This photo shows the REI Co-op Silk Long Underwear Crew Shirt.

REI Co-op Silk Long Underwear Crew Shirt Review

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The REI Co-op Silk Long Underwear Crew Shirt is a lightweight, breathable silk base layer. It’s super soft and surprisingly warm.

To get us a closer look, REI sent Man Makes Fire a review unit. This is what we learned:

REI Co-op Silk Long Underwear Crew Shirt Review

I’m generally a fan of natural fibers for my shirts and base layers because most synthetics have a tendency to cause me to sweat. Weird, I know. Either way, if you also avoid synthetic base layers, you have only a few commonly-available choices: merino wool, silk, and cotton.

This photo shows the men's REI Co-op Silk Long Underwear Crew Shirt.
The REI Co-op Silk Long Underwear Crew Shirt delivers ultralight warmth.

Cotton, of course, is a terrible insulator when it gets wet, so it’s not good for layering in the backcountry.

Merino wool is relatively soft (but can be somewhat scratchy to some people). It’s a fantastic all-around insulator, still works when wet, and it almost never gets stinky after repeated wearings.

But what about silk?

Silk is super soft, which is hard to beat. Plus, it’s surprisingly warm. What I wanted to know, though, is this: Could silk become my new go-to base layer?

Silk as a Base Layer

For starters, it’s hard to understate how thin the REI Co-op Silk Long Underwear Crew Shirt is. It’s super thin. In addition, it’s super shear. You can nearly see through it. This means that if you wear it in the house or peel off a midlayer at the bar in the ski lodge, everyone is going to see your nipples and maybe even your chest hair. Not cool.

This photo shows how thin the REI Co-op Silk Long Underwear Crew Shirt is.
Warning: The REI Co-op Silk Long Underwear Crew Shirt is very sheer.

Even in the privacy of your own home, there are some images wives and girlfriends should never have burned into their retinas — and a dude wearing a sheer silk base layer top is one of those images. Just saying.

Consequently, the REI Co-op Silk Long Underwear Crew Shirt is best worn as a base layer always underneath another shirt. For example, you’re not going to want to wear this base layer on its own (like a shirt) underneath your shell or even under a lightweight down jacket like you can with a merino wool base layer.

As a base layer, the REI Co-op Silk Long Underwear Crew Shirt is awesome. It’s soft and so lightweight you barely know it’s there . . . and yet, it provides great warmth.

Warmth, Breathability and Drying Out

Warmth is subjective to many variables, but from what I can tell so far, the REI Co-op Silk Long Underwear Crew Shirt is nearly as warm as lightweight wool base layers . . . as long as you wear a shirt over the top. Because the knit is so thin, I think the silk really shines when something over it helps keep warm air trapped inside of the tiny gaps in fabric’s weave.

It’s hard to say, but as a base layer, it’s far warmer than I would expect based on the weight and thinness of material.

As for breathability, because it’s so sheer and thin, it’s fantastically breathable. I’ve worn it over multiple days without washing it, and like wool, it doesn’t get funky — at least, it hasn’t yet.

If you are working hard and your silk layer gets wet, it dries fast.

Fit and Finish

Out of the gate, the REI Co-op Silk Long Underwear Crew Shirt has excellent fit and finish. It fits perfectly to the size and has a bit of natural stretch to make it sort of form fitting. So far so good.

This photo shows the REI Co-op Silk Long Underwear Bottoms.
REI also offers silk bottoms.

As you wear it, though, it starts to stretch out. The cuffs and sleeves by your forearms start to get loose unless they are protected by a long-sleeve midlayer. This doesn’t seem to affect performance much, but still, tighter sleeves and cuffs get looser over time.

Once you wash it, it springs back to a tighter fit. Nice. On the downside, I think it also shrinks just a bit. Not bad, but if you have longer arms, definitely choose the tall version to give you longer sleeves to start with. And if you’re on a borderline size, I’d size up just to give yourself some protection from likely shrinkage.

One more note, and you can see this in the feature photo, if you machine wash the silk shirt, you’ll likely get some pilling. It’s more cosmetic than anything else — the shirt is still soft to my skin. You might be able to avoid this with hand washing in the sink, which, given the ultralight weight of the shirt and fast drying, is what I would recommended going forward simply because it’s so easy and would mitigate wear and tear.

Shop all men’s base layer tops at REI and get FREE shipping on most orders over $50!

The Verdict: Excellent ‘Ultralight’ Base Layer

If you’re looking for an ultra lightweight base layer, try silk. You’ll be shocked at how thin the material is but also how warm it is. The overall warmth performance is great. It’s not rugged, but it’s so light it’s perfect for situations where you want a bit of extra insulation but don’t any bulk. You could, for instance, wear the grey ‘Quiet Shade’ version underneath a white dress shirt without anyone even noticing. As a backpacking layer, wow, it’s totally worth the weight. So, if you know what you want and what you’ll be getting with the REI Co-op Silk Long Underwear Crew Shirt — highly recommended. If you’re not sure, stick with lightweight merino wool base layers.

Get the Gear:

REI Co-op Silk Long Underwear Crew Shirt
Surprisingly warm for its weight and thickness
Dries fast and doesn't get funky like many synthetics
It's soft
You can nearly see through the sheer fabric
You can machine wash it, but we recommend hand-washing it for longevity and best results
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