I'm thinking of using an old silk shirt as a base layer for strenuous rides in temps 30-45F.

What sort of silk base layer do you wear, is it an ultra thin undershirt, long sleeve or short?
How well does a silk base-layer 'play' with a mid-layer wool or fleece?

You're probably talking about something a little thicker perhaps, but I used to love silk longjohns, they are comfy and warm. The problem is that real silk is rather fragile. My silks always end up with huge rips and holes in them after only one or two seasons.

What I use now is the 'silkweight' bergelene longjohns from EMS, or if I want a thicker base layer, any shirt made from one of those new wicking fabrics. They're just as comfortable and warm as silk, but are more durable and keep you dry. Long vs. short sleeve is basically according to temp.

The only Silk I use... is a Silk Sock Liner

I use Under Armour Cold Gear for everything else as a Base Layer

I'll second the Under Armour Cold Gear. But only when it is below -10C for the top and -15C for the bottom. Otherwise I use one layer of Sporthill winter gear with a somewhat breathable wind stopper style high vis jacket.

You're probably talking about something a little thicker perhaps, but I used to love silk longjohns, they are comfy and warm. The problem is that real silk is rather fragile. My silks always end up with huge rips and holes in them after only one or two seasons.

What I use now is the 'silkweight' bergelene longjohns from EMS, or if I want a thicker base layer, any shirt made from one of those new wicking fabrics. They're just as comfortable and warm as silk, but are more durable and keep you dry. Long vs. short sleeve is basically according to temp.

I second what threephi says about silk. It is wonderful, functionally speaking. It is warm and it breathes. It seems near perfect. However, it is fragile. You get MAYBE one season out of it and it is full of holes.

I had a silk knit sweater, rathe bulky. It was beautiful and very comfortable in a wide temp range. After 3-4 INDOOR wearings, it already looked shabby. Woven silk shirts are nice but they stain easily if they get sweaty or even wet, in my experience. Rayon shirts look just as slick but are a little more durable. i never wore one during strenuous exercise, so I don't know about that.

I have a silk long sleeve undershirt and long johns I got cheap from Sierra Trading Post. I love it. It's super-thin but as warm as any of my much heavier polypro undershirts. Also makes for great PJs in the winter time.

You can wash them in the machine on the cold/delicate cycle. Use a mesh lingerie bag if you have one.

I have an old cashmere sweater that I wear as a layer on really cold days. I also have silk socks that I wear under wool socks. Works great.