I can’t get quality EPS now so I’m going back to XPS for a bunch of reasons.
I’m familiar with the drawbacks but the main problem now is weight. How much should XPS weigh compared to EPS of the same density?
How much would a 6’x23" blank weigh?
I’m aiming for ~7’ board(s) and less than 3-4kg total weight.
Making the whole thing out of XPS makes the process much, much easier for me due to the very limited access to tools and space I have. I could easily chamber the foam too if that’s feasible?
for same volume, same density = same weight
It might also be worth pointing out that in spite of claims by many to the contrary, boards glassed with Epoxy,Poly, or urethane resins and shaped from EPS, XPS, Polyurethane, Balsa, etc. will have the exact same buoyancy if the weight and volume are the same among the different construction methods. I.E a board of 32 liters that weighs 7 lbs has a certain buoyancy** **regardless of core or shell materials.
“How much would a 6’x23” blank weigh?" - that would depend entirely on overall volume and density + stringer + glue — 6’ X 23" doesn’t give enough info to calculate.
Thanks 
Both good points.
I should have given a more exact example. Let’s say, a 36L blank with 2.5lb density.
I have been using horrible 2lb EPS but the XPS is supposed to be 2.5lb density. What I don’t understand is that the XPS seems a lot heavier. It would be nice to measure and weigh both but they’re both a long way to get to at the moment. Possibly something has been lost in translation and I should be using less dense XPS.
I’m not familiar with imperial measueres, but if you have a given volume, you just multiply it with with the density and you get the weight.
metric density is expressed in kg(or N)/m³ and volume is m³ so the equation ?=kg/m³ x m³. Its the same with lbs and inch³. This means if your blank is 36l and the xps density is 2,5lbs/x³ compared to EPS 2,0lbs/x³ your blank of xps is exactly 25% heavier than the eps (2,5/2=1,25)
With a normal density of eps of about 25g/dm³ you get a blank weight of 900g, with 30g/dm³ you get 1,08kg at given 36 litres. That means no matter what material you are using your blank with 36l is anywhere between 2 and 3lbs. For a total weight of 3-4kg a one lbs difference of the foam blank weight is neglectable. If you want to build as light as possible, the lower the density of the foam should be, but you may need more glass and resin to get the wantetd mechanical properties…
Hi Jago, I do not envy you having to order materials via translation. A sample would be so nice in this situation.
Once your units are in the same system density calcs are easy.
Convert L to CF 36L=1.27CF (1L =0.03531467 CF according to Bing’s online calculator).
1.27CF times 2 PCF=2.54 pounds for the EPS above
1.27CF times 2.5 PCF=3.175 pounds for the XPS above, or 25% more as Surfdude mentioned.
36L volume will displace 36KG ( 79.2 pounds) in fresh water and about 2.5% more or 36.9 KG (81.2 pounds) in salt water.
But there would need to be a small correction for finished board weight to be accurate as a system, yes?
Density = weight divided by volume (weight/volume)
Standard densities for commonly available Dow (blue) and Owens-Corning (pink) XPS insulation foams are 1.6, 1.8 and 2.2 pcf (pounds per cubic foot). Use Jrandy’s calculator. Or 1.0 cubic foot = 28.317 liters.
36 L/28.317 L = 1.27 cubic feet
1.27 cubic feet x foam density (pcf) = foam weight in pounds
Foam weights for a 36-liter volume:
1.6 pcf = 2.03 lb
1.8 pcf = 2.29 lb
2.2 pcf = 2.8 lb
One pound of feathers weighs the same as one pound of lead. The density is different. Feathers are less dense than lead. Because density is different, 1.0 lb of feathers will occupy much more space (volume) than 1.0 lb of lead.
I think XPS comes in different densities. Or maybe they mark it by thickness. I used some blue Dow XPS years ago that was 4" thick and it did seem heavier. I chambered it and it eventually broke in half. Overall, I think XPS makes lighter boards, I think it is because the foam doesn’t absorb as much resin. I like working with XPS, but I think the EPS flex feels a bit livelier.
Plus side of XPS is you don’t have to worry about dings compared to EPS. Down side is it usually costs more.
Thanks.
The XPS is definately more than 25% heavier than the EPS. Perhaps the XPS is actually 3lb and the EPS is actually 1.5lb.
I’ll have to find a way to get to the yard and find a way to measure it. I’m sure there must be something awry here.
Thanks for all your help guys
In general eps (from large bloc) real density is lower than nominal one, so 2lb is in fact around 1.75. Even more for construction grade. Cut a small bloc well cubic, measure it, calculate volume, weight it , divide weight by volume and you find density.