Running in a shirt often makes me feel overheated, constricted, and weighed down, but when the temperature starts dropping below 50 degrees and the wind picks up, it becomes a necessary evil. Everyone's temperature tolerance is different so I imagine there are people who are comfortably running shirtless in the 30s and people who like to wear a sweater in the 60s; whatever is comfortable for you is clearly the most important aspect of your run wear.
I've got the long sleeve, crew neck t-shirt that I got through Costco and it's my go to for temperatures between 50 and 35 or so. In that range, I don't even have to layer with this shirt to stay warm because it insulates body heat really well. It's a super thin material, surprisingly so for how well it retains heat, so it layers well when it starts getting even colder or if you want a windbreaker or something on top of it.
The shirt fits fairly close to your body, but isn't a compression shirt by any stretch, which I like for the temperature range that I use it in. I have a couple Under Armour Cold Gear shirts that have compression, but until the weather starts getting a bit more rough, I prefer the looser fit of the 32 Degrees Heat shirt. It's also markedly less expensive than the Under Armour alternative.
The lightweight material wears really well, far better than regular cotton t-shirts. It isn't abrasive to your skin so it's great for longer runs too. If you've never gotten chafed under your arms or on your nipples - odd as it may sound - it may not strike you as a big deal, but once you have it happen halfway through a run, you'll think carefully about what you wear the next time out.
I'm really glad I found this shirt as it gives me a great, comfortable option for midrange temperatures. If you're interested in the shirt and want to try it out, you can find it here on Amazon - they have women's styles as well - with Prime shipping, if you're into that. If you shop through my Amazon links, your price and shopping will be the same as ever, but I'll get a little referral kickback!
If you try this out, I'd definitely be interested in your thoughts on it or if you have a recommendation of your own, let me know! Shoot me an email with any feedback or post a comment below.