Refrigerator box, new insulation question

[skipmac]

Have pulled out the old box, stripped off the 1-2" of sprayed on foam and am ready to install new insulation, then re-install the old box and new Frigoboat system.

I have enough room to add 4" of new insulation and still keep the old box which will save me a LOT of extra work but have one question.

The original box is fiberglass and the back side is unfinished, kind of rough and also has some lumps and low spots where the builder used extra layers of glass to reinforce the corners, bottom, etc. So attaching the new sheet insulation will probably leave a few small spots where there will be a gap between the outside of the box and the first layer of insulation.

I know enough to understand that there should not be an air gap between the cold box and insulation to prevent condensation and moisture accumulation that over time can reduce the R value of the insulation. BUT, how critical is this? If due to the uneven surface of the box I have a few small areas, say 2-3" diameter where there is an air gap of 1/8-1/4" but sealed tight all around it, will that be a major problem?

Also the corners and edges of the box are rounded and unless I spend a good bit of time shaping a strip of insulation to square them off I will also have a long, thin gap at every edge. Again, how bad is this? How much long term benefit if any, would I gain by using a filler or resin to make the whole box surface totally flat and square so I will have no voids at all between the insulation and the outside of the refer box?

One other question. Air gap between the insulated box and the side of the hull or not?

[Celestialsailor]

I know this does not exactly answer your question but I am doing mine at the moment also. I made a mold and produced a new box, then wrapped it in Aerogel (1.1") = R11.5. I set it in then used 1" Styrofoam and layered that into remaining spaces. Lastly (and where I'm at now) I'm using pour foam to fill the voids. Some of them a 2" and some less than 1/4". I am also going to use spray foam from a can for difficult areas. Theoretically, I should not have many air space. I should end up with insulation thickness's of 3" to 4"s.
To address your air question, I guess we all just do the best we can. Surely the builders do the same. Hope this helps.

[skipmac]

Have not seen the Aerogel in person. What is it like? From reading the web site I can't tell if it is a mat like consistency, like a dense form of pink fiberglass insulation or more solid but flexible like the Dow blue foam insulation.

[senormechanico]

I highly recommend Aerogel. Use Aluminum tape (available at hardware stores) not duct tape to seal the seams.
Fold back any aluminum facing at the seams if using multiple layers to prevent thermal bridges.

[sea1ljs]

There are a gazillion examples here and elsewhere on the web showing how an owner rebuilt the refer box, here's my 2 cents worth. Having done this necessary evil job last July I followed the advice of many a boat owner before me. Key points are install a ONE PIECE vapor barrier on the outside of the box, floor-sides-top. Next layer - insulation panels. Overlap the insulation panels, no butt joints. In my case I too surrounded the box with 4 inch walls using the higher "R" rated foil backed panels but I increased the floor insulation to 6 inches. This also raised the floor up to ease the dumpster diving effect. I used pink board against the hull because of the curve which required having to cut the pink board into multiple pieces. I installed two layers of double sided foil "bubble" insulation over the one piece vapor barrier then the pink board. Of course there were voids in that hull section to which spray foam insulation filled these voids. Now here's the difference in your project vs. mine. I wanted to save the counter top so I removed the side wall and carved everything out including the existing fiberglass box. I rebuilt the box using prefabed FRP panels from Home Depot. These are the same type of panels used to make a shower enclosure. It was very easy to cut, shape and install. Tool Tip - I bought a Dremel plunge cutter ($100.) and used the dry wall cutting blade to cut the insulation and FRP panels. The blade would not cut entirely through the 2 inch thick insulation panels so I would make the first cut along the line made with a Sharpie pen. When the first cut was made I would take an ice pick and poke holes every few inches along the cut, now flip the panel over and connect the dots for the final cut. So unless you really want to save the existing box the prefab FRP panels are really easy to work with and all you have to do is mix up some West System and micro bubbles to a thick paste and fillet the seams. You now have the ability to customize the interior of the box with a new shelf configuration as needed. I made a divider to create a freezer side, no spill over just a solid wall and don't forget a drain hole for defrosting/cleaning. I also installed an interior light on the refer side. I then reinstalled my Grunert holding plate system. With the Grunert system the compressor runs 1 hour in the morning and 1 hour in the evening. The refer side is at a constant 38 degrees F and the freezer ranges between 0 and 10 below F. I have no doubt when I get into warmer climates with the water cooled system I'll have to add some run time to keep the rather large freezer in that temp range. As far as the air gap between the hull and box I think the key there is the vapor barrier. You need to keep the cold separated from the moisture in the air. Good luck with the project, it'll be worth the time and effort.

[barnakiel]

Hey guys/ladies:

If we double the insulation thickness (keeping the chest's internal volume intact) then how much less energy are we going to use?

Is it anything like twice the insulation thickness = half the energy?

b.

[Cotemar]

skipmac,

If you go with Aerogel .4 thick X 8 layers = R33 at 3.2 inches thick.
Aerogel will conform very well to the rough surface of your box.
Once its all rapped then do a final rap of aluminum foil and foil tape any seams.

You can buy the Aerogel on eBay by the foot.

[Cotemar]

Originally Posted by barnakiel

Hey guys/ladies:

If we double the insulation thickness (keeping the chest's internal volume intact) then how much less energy are we going to use?

Is it anything like twice the insulation thickness = half the energy?

b.

Most believe that R30+ insulation on a boat fridge/freezer is where you want to be. Anything over R30 is probably overkill.

[barnakiel]

Originally Posted by Cotemar

Most believe that R30+ insulation on a boat fridge/freezer is where you want to be. Anything over R30 is probably overkill.

OK but we have limited energy on passages. Can I save considerable amounts of energy by doubling the insulation or am I just exposing my ignorance of thermodynamics?

We never used the fridge before but since it is here I though to take some butter on our next passage. I believe I can divert about 20 Amps per day for the fridge.

It is a 30 liters volume, 4 inches foam insulated unit, I have space to double the insulation.

I understand I may be daydreaming.

b.

[Celestialsailor]

Its a curve I believe. R20 is the minimum and like Cotemar stated, R30 is optimal. I believe it was Kollman that said more than R30 has little effect.
BARNAKIEL...If you're up to R30 now, maybe another battery and solar panel might help you out.

[senormechanico]

For a given insulation, you have to double the existing thickness to halve the heat transfer. 2 is twice as good as 1, then 4 then 8 etc.
When I asked about how much I needed to use on my box, the sales person told me I'd be committing overkill with more than 3 layers of 4mm Cryogel.
I used 3 layers everywhere except the door.
I could only fit 2 layers in there because of space.
I installed 4 around the freezer compartment because I could.
Batteries are always full from solar.

[barnakiel]

We already have plenty of solar energy. Actually we have doubled the amt of solar juice while also made some savings on cabin and nav lights and electronics.

I think I will go as it is then and see what happens. No problem to shut the fridge on and off.

Just I would hate to let the butter go to waste and be left with fresh bread but no butter to go with it.

b.

[barnakiel]

Originally Posted by senormechanico

For a given insulation, you have to double the existing thickness to halve the heat transfer.(...)

!

OK. Then I will double (now 4'', will make it 8'' everywhere except the lid - but I can place an extra layer of foam over the lid/top).

I would be very happy to use the fridge next time we go across as the canned butter does not seem to taste equally as good as the fresh stuff.

THX
barnie

[hellosailor]

Skipmac-
If the air gap is sealed space, it won't matter much. If you can't seal that space up, consider using some spray foam insulation, quickly spreading it with a notched trowel, and then putting on the first layer of real insulation, with some tape bands, etc. to hold it in place while that foam cures. The foam also makes an excellent adhesive, so it will fill and bond and get rid of that air space.
The trick is moderation, use "just enough", practice on something else first, and don't let the real insulation get too compressed during the process.
You can also use real (mylar or metal) duct tape to seal around the panels when the job is done, or again, foam spray, so it all is really sealed tight.