His eyes on this untypical winter day are as bright as the sky, his head shaved to a No 1 rather than the full-on egghead he has favoured on occasion. Anywhere between 25 and 35, maybe. Not bad, given he's spent much of the past two decades entombed in the studio and tour bus, or sweating under spotlights as he alternately wreaks diabolical levels of magmic fuzz and delicate spun-sugar melodies from his trusty Telecaster. "Thirty-seven," he says, "and a half, if you're being picky."
Refuting the theory that life begins at 40, the guitar player and de facto frontman of Mogwai - not forgetting Scottish independence supporter, anti-monarchist, Celtic fan, sci-fi geek and vegetarian - could be forgiven for decelerating after multiple circumnavigations of the globe in support of seven studio albums and a panoply of live recordings, soundtracks (the most recent being Canal+/Channel 4 drama The Returned, or Les Revenants), EPs, remix albums, singles and compilations.
Nothing could be further from the truth, however. In a matter of weeks the Glasgow quintet - four of whom still live in or near the city while multi-instrumentalist Barry Burns calls Berlin home - head out on tour, starting in Britain before transferring to Europe, then Asia, back to Europe, then the US and Canada. Before all that, though, comes the release of Mogwai's eighth studio full-length, Rave Tapes, which coincides with the group playing Celtic Connections.
A sparser, more glacial record than its predecessors, Rave Tapes marks a departure for the band, large tranches of it being embellished by an almost Nordic serenity, neatness and symmetry. Is there an overriding reason for this?
"There are a couple of factors," says Braithwaite as he awaits lunch - a mushroom burger, the ordering of which is preceded by an outlandish digression on the rights of funghi - in Glasgow's Mono cafe-bar. "Equipment has a lot to do with it. Barry got a modular synth and used it to create a lot of the music. Also, there is an element of minimalism I don't think we've embraced since [second album] Come On Die Young, where rather than chucking everything on every song there are bits where there are only one or two people playing, so there's a lot more space."
This wasn't wholly planned, however. The group elected to narrow an already limited window of opportunity for recording the album last summer by playing a short run of shows where they performed their 2006 soundtrack to Douglas Gordon's Zidane: A 21st Century Portrait in real time along to the film.
"We were pressed to get Rave Tapes ready to record quite quickly because we did those Zidane gigs," says Braithwaite, toying with a glass of juice. "We'd almost bitten off more than we could chew, but in a way it worked to our advantage because we didn't have time to swathe everything in extra music. I think the minimalism works with those songs. The melodies and sounds are more apparent because there aren't more of them." He pauses to think. "Scandinavian efficiency born of Caledonian bad planning," he says before collapsing into laughter.
Now there's a topic Braithwaite hasn't been shy about recently. Scotland. Independence. Nationhood. You can trace his enthusiasm for the Yes campaign back to his group's infancy - the Saltire and Lion Rampant have long been on-stage ornamentation at Mogwai shows. And mindful of the upcoming Celtic Connections concert - an alliance that would have seemed preposterous even 10 years ago - it seems apposite to probe whether there's anything intrinsically Scottish about Mogwai.
"I'd say we're pretty international, musically," says Braithwaite. Which, when you view Mogwai in the context of this country's post-second world war musical history, seems fairly Scottish itself.
"I think it is a Scottish thing," agrees Braithwaite. "As people in our characters and in our outlook we're brutally Scottish but musically we're very international. We grew up with Sonic Youth and Nirvana but there was always a connection between these bands and The Vaselines, The Pastels and Teenage Fanclub. There has always been a web connecting the American underground music culture with the Glasgow underground music culture."
What does he believe piqued Mogwai's global curiosity? "It's maybe because, largely speaking, the London media have had sporadic bursts where they get excited about Glasgow but in general they're very insular. Maybe that's been apparent up here and we've looked elsewhere because London hasn't been such a big part of our experience. So we've looked around the world - to Tokyo, to Seattle, New York. Even though we've stayed put, there's been an migration of sound."
Braithwaite is less convinced by the suggestion that his group's youthful tendency to provoke controversy in the music press might be attributable, in part at least, to their nationality. In 1999, when Mogwai unveiled a T-shirt bearing the slogan "Blur: are s***e", Braithwaite told NME: "We decided to proclaim our dislike of one of the weakest bands on the planet by putting out these shirts … It's like a dictionary definition. It's factual and if there are any legal problems I'll go to court as someone who has studied music so I can prove they are s***e."
Now, he says, it was merely youthful mischief for which he doesn't apologise. "You get some pretty gobby English folk as well. We were young and everyone was giving us beer and putting microphones in front of us. How could it possibly go wrong?
"Even in my relative dotage I have no regrets about some of the daft things I've said. If a band came out with some of that crap now I'd think it was absolutely wonderful. Although I'd probably cringe thinking about some of the awkward situations they'd be bringing upon themselves that I've experienced."
It was, then, mischief bolstered by deep-rooted artistic ambition. "When we started, what we were doing - certainly in the mainstream - was seen as something not to be taken too seriously," Braithwaite recalls, "whereas other than saying daft things in interviews we couldn't have taken ourselves more seriously. We were practising every hour of the day, trying to get better at what we did, beating ourselves up if we did something that wasn't as good as it could have been or should be, so it did piss us off if people didn't take it as seriously as we did."
Has that changed? "Not really. There's a level of accomplishment that wasn't there, certainly before Barry joined. Barry's by miles the most musical member of the band and he wasn't there at the start, but the general 'all get together and try to make something we feel is special' has not changed at all."
Braithwaite seems to be taking his impending schedule chaos well. It will help that his private life is on an even keel. In the summer his girlfriend Elizabeth, an artist and musician, will leave her hometown of Hexham in Northumberland to join Braithwaite and his dog Rambo in Scotland. He is clearly relishing the prospect.
Around this time, Mogwai - comprising John Cummings, Dominic Aitchison and Martin Bulloch, as well as Braithwaite and Burns - will embark on the festival circuit before writing music for series two of The Returned ahead of more touring in the autumn.
"We knew the vibe," Braithwaite says of composing the music for the first series of the otherworldly drama. "We'd seen pictures and it was all very vague. It was a vague concept - it's French; vagueness everywhere - but I've rarely been as happy with something I've been involved in as when I saw it.
"I didn't think I'd be able to get absorbed by it because I thought I'd remember the songs, and it was such a good story and so well acted I forgot it was even our music. That takes quite a lot. Even when I hear one of our records I can pretty much - unless it was a long time ago - remember playing it, or sitting in the room when it was getting mixed." The next step for series two, he says, is to "start demo-ing some creepy wee songs and see how it goes".
Consider all of the above, plus the fact that Chemikal Underground will release a remastered version of Come On Die Young before the year is out, and you might think Mogwai are in danger of entering their 20th year in a state of exhaustion. Maybe, but for now there is much for them to revel in, starting with a double debut - both at Celtic Connections and in the splendour of Glasgow Royal Concert Hall.
"It seemed too good an opportunity to pass by," he says, "We're releasing a record round about this really big, really great event in our hometown celebrating, among other things, Celtic music."
Supporting them at the show will be RM Hubbert, whose dog D Bone is Rambo's brother, and whose acoustic guitar-playing Braithwaite views as especially well suited to the venue.
"Playing this prestigious hall is an opportunity to have someone whose music would sound great in it, who thus far hasn't had the opportunity to play this kind of hall. So it makes sense, rather than getting a band we love who are noisy and would maybe suit the Barrowland better than the Concert Hall."
Mushroom burger blitzed and his thirst slaked, Braithwaite zips up his parka and shakes my hand before darting into neighbouring record shop Monorail to hunt down sonic manna for his various DJ gigs. Thirty-seven (and a half) going on 17, I reckon.
Mogwai and RM Hubbert play Glasgow Royal Concert Hall on January 28 at 7.30pm