R-values of deck coring?

[Zach]

Hi guys,

I'll be re-coring the deck of my Triton this coming summer, and in hopes of killing two birds with one stone, I'm wondering what cores offer the highest insulating value!

Where would one find a comparison between the balsa (mine is composting... ) and the various foam products?

I've been reading a little about home built boats, skiffs mainly, that use dow board... styrene foam instead of plywood. The price is right, anyone fooled around with the idea of using it? In the "Fiberglass Boat Repair Manual" Allan H. Vaitses talks about coring, stating that originally boat builders used polystyrene... Just not so sure that under compression it'd be up to the task. The dow board dents easily, but maybe with an extra layer of glass or two it'd be up to the task?

[knottybuoyz]

Polystyrene foam won't perform as required for a core material. It has virtually no structural strength (I'll avoid all technical lingo). Resins will bond to the foam but the foam itself will fail under stresses especially sheer.



This was from a little experiment with it as a core for a non-structural part and it failed from just the vibration of the jig-saw blade cutting it.

Although this article is a little out of date it'll provide insight into the physical requirements of structural foams.

Foam Core Materials in the Marine Industry

[Zach]

Thanks! Excellent picture and article.

I'll throw that idea out the window.

It sounds like the core is only a 1/2 inch thick, so it may be an exercise in futility to get any insulating value from it. Nevertheless, every little bit helps.

[Zach]

10lb per square foot end grain balsa has an R-value of 1.6 per inch.

Corecell has an R-value of 4.16 per inch.

... and thats as far as I've made it. Got an email in to a few other manufacturers asking about their ratings. Soon there ought to be a definitive answer to this question.

Though I just learned that my core is 3/8ths thick and not 1/2 inch. Balsa is sounding pretty cheap, but investigation will continue!

[jdoe71]

Originally Posted by Zach

Balsa is sounding pretty cheap, but investigation will continue!

Balsa rots, as does plywood. If I ever have to open my deck to fix core I won't be putting wood back in. Please don't tell me that corecell or other synthetics have issues too.....

[Whimsical]

Originally Posted by jdoe71

Please don't tell me that corecell or other synthetics have issues too.....

Should I or shouldn't I
OK I won't then
Mike

[OldYachtie]

They say some foams soften in the tropical sun. Never had any problems with balsa despite years of using a boat with balsa deck for heavy duty cruising. When you mount something in balsa, you must protect the holes if you have balsa in your decks. Oversize hoes with epoxy putty are recommended. Use an EL shaped bit and take out a bit more balsa than skins, if you can. My next boat will have balsa, but I will keep water away from it.

[Zach]

Update: Nidacore has an R-value of 4. I'm emailing back to ask if that is per inch, but at this point I'd assume it is.

[BigCat]

http://www.fiberglasswarehouse.com/tds/ProBalsa.pdf says that Probalsa has an r value of 2.2 per inch, in the commonest and cheapest density, which is not quite 10 pounds per cubic foot. It is stronger and denser than the foam usually used in boat building, which is commonly 6 pounds per cubic foot. My impression is that if you use foam as strong as standard balsa, it has similar weight and r values.

[GordMay]

Originally Posted by Zach

Update: Nidacore has an R-value of 4. I'm emailing back to ask if that is per inch, but at this point I'd assume it is.

Because R-values are not exactly linear, the US FTC R-value Rule generally prohibits calculating R-value per inch of thickness (although we all do it, in practice).

Accordingly, DIAB will likely specify a different R-value for each density &each thickness of Nida-Core (Divinycell, Corecell, ProBalsa, etc).

Ie: LD7 PB HW Light Weight ProBalsa core is specified (see BigCat's link):
12 mm / 0.5 in = R 1.4
25 mm / 1.0 in = R 2.9
51 mm / 2.0 in = R-5.7
Were the R-Value exactly linear, the 51 mm should be rated R - 5.95, (not 5.7) or R-2.975 per Inch.
(51 ÷ 12) x 1.4 = 5.95

[BigCat]

Yes, Probalsa standard weight's R value for an inch is actually 2.3. This, of course, isn't much. In the US, R 19 in walls and R 30 in ceilings is common, and that doesn't include any R value obtained from the gypsum board, sheathing, and siding. My own house has R 40 in the walls and R 46 in the ceiling, and I don't even live in an especially hot summer/ cold winter area. Code here now requires R 21 walls, R 30 in vaulted ceilings, and R 38 in attics, since it has been made more stringent for energy conservation.