How much insulation does a hot tub need?

Question:

"I just purchased a hot tub and I noticed that there doesn't appear to be a lot of insulation on the underside of the hot tub shell. How much insulation is needed? Should I purchase some expanding foam insulation and add more to what is there? It looks like an inch of insulation."

Answer:

While I am not sure of the type of spa, I assume by your question that you have purchased a full foam spa and the area you are speaking about is where the equipment is. If there isn't insulation there, the manufacturer probably left it out for a reason (like needing service access to plumbing areas later).

It is unlikely that you should need to add insulation under the shell. If you purchased a good cover with your spa, and that cover overlaps all of the top surfaces of the tub, this adds a value of about R12 to the area I think you are speaking of.

Remember that heat rises, so a good, insulated cover that seals the top of the hot tub is much better than having a lot of insulation glued under the shell.

Also, the outer cabinet of the hot tub serves to trap the air around the shell, creating an insulating barrier of air. Make sure the cabinet walls do not have a lot of leaks for air (and rodents) to slip in. You can find foam strips (for air conditioners, etc.) that will compress into cracks that allow air to seep in.

This, combined with a high-R rated, tightly fitted cover, is much more effective (and a lot less messy) than adding expanding foam insulation to the shell.

: )