Does cuben fiber cast a shadow and provide UV protection?

It casts a shadow, not sure about UV, feels like a greenhouse in the sun.

so you can still feel the sun's heat under it?

With silnylon tarp I can somewhat escape the heat of the sun as long as there is a breeze.

I've used the MLD Cuben SoloMid for 5 months on a thru-hike, and yes you can feel the heat from the sun under it. A greenhouse is a good description. There is some amount of shading from the sun – but a fair amount leaks through. Same goes for the Hexamid Solo I've used, and for the BPL Stealth Nano that I have now (assume the Hexamid Twin I have in grey would be the same). I look at it as an assumed tradeoff for using a see-thru material that saves weight.

I've had similar experiences. I've found that it's often cooler outside a cuben shelter on hot summer days, particularly if there is any breeze. I put velcro tabs up in my Hexamid to hang a space blanket inside for some shade.

The aluminized/reflective cuben took care of this, but unfortunately they had problems with the material delaminating so no products are on the market anymore (ie. Brook Range Rocket).

I've never found it to be an issue, but it is warmer inside a cuben shelter than a silnylon one. Depending on your locale this could be good or bad. I supposed in the desert SW you'd want to make sure you have 100% shade.

I forget where, but I do think I heard somewhere that it does block all UV. I can't back that up, but an email to cubic tech might shed some light.

One thing I like about cuben fiber is I can see the trees above me. I don't know about the UV protection.

According to the below links, Cuben Fiber (CTF3) does have UV protection.

Mountain Laurel Designs

Shore International – Sail Makers

Quest Outfitters

Oh. I forgot to mention that I'm not using a tent but a tarp so if it's just hovering 5 feet over your head in a 'patio' configuration with some breeze I think the greenhouse effect would be minimized.

Note that this would be a BENEFIT in the winter. It would be warmer inside a cuben shelter.

To create a cool shadow under a cuben (or other) tarp or shelter in warm conditions I drape my quilt or sleeping bag over it. Casts a cool shadow and airs the quilt at the same time.

There are 9oz space blanket all-weather tarps on Amazon for 12 bucks. If you're really worried about the sun, you can use the reflective side out and keep yourself cool. I don't know of a lighter weight alternative tarp.

A little off-topic, but the below post reminded me of something I did recently…

"To create a cool shadow under a cuben (or other) tarp or shelter in warm conditions I drape my quilt or sleeping bag over it. Casts a cool shadow and airs the quilt at the same time."

I did something kinda' like this recently with my Spinnaker tarp: I was on a late summer trip in high temps (low 100s F) that we ultimately had to bail on and hike out to a TH. This being chaparral country, there's no trees to get in the shade of, so while waiting for a ride I pitched my tarp in an A-frame configuration somewhat high off the ground to allow better air circulation and then used an SOL Emergency Blanket (made of metalized polyethylene) to drape over the roof of the tarp (metal side facing the sky) to reflect the heat and create shade.

It was certainly still warm, but it seemed to somewhat help as it was noticeably cooler under the tarp as compared to sitting out in the full sun.

The Emergency Blanket is primarily used as my groundcloth. Weighs ~2.5 oz, costs $4.00 and is the perfect size for one under a tarp. Added bonus: It can double as an… emergency blanket. And now shade structure.

I suppose had it been windy, I would've needed to tie off some cord to the corners of the blanket and secure it to the ground to stay put.

So, "in 2001, a new soft hand Tedlar coating replaced the mylar laminate of previous Cuben Fiber."

And Tedlar is … http://www2.dupont.com/Tedlar_PVF_Film/en_US/

Who would have thunk it? No wonder they sell the Mylar stuff for backpacking.

On my 2009 PCT thru-hike,I once tried to set my cuben fiber tarp up for shade during the heat of the day. It sucked. It dimmed the light some, but not near enough. Now, the tarp did provide support for some of my other gear like my down quilt draped over it which did a better job at blocking the sun. But the wind blew it off after awhile.

White cuben fiber in tropical conditions implies you get out from under the tarp shortly after the sun rises, or you fry.