Hi everyone, new member here looking for some advice/help. I live in Kamloops, BC in zone 5 (bordering 4) and am looking to extend my growing season for backyard veggies (have a 3yr old and twins on the way so fresh healthy food is a priority).

I have a raised bed about 80' long (concrete garden blocks on the front side and heavy fir on the back side) which I am looking to add some cold frame covers too (about 20 lineal feet worth). The current plan is to build the back (north side) up 10-14" with wood and then have a hinged cover sloping down to the front (south) side.

1- What type of transparent material is best ? I have 6mil poly film on hand that I was going to use for a double layer (each side of a 1.5" frame). I could get Solexx or double walled polycarbonate panels if there is a significant benefit.

2- how much could I expect this to extend my growing season (mostly for cold hearty kale, chard, beans, beets, carrots) and possibly

to help with hardening tomatoes and cucumbers.

3 - Is there a benefit in adding some concrete paving stones (painted black) as a thermal sink?

I don't have any experience with this type of 'assisted' gardening so looking for guidance. If I am only going to get an extra week or two it probably isn't worth the hassle. Please poke holes, give advice or point me to reputable references / other garden forums if available.

Thanks.

Pic to give reference of the garden