Hot Tub Insulation Systems By The Numbers
Hot Tub Insulation Systems By The Numbers
I know a lot about hot tubs, and insulation systems, and R-values, and Thermal loss, and Reflective heat barriers, and Convection loss, and Conduction loss, and Thermal mass. and even I get confused when watching the videos about different hot tub insulation systems! I mean holy jumpin, R values, Open Celled Foam, Closed Cell Foam, Thermal drift, Diminishing Returns, Motor heat reclamation… it can get very confusing even for an expert! Especially when most of the talk is wishy washy non-tech talk! It’s all about buzz words and intentionally confusing the issues.
Now deep down My usually very reliable logic side says the more insulation there is in the system the more efficient that system is going to be, after all, let’s face it, it costs twice as much to do a full foam insulation system on a hot tub than it does to do the best perimeter systems and the base R-value is two, three, or even four times higher on the full foam spas! But, is it as Arctic Spas says in their video that the extra foam in a full foam spa is past the point of diminishing return and a waste of money and just for show?
So I went back to my tech background and decided to do it by the numbers. Science and numbers don’t lie and I really was curious as to when do we reach a point that a system has enough insulation in it? At what point is adding extra insulation into the Spa become meaningless? The law of diminishing returns says that eventually, the insulation value would be high enough that the extra insulation would be a waste of time and money but were hot tubs at that point? or was it a real meaningful difference?
To make this realistic I did two things. First I chose what I think are the best examples of each type of insulation system.
So I used Arctic Spas, to represent the Perimeter insulation systems because they use a Closed cell high-density foam approximately 3” thick as opposed to the cheaper systems using mylar, attic insulation, and styrofoam board. Arctic Spa is arguably the best Perimeter insulation system out there.
Then I choose Master Spas for the full foam system because they use the Icynene open celled foam so they can put 3x more insulation in and still get easy access to plumbing, again they are arguably the best full foam system out there.
I ignored the floor of the hot tub because both of the hot tubs I modeled use high-density foam under the floor, and I ignored the cover because any spa can have any cover and both are available with a 6” FRP covers. I also ignored the Heat reclaiming systems from the motor waste heat because even though the Master uses an active system and is slightly more efficient the Arctic uses a passive system and is a close second. Both spas are pretty good at reusing the waste heat from the motors so that’s not going to be an effector on the running costs and I really wanted to know just how the insulation by the numbers alone without all the bullshit.
So… First off I’m not going to go into How R values work and how these formulas were derived, they are accepted calculations from approved rating agencies and accepted in building standards, In other words the science is sound, there is no arguing that this is the right way to calculate heat loss in a system and if you are an anorak and really want to go to the deep end of the pool follow the links at the bottom of the article.
So here is the comparison by the numbers.
Arctic claims to use 3” of high-density foam and we will take them at their word. High-density Closed cell foam is right around R-5/inch to R-7/inch when newly sprayed so we will go with an average value of R18 for the system.
Master Spas uses a half inch of closed cell foam on the shell and an average of 8” of open celled foam in the cavity between the spa and the skirt so the R-value of the high density is about 3 and the open celled at R-3.6/inch to R-4.5/inch per inch is another R-32 so let’s round down to R35 for the whole system.
Now Arctic spa claims in their video that the extra R value is meaningless as it is past the point of diminishing returns but what do the numbers say?
We will assume four sides of a 2m wide spa 1m deep or 8m2 of surface area.
We will run two sets of numbers one to assume winter running conditions of 40 °C (104°F) inside the spa and -10 °C (14 °F) outside. And one to assume spring or fall conditions of 40 °C (104°F) inside the tub and 10 °C (50 °F) degrees outside
The energy required to keep up with the heat loss in a system is calculated as follows…
(The surface area of the sides) X (Temperature difference °C) / (RSI (r value factor for metric)) = Energy required to keep up with the heat loss in watts.
Perimeter Insulation ~ R18 (RSI-3.17)
***Assumes brand new closed cell foam this will diminish over time see below***
@ -10C 8m2 X 50 / 3.17 = 126 Watts
@+10C 8m2 X 30 / 3.17 = 76 Watts
Full Foam Insulation ~ R 35 (RSI-6.16)
@ -10C 8m2 X 50 / 6.16 = 65 Watts
@+10C 8m2 X 30 / 6.16 = 39 Watts
The perimeter insulation system needs almost twice as much energy and therefore costs twice as much to replace the heat lost through the lower R-value insulation system!
So this clearly is a significant amount of energy difference required to keep up with the heat loss but what does it mean in dollars and cents? The perimeter system uses almost twice as much energy to keep up with heat loss as the Full foam system.
What about the R-value over time?…
Open celled foams do not diminish over time, so the value it has 10 years down the road is 99% of the original value
The closed cell foams on the perimeter systems use gasses other than air in the cells, known as “captive blowing agents,” these gasses increase the thermal resistance of the foam. However, the gasses leak out of the cells over time and are replaced by air thereby decreasing the R-value. This phenomenon is known as “thermal drift” and applies to all closed cell foams and is covered by ASTM C 1303 in the USA and CAN/ULC-S770 standards in Canada which calculate the loss of R-value over time. A foam starting at R7 will within a few months drop to R6 and CAN/ULC-S770 certification says that this foam should be considered around 5.5 to account for thermal drift over a 5 year period. So the truth is that while the closed cell foams are rated from R-5/inch to R-7/inch at the time of blowing they really should be rated between R-3.5/inch to R-5.5/inch to account for lost R-value over time! So while the pitch is that the hhigh-densityfoams are higher R-value within a 5 year period they are the same as open celled foam and after that are less!
Re-calculating the numbers to account for the loss in R-value over 5 years to give you a real picture of the average R value makes a massive difference.
Perimeter Insulation ~ R13.5 (RSI-2.83)
***Assumes R-value loss through thermal drift as outlined in Underwriters Laboratory of Canada standard CAN/ULC-S770***
@ -10C 8m2 X 50 / 2.83 = 141.34 Watts
@+10C 8m2 X 30 / 2.83 = 84.81 Watts
So when we take all the factors into account the reality is the full foam spa should perform more than twice as well as the best perimeter systems out there. And three to four times better than the worst ones!
This really is no surprise. There are no magic bullets folks just magic spin and marketing, there is a good reason why the full foam systems cost twice as much as the perimeter systems, the numbers say they will perform more than twice as well.
I guess there is no Santa Claus 🙁
Further reading for my fellow anoraks…