I am hopnig someone can provide me with some guidance as to the engineering behind spa covers. We recently aquired a old CalSpa for free and have spent the summer fixnig leaks, building new frames and skirts, and in general restoring this unit on an economical budget. We have used it a few times and I have been using a couple sheets of Dow Pink Foam as a cover temporarily. I have looked at covers and it appears I will be money ahead to build my own and build it thicker then standard. I have built a light wood frame and put insulation on it and am beginning to infill between the frame members. My question comes in the cover. I understand from an older cover I have that is broken they wrap the foam core in plastic and then there is a mesh on the hot tub side to let water drain into the tub. The pink foam does not absorb water so I wonder if the plastic wrap is necessary? As well as, if you put vinyl on the hot tub side of the cover will you ever get enough water into the cover to warrant the mesh? That just seems to be allowing water to go into the cover. Unless there are enough condensation issues at the dewpoint of the cover to create a moisture problem. Thanks Tim

Covers vary as to the construction. Some are simply solid vinyl on both sides, some with mesh on the bottom, others with eyelets in the vinyl to allow for breathing. I can't see the plastic being necessary if using the extruded polystyrene insulation as you suggested. The trick is that most covers are thicker in the center than at the edge, and often have an aluminum "TEE" channel insert at that center edge for support. Most covers fail over time at the seam where the two halves bend. Many suffer from waterlogging. I'm in the belief that a few eyelets in the vinyl on the underside while using the polystyrene insulation should be an effective and efficient cover.

The extruded polystyrene you have, like expanded polystyrene (that the typical spa cover is made of) won't absorb water if in direct contact with it. It WILL, however, just like expanded absorb the smaller molecules of steam and evaporating water.

Thanks for the help and guidance. I did address the slope for drainage and had build a lightweight wood frame to support at the center. However in my attempt to become more energy efficient I am almost 6" thick at the center and getting a bit heavy. Maybe I should have just bought one. Thanks for the help. Tim

I wonder why no one has come out with a spa cover that is made of a sandwich of fiberglass with a injected foam insulation core. All this vinyl stuff is nonsense, and is a mildew trap. Look what they make the tub out of.

Funny you should mention that. I was at a home show recently and saw a new model with a cover exactly as you have described. It was totally sealed so that no water could ever get in. The sales rep had just gotten it in the week before so he know very little about it. Sure woudl be nice if they started making them for older units.

I just bought a used tub with no cover so I made my own. I didn't have $300+ to buy a real cover so I spent under $100 on materials instead. Hope it holds up for a year or two at least.

I used 2 layers of 2" thick pink insulation board that claims R20 at that thickness. What's the R value of most spa covers?

I also have a little foam blanket that rests on the water that I'm sure adds 1 or 2 to the R value.

Seems to be working pretty well but it's still early. We'll see how it holds up when the temps really dip.

Lunchbox1973 -

Let me know how this goes. I have yet to get my used Sundance in service but I am curious how others in WI hold up in the frosty winter months. What are you using for a floating foam blanket?

AGE

I don't know, it came with the tub. It's white, about 1/8" thick maybe. I'm trying to compare it to something else but can't think of any material similar.

Since I wrote the first post I put together scraps fro the foamboard to create a piece that floats on top of the blanket for extra insulation.

I have a very waterlogged cover, only on one side, and can't believe the cost of replacement foam or a new cover. Apparently the plastic covering the styrofoam deteriorated which made the foam come in contact with the chemicals and steam from the water...and the styrofoam absorbed alot of water. It is unbelievable how heavy my cover (1/2 of it) is. possibly 100 lbs? (I wasnt aware that styrofoam was able to absorb water.)

If you use the 2 layers of 2" thick pink insulation board, do you cut it to fit inside the top of the spa (floating on the water) or do you cut it to lay across the top with a few inches of air space above the water level? Wont this insulation absorb water also?