amount to much if you are using the hot tub even semi-frequently (e.g. 2-3x per week or more), as >90% of heating costs come during use and not while the hot tub is closed and in stand-by.  If you aren't using it much in the winter, a simpler solution would be to either drain the tub completely or let the temperature drop to the lowest the unit will allow (usually 64F).  The latter is only energy efficient if you use the tub infrequently, once-per-week or less (depending on volume).

experience:  someone who used to service hot tubs and who also lives in Maine.

Prairie Stash

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Re: Anyone super-insulate a hot tub?
« Reply #4 on: December 17, 2018, 08:15:26 AM »
Quality modern hot tubs have significant insulation.  I can't see surrounding the outside with foam would do much other than make it look like an eye sore.

Well you could say the same thing about homes - that the typical R-38 is enough in your walls and R-60 in the ceiling. But if you want to be super efficient and use as little energy as possible (especially in cold climates), you could make it R-60 in the walls and R-90 or even R-140 in the ceiling. Maine gets COLD.

Except that the volume of a hot tub and the thermal capacity of water is orders-of-magnitude different between a home and a hot tub.  modern hot tubs already use closed-cell foam inside the shell as well as throughout the cabinet, and when the hot tub remains closed the heat loss is fairly minimal given its surface-area:volume.

When you are using the hot tub, heat-loss is going to be very high due to the fact that the top will be off and the water will be in motion (convection).
 
When its not in use, the majority of the heat loss is through the cover (analogous to the roof in your example), so having a high quality cover is key.  Next up is adding a bubble-mat to the water's surface (which slows down thermal loss from the water to the air space above) and (if you really want) placing an additional level of rigid foam on the top. 

Ultimately further insulation efforts won't amount to much if you are using the hot tub even semi-frequently (e.g. 2-3x per week or more), as >90% of heating costs come during use and not while the hot tub is closed and in stand-by.  If you aren't using it much in the winter, a simpler solution would be to either drain the tub completely or let the temperature drop to the lowest the unit will allow (usually 64F).  The latter is only energy efficient if you use the tub infrequently, once-per-week or less (depending on volume).

experience:  someone who used to service hot tubs and who also lives in Maine.
I don't dispute anyting you say.

However, what is the dollar value associated with insulating more vs. the energy loss? Are we discussing $300 of insulation and cover up material to save $40/year? will the savings be greater or less? I really don't know, it would be interesting to have numbers.

R-120 isn't entirely unreasonable in a ceiling for the cost where I live. Obviously there is a price point where more insulation isn't saving much money, but it will be different for every locale. Saying that a hot tub in Maine requires the same insulation as Florida means either the one in Florida has too much or the one in Maine has too little relative to the cost of extra insulation. Or do they ship them with varying insulation levels to different parts of the country? I really don't know and I'm curious.

nereo

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Re: Anyone super-insulate a hot tub?
« Reply #5 on: December 17, 2018, 08:48:33 AM »
Quality modern hot tubs have significant insulation.  I can't see surrounding the outside with foam would do much other than make it look like an eye sore.

Well you could say the same thing about homes - that the typical R-38 is enough in your walls and R-60 in the ceiling. But if you want to be super efficient and use as little energy as possible (especially in cold climates), you could make it R-60 in the walls and R-90 or even R-140 in the ceiling. Maine gets COLD.

Except that the volume of a hot tub and the thermal capacity of water is orders-of-magnitude different between a home and a hot tub.  modern hot tubs already use closed-cell foam inside the shell as well as throughout the cabinet, and when the hot tub remains closed the heat loss is fairly minimal given its surface-area:volume.

When you are using the hot tub, heat-loss is going to be very high due to the fact that the top will be off and the water will be in motion (convection).
 
When its not in use, the majority of the heat loss is through the cover (analogous to the roof in your example), so having a high quality cover is key.  Next up is adding a bubble-mat to the water's surface (which slows down thermal loss from the water to the air space above) and (if you really want) placing an additional level of rigid foam on the top. 

Ultimately further insulation efforts won't amount to much if you are using the hot tub even semi-frequently (e.g. 2-3x per week or more), as >90% of heating costs come during use and not while the hot tub is closed and in stand-by.  If you aren't using it much in the winter, a simpler solution would be to either drain the tub completely or let the temperature drop to the lowest the unit will allow (usually 64F).  The latter is only energy efficient if you use the tub infrequently, once-per-week or less (depending on volume).

experience:  someone who used to service hot tubs and who also lives in Maine.
I don't dispute anyting you say.

However, what is the dollar value associated with insulating more vs. the energy loss? Are we discussing $300 of insulation and cover up material to save $40/year? will the savings be greater or less? I really don't know, it would be interesting to have numbers.

R-120 isn't entirely unreasonable in a ceiling for the cost where I live. Obviously there is a price point where more insulation isn't saving much money, but it will be different for every locale. Saying that a hot tub in Maine requires the same insulation as Florida means either the one in Florida has too much or the one in Maine has too little relative to the cost of extra insulation. Or do they ship them with varying insulation levels to different parts of the country? I really don't know and I'm curious.

You can buy either a partially insulated hot tub or one with full insulation.  For this I'm assuming the OP is using one that's already fully insulated - which are typically the only ones marketed in colder climates (if it wasn't fully insulated, it would be unlikely that the in-line heater could even keep the tub at temperature in colder temps).

As I said, the most value comes from having the best cover and an insulated bubble mat. Part of what's missing here is the size of the hot tub in question and the number of times its used and what the external temperatures are.  The best insulated covers are $400-600, and they last 2-4 years outside in direct sunlight (longer if its protected from sun).  A bubble-mat is $50.  Using these a hot tub will use only a few kilowatts per day (so a monthly electricity bill somewhere between $5-10). But the key is that most of the electricity use comes during use, while the top is off

I would certainly use the highest insulated cover and a mat in a cold-weather climate. But unless you stop using the spa entirely any additional insulating efforts would be trying to minimize a small fraction of the total electricity use.  At best there's only $100 per year in standby energy costs to be recouped (the tub will use even less energy during the warm summer months) - to double the insulation all around the tub would take (by my estimates) at least 4" of rigid foam on all four sides - so $500+ in expenses and considerable amount of work.  You also need to make sure that there is access to the pumps and the motors themselves can't be fully enclosed (which leaves a big gap in any super-insulating plans).

BudgetSlasher

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Re: Anyone super-insulate a hot tub?
« Reply #6 on: December 17, 2018, 03:52:08 PM »
Quality modern hot tubs have significant insulation.  I can't see surrounding the outside with foam would do much other than make it look like an eye sore.

Well you could say the same thing about homes - that the typical R-38 is enough in your walls and R-60 in the ceiling. But if you want to be super efficient and use as little energy as possible (especially in cold climates), you could make it R-60 in the walls and R-90 or even R-140 in the ceiling. Maine gets COLD.

Except that the volume of a hot tub and the thermal capacity of water is orders-of-magnitude different between a home and a hot tub.  modern hot tubs already use closed-cell foam inside the shell as well as throughout the cabinet, and when the hot tub remains closed the heat loss is fairly minimal given its surface-area:volume.



It's Maine, wait 5 minutes and you'll find a free perfectly working hot tub, the only catch is you have to removing it from the last owner's deck now that he is sick of paying for it's upkeep and heating. So perhaps the hot tub is not that new.

agoraphone

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Re: Anyone super-insulate a hot tub?
« Reply #7 on: December 18, 2018, 11:38:01 AM »
Thanks for the thoughtful responses everyone! Sounds like it's probably not worth it unless I somehow inherit a bunch of insulation. Another consideration is hiding all that ugly pink insulation behind something more attractive....

Guess I'll focus on making sure I have a thick, quality cover.