The Snowline Bivvy Bag

An insulated bivvy bag / sleeping bag cover to protect and boost the warmth of a down sleeping bag, increasing its warmth by a further 7°C while keeping off the muck and damp of the bivouac.
Typical Operating Temperature
Guideline minimum temperature. Sleeping Bag TOT: you should get a night’s sleep, under normal circumstances. Clothing TOT: you should feel warm enough, assuming low/moderate activity. Sleep System TOT: relates to full system. For more details, see our Technical Briefing.
15°C / 59°F
For clothing, weight is for size: M. For sleeping bags, weight changes with selected length/width. N.B. there may be some variation in the weights of finished products.
560g / 20oz
The Snowline Bivvy Bag

"My favourite piece is the bespoke, waterproof, synthetic-filled bivi bag."
Dr Mark Hines



  • An insulated bivvy bag
  • Outer Fabric: Waterproof Dri-LX (fully taped) shell
  • Inner Fabric: Superlight MX
  • Insulation: 60g Primaloft Gold
  • Stuff sac
  • Packed size: 13cm x 21cm
  • Options:
    • Lengths: Short, Std, Long, Extra long
    • Widths: Slim, Standard, Wide, Extra wide
    • Colours: Black, Red, Green, White
    • Expedition bag size (+£25). Adds 10% (approx. 60g) to the weight.


In a snowhole, tucked away under an overhang or simply beneath a big boulder with a slot under it: the classic bivouac spot keeps you out of the wind and rain, but can be a bit grim, mucky and damp. Not kind to a down sleeping bag, but a very simple way of sleeping out.

The PHD Snowline Bivvy is an insulated bivvy bag or sleeping bag cover designed to protect and boost the performance of a down sleeping bag. Like our Filler, the added insulation (in this case Primaloft Gold 60g) boosts the sleeping bag’s warmth by a further 7°C but, in addition, the PHD Snowline Bivvy protects your sleeping bag from the damp and drips found typically in a snowhole, bivvy cave, tarp, or the big boulder with a slot under it.

While the PHD Snowline Bivvy is not intended to be used alone out in heavy rain – after all, the hood area is open to the elements, and a tent or hooped bivvy bag does that job far better – the Snowline is made in Dri-LX waterproof/breathable fabric with fully taped seams to give a high degree of protection to your sleeping bag.

Usable with or without a sleeping bag, when used on its own the Snowline Bivvy has a TOT (Typical Operating Temperature) of 15° (British summer nights) but when used over your regular sleeping bag it provides an added boost of 7°C, while protecting it at the same time – keeping it cleaner and drier.

For Example:

  • M.Degree° 100 K bag (TOT 10°C), plus the Snowline Bivvy = combined TOT of 3°C
  • Minimus K Bag (TOT 5°C), plus the Snowline Bivvy = Combined TOT of -2°C
  • Minim 400 K Bag (TOT -5°C), plus the Snowline Bivvy = combined TOT of -12°C
  • Hispar 400 K bag (TOT -9°C), plus the Snowline Bivvy = combined TOT of -16°C

And of course, the lightweight versatility of a half bag & down jacket “Sleep System” – boosted by the Snowline – really opens up an exciting wealth of possibilities.


Sized to take lightweight sleeping bags, we also offer a larger expedition bag option.

Standard bag option

  • Length: 210cm
  • Shoulder width: 80cm
  • Waist width: 70cm
  • Knee width 55cm
  • Foot width 55cm (half - circumference)

Expedition bag option

  • Length: 210cm
  • Shoulder width: 85cm
  • Waist width: 73cm
  • Knee width 57cm
  • Foot width 55cm (half-circumference)

And for both options we can make short, standard or long lengths and slim, standard, or wide widths.

The Inside Story: the insulation does far more than just insulating!

Traditional bivvy bags are often troubled by condensation, which leads to a wet sleeping bag. But a lot of modern lightweight tents can be condensation prone too, so what’s the problem with bivvies?

It’s contact with the condensation – the rubbing of fabric against damp fabric – which is the problem. Page one of the camping rulebook says touching your tent fabric means a wet sleeping bag, but obviously it’s unavoidable in a bivvy.

In the Snowline though, your sleeping bag is isolated from the outer fabric by the network of superfine fibres which form the Primaloft Gold insulation. There’s no contact with condensation, so your bag stays dry.

Plus, bivouac sites can be grimy and/or damp; dirt doesn’t help the crucial breathability of a bivvy bag’s fabric, and water-soaked fabric “wetting out” also prevents breathability and is colder than dry (beading) fabric…and where there’s cold, there’s condensation.

Keeping a traditional bivvy bag clean and re-applying its DWR treatment is therefore vital if it’s to have a chance of performing, but this really isn’t all that practical in the field.

Aware of these problems of damp, dirt and contact with condensation, we realised an insulated bivvy bag/sleeping bag cover (especially with Primaloft Gold, which we know copes well with water, sweat, and condensation) is a far better bet.

"I always found bivvies rewarding, the comfortable, the bearable, even the grim (in retrospect). With the new kit available they offer a wonderful alternative way of spending nights outdoors. You can have the magic without the misery." Peter Hutchinson, founder of PHD, reflecting on a lifetime of bivvies.



Dri-LX (weight: 64 gsm)

At just 64gsm, Dri-LX is our lightest 2-layer breathable waterproof rip-stop nylon fabric. Its remarkably low weight not only makes for a lighter load, but also reduces pack size. Its excellent waterproofing qualities are in part due to how it can be taped at the seams to create a waterproof shell for sleeping bags and clothing.

Superlight MX fabric (weight: 32 gsm)

A mini-ripstop nylon with excellent tear strength, MX is the pick of the superfine 15 denier downproof materials. We have tested lighter fabrics, but most have had significant drawbacks, while our confidence in MX is based on several years of extensive use in both our ultralight and our high mountain gear.

Primaloft Gold®

Primaloft Gold® is the ideal microfibre insulation. Ultrafine fibres produce an insulating core that is incredibly soft, lightweight and water resistant. In our experience it absorbs water much more slowly than other waddings, a marked advantage in wet-cold conditions.

The nearest of the synthetics to the feel of down, yet tougher than you would expect. Primaloft Gold® offers the best warmth/weight performance in the Primaloft range

"My favourite piece is the bespoke, waterproof, synthetic-filled bivi bag. I've had issues in the past with the amount down compresses, so always wanted a synthetic base to a bag - this is a great way of doing it."
Dr Mark Hines using the Snowline Bivvy Bag on the Iditarod Trail Invitational.
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