You can't get it. I've looked and looked. The closest you'll find is clear vinyl, but it doesn't last very long and it is very expensive (it is used for outdoor eating areas and the like. You can get 0.25mm thick, 1.2m wide flat polycarbonate in rolls from dupont which are beautifully clear, but it isn't a 'film', more like cardboard. (i.e. it doesn't bend around 90 degree corners)

I used the PC to replace windows in my garage.

The hardware store polyethylene (what you're talking about) is a waste of money for a greenhouse, it lasts less than 6 months in the sun.

Not realy concerned with how long it last. I only need it for my vegetable seedlings until their ready to plant. Will my seedlings get enough light through the hardware store poly?

should be ok. Here we also can get plastic film drop sheets, 4mx4m for $1.5, much clearer. I've used that in the past - it usually lasts about a year outside.

Yes you can use the plastic sheathing from the home stores. It does NOT block any light. It wont last long but its cheap. I use it on my 14 x 16 temp GH.

They all block light. Even the most clear that you have ever seen blocks about 4% of the natural light. Some frequencies more than others. Is it enough to care about? Depends on what you want to do. I don't have the numbers in fron of me but i believe the typical greenhouse plastic blocks something like 8 to 14 percent of the light.
Does it matter? Probably not. Might even be beneficial.

If you really want to know deatils, web searches using the plastic name and words like "Technical specifications" and "transmission" and "Optical" usually can get a page with the specs your looking for.

Poppa

Just use what ever cheap clear plastic you can get because it is only for protecting your seedlings until the danger of frost is gone and they are not picky at all about the small percentage of light that is blocked. If you are planning on using year round or for late winter starts and prolonging fall, then I would go to your local nursery and ask if you can buy some of their scrap greenhouse film(plastic). It is UV protected and will last for four years. For a small GH like yours they may even just give it to you.

Just remember that it will get too hot in there when in full sun and burn or kill the plants once they get growing. Have a way to vent it like proping open the door and or drapping a thin white sheet over it as a tempory shade cloth.

Good luck and have fun.

Why not go to a greenhouse supplier for the real thing. I bought another 40x120' roll of the 6 mil greenhouse film this spring and I think the price per sq foot is very similar to the Lowes construction plastic. The drawback for many of you will be trucking costs. So there is no point in referring a supplier in my area but check with commercial outfits in your area. The greenhouse film is usually rated for 3-4 years but I have a double layer on my first greenhouse since 1994 (12 yrs). The construction clear plastic isn't usually UV resistant as others have indicated.

If you want to test the newer technology in greenhouse films for a few extra $$ I'd be interested in responses. Newer films have light dispersing and non-drip properties that some of you may feel are worth the extra $$.

Thanks for the helpful answers. Since I only need it for a few weeks I think i'll use the home depot stuff I already have. Thanks again
Keith

If the plastic didn't block any light you wouldn't be able to see it!

I stand corrected. The plastic does block some light!

The Home Depot here has twin-wall polycarbonate (Lexan) in different size sheets. One side is UV protected.