
Exped
MegaMat Series
Car camping comfort champ, but some side sleepers find it thin.

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I carry this type of pad for added cushion and as a backup for a possible failure of my main Exped Synmat 7. I would not use something like this alone because I would feel the hard ground and not sleep. Add the few pounds and get an inflatable and sleep like a baby.
I’m not your size, but I do have a really hard time sleeping on the really small pads that people seem to love. Exped sleeping pads are the only thing I’ve found that I can sleep well on, and I happily carry them knowing there are lighter pads out there. For reference, I’m an athletic 5’9” 190 and my Exped SynMat is a Regular length, but “Wide” version. It takes up most of the floor space in my Nemo Dragonfly 2p.
I have an Exped as well as the bestway one often mentioned here and the zen bivvy one, and the exped is the comfiest and warmest hands down. However I can’t remember which one it is exactly (it’s an old model, now deprecated, but 7 cm deep). Also has by far the best inflation system.
If you get a Big Agnes it will squeak against the tent floor all night when you move. I use Exped. Not completely noiseless, but built differently than the Neo pads and not a Big Agnes. I have a discontinued Hyperlite, will replace it with an Ultra when it develops too many leaks just as I did the Synmat. I wish they still made the Hyperlite.
I’ve been using a similar set up for years now. Robens z mat, Exped Synmat and a Criterion sleeping bag. Haven’t had an uncomfortably cold night yet and I tend to camp in autumn and winter.
What time of year are you going? I've done a lot of camping in Iceland in a range of seasons over the course of about 7 trips, if you're going in summer you don't need anything particularly large, just think UK spring or Autumn really. I took a Synmat in May/June and it was fine. All I would say is that it's very windy and there are very few trees or vegetation so your tent needs to be able to stand up to the wind exceptionally well. I stayed at a campsite on the south cost where someone's tent literally blew away with about half their stuff in. It ended up in the sea. So get good pegs. The ground isn't very soily! Definitely take a ground sheet protector there's some spikey rocks.

Exped
MegaMat Series
Car camping comfort champ, but some side sleepers find it thin.

Exped
Ultra Series
Lightweight side-sleeper comfort, but a bit pricey and bulky.

NEMO
Tensor Series
Ultralight side-sleeper comfort, but slippery and prone to leaks.

Big Agnes
Rapide SL Insulated Sleeping Pad
Comfortable with side rails, but heavy and warmth claims disputed.

Therm-a-Rest
NeoLoft™ Sleeping Pad
Very comfortable, good service, but heavy and bulky for backpacking.

Ranked #1
Exped - MegaMat Series

Ranked #1
Exped - MegaMat Series

Ranked #1
Exped - MegaMat Series

Ranked #1
Exped - MegaMat Series

Ranked #1
Therm-a-Rest - NeoAir Xtherm Series