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Mithril 05-24-2007 09:11 AM

Icebox question
How thick do you think a toploading icebox should be? Dimensions, roughly 16" x 24" and tapering down on one side. Because of size constraints, the largest I can make it is about 4 cu ft. I am planning on using FRP Sheets (Fiberglass Reinforced Plastic). Will 1/8" be thick enough?


sailingdog 05-24-2007 09:29 AM


How thick does the FRP have to be to support the loads? How thick does the insulation have to be to keep the heat out...and prevent the ice from melting? Your question doesn't make any sense. Might want to re-phrase it.

Mithril 05-24-2007 09:41 AM

Sailing Dog... I have already determined what I need for insulation to achieve R-30 so it doesn't come into play in this question. The insulation will be added afterward. Your first question "How thick does the FRP have to be to support the loads?" comes closer to what I am asking.

Is 1/8" Fiberglass Reinforced Plastic sheets thick enough for a 4 cu. ft. icebox, if anyone has used this material before? Otherwise, how thick is the material you used in your icebox of about this same size?

An online marine refrigeration company suggested that this should be thick enough but I wanted others opinions...

- Mithril

sailingdog 05-24-2007 09:52 AM

It really depends on how the sheets are made and how they will be joined together to make the icebox. Since you don't say what the 1/8" FRP is made of, or give any more information than that, it is hard to say whether they will be strong enough.

Are you going to fiberglass the sheets together and make it into a single piece laminated box?

Your box is going to be about 16" x 24" x 18" or a bit more depending on the tapering... If you've got the icebox loaded with beer...think of how much four cubic feet of water weighs... comes in around 250 lbs.

An 18" span of 1/8" fiberglass with 250 lbs on it is going to be a bit on the weak side. I'd over-engineer rather than under, but that's just me. Also, you want the bottom of the box to be strong enough so that if someone were to drop that half-gallon metal can of pineapple juice into the empty icebox, it wouldn't crack or break under the impact. I don't think 1/8" is going to hold.

Mithril 05-24-2007 10:09 AM

Thanks Sea Dog... that was my concern as well.. The information on FRP sheets can be found at: White Fiberglass Sheet

As far as the construction of the box is concerned, they instruct to: lightly sand the edges then apply a strip of fiberglass cloth and resin (polyester or epoxy) to the outside edges to hold it together. Once it is dry, use thickened and white pigmented resin to create 1" fillets on the inside corners.

- Mithril

sailingdog 05-24-2007 10:13 AM

I would use epoxy resin, rather than polyester, as it will probably give you a stronger bond and is more resistant to most chemicals. Should probably gelcoat or paint the interior of the icebox just in case.

I would go with the 1/4" just to be safe... ;) Don't want your beer falling through the bottom of the icebox into the bilge.

Mithril 05-24-2007 10:16 AM

It looks like their site just went down. Here is what they have...

Fiberglass Reinforced Plastic
(FRP) Sheet

Of all the materials used to build refrigerator boxes, none is better than strong fiberglass sheeting. Stainless steel looks nice but it is difficult to work with and thermally conductive. Plywood is popular but needs to be fiberglass coated and often ends up with a rough, amateur appearance when you are done. Rparts has the answer - Bright white, pultruded fiberglass sheet.

What is "pultruding"
Pultruding is a high-volume production method of making certain types of fiberglass products. In pultruding, continuous fiberglass strands are run through a resin bath and then fed into heated high-pressure dies. The result is a consistently optimized resin/glass ratio unobtainable by any other method. Anyone familiar with fiberglass knows that an optimized resin/glass ratio means the highest strength attainable - often 3x higher than hand-laid glass.

How do I build a box with it?
The details of box construction are outside the scope of this web site at the present time (we'll be changing that in the future). However, in general, you would use this sheet in pretty much the same way as you would plywood. Use a regular saw and carbon-tip blade to cut individual sheets to size and create the box that you want. Lightly sand the edges then apply a strip of fiberglass cloth and resin (polyester or epoxy) to the outside edges to hold it together. Once it is dry, use thickened and white pigmented resin to create 1" fillets on the inside corners. If you are reasonably careful with the filleting, the result looks like it was just popped off a production mold. Once everything is set, you can paint the inside if you wish but it is not necessary. Even unpainted, the FRP sheet is very bright and stain resistant.

Does it bend?
RParts FRP sheet is available in 1/4" and 1/8" thickness. The 1/4" material is far too stiff to bend much at all The 1/8" material can be comfortably bent to a radius of about 18". Which thickness you should use in constructing your box depends on the size and shape. In most cases the 1/8" material has more than enough strength to rigidly support even the largest vertically hung holdover plate.

PLEASE NOTE: All FRP sheets are subject to a shipping charge adjustment. We will contact you with details shortly before the order ships if this adjustment is applicable.

sailingdog 05-24-2007 10:23 AM

The only issue I have with their pultruded FRP sheets, is that it sounds like the glass is all oriented in a single direction. IMHO, it would be better to have the fiberglass fibers oriented in more than a single direction, since it will be far stronger at resisting loads in multiple directions than FRP with all the fibers oriented in the same direction. This is why a good sheet of plywood is often far stronger than a board of fir of equal thickness. Even though plywood is usually made of fir, the fact that the fibers are laid in two directions, perpendicular to each other at every other ply... it gives the plywood far more strength than the wood has naturally.

Mithril 05-29-2007 06:18 PM

Well… I asked a few more questions, most notably from a refrigeration expert at Miller and Miller in Seattle. It turns out that the fiberglass in the FRP sheets is actually laid out in all directions not just one and because of the optimal resin/glass ratio and the high pressure it is in fact very sturdy material. He commented that throwing a gallon can of pineapple juice against it with all your might would not damage it at all. As such he suggested that ¼” was much too thick and in fact he had used as little as 1/16” to construct iceboxes.

The only downside he mentioned about the material was that it is quite heavy.

- Mithril

sailingdog 05-29-2007 09:46 PM


1/16" FRP doesn't sound like it is strong enough to make an icebox. Even if it was an ideal ratio of glass to resin, it is still only a 1/16" of an inch thick. I seriously doubt that 1/16" FRP can take a gallon can of pineapple juice being thrown at it at maximum force.

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