This bushcraft course that I attended a few years back was the first time I came across the Lifeventure thermal mug and I thought 'What a shapely little number' (I'm talking about the mug here obviously). The good thing about this mug is that it doesn't have a handle of any kind so it can be slipped neatly into a carry pack or a side pocket on outdoor trousers. The bad thing is that it doesn't have a handle of any kind so it can't be hung up to store it when not needed so delete as applicable.
To open one of these mugs you turn the disc like top first and then unscrew the wider (lower) part and reverse this when tightening the lid. This two part lid is actually conceals a small chamber which allows the pressure to balance and if you don't seal it well you may get the tell-tale 'Piff' as the lid pops off.
Now this chamber was the subject of a flashback to a football match (Nottm Forest -v- Middlesboro', back in the day of Pearce and Juninho). It was a freezy cold Saturday but I had a new thermos although I didn't fancy the chances of the in laws having a hot half time brew with their ancient flask, how wrong I was as my tea was luke warm and the knackered looking tartan number delivered a steaming hot beverage. I worked out that the little storage compartment in the lid was the weak point and a copious amount of little polystyrene balls in it solved the issue.
I viewed this chamber with the same contempt and decided to fashion a loose fitting polystyrene plug to insert into it which insulated and allowed pressure balancing at the same time. When I take a brew to Cubs on a Tuesday evening I have to leave the lid off for a few minutes or else it's too hot to drink.
I have two of these mugs and in a light bulb moment I decided to see a) Just how good they are and b) How much difference (if any) the polystyrene makes.
So the two cups are prepared with the insulated chamber being on the left, and the standard uninsulated cup on the right.
I boiled the kettle, filled the first to around a centimetre from the top and popped the lid on. I then re-boiled the kettle and filled the second in the same way. I decided that 90 minutes would be a good time to probe the liquids because not only would that give the two cups time to lose heat but also if out and about I've drunk tea from one after this period of time. I've even made a brew to take to work, forgotten to take it and returned to find it still lukewarm. Now you may notice the black and silver colour thing going on with the lid, I only ever rinse the mug(s) and kitchen towel dry to keep the memory stains of a thousand brews alive. My wife was only helping by putting them in the dishwasher...
Whilst waiting for the time to elapse I made a mug of tea and probed it before I added milk and it was 88 degrees, so to find it was 79 degrees in the insulated mug was pleasing but not surprising.
The second uninsulated mug still registered a healthy 75 degrees so it looked like the polystyrene did aid the temperture but equally the difference isn't a deal breaker and certainly not tepid tea at a football match vibe.