The British damp challenge: why your sleeping bag matters

Wild camping in the UK is rarely a dry affair. Whether you’re pitching beside an upland tarn in the Lake District or bedding down in a remote bothy in the Cairngorms, moisture is your constant companion. Damp rises from the ground, condenses on the inside of your shelter, and hangs in the air after a day of rain. In bothies, wet gear and breath from other people add to the humidity. A sleeping bag that performs well in these conditions needs to do more than just keep you warm. It must cope with moisture without losing its loft, and it must be light enough to carry on long approaches.

Fill power and the down versus synthetic debate

Fill power is a measure of how much space one ounce (or gram) of down occupies. Higher fill power – 700, 800, or even 900 – means more loft per gram, so you get more warmth for less weight. That’s brilliant for backpacking, but untreated down collapses when it gets damp. Once wet, it clumps and loses its insulating ability, and it takes ages to dry. Hydrophobic down treatments help by coating each filament, but they’re not a magic bullet. Synthetic insulations, such as continuous filament sheets, handle damp far better. They absorb almost no water, retain most of their warmth when wet, and dry quickly. The trade-off is weight and bulk: for the same warmth, a synthetic bag is heavier and packs larger. For British damp, a good compromise is a hydrophobic down bag with a fill power of 650–850 if you’re diligent about keeping it dry, or a quality synthetic bag if you want peace of mind in a bothy where condensation drips from the ceiling.

Shape and features: mummy, semi-rectangular, and the humble hood

Shape affects both warmth-to-weight ratio and comfort. A mummy bag is tapered to reduce dead space, so your body heats less air. That makes it the most efficient choice for cold, damp nights. However, some people find mummy bags restrictive, especially in a bothy where you might want to sit up and read. A semi-rectangular bag gives more room but weighs more and is less thermally efficient. For UK conditions, prioritise a mummy shape with a well-fitted hood and a draught collar. The hood traps warm air around your head, and the collar stops warm air escaping when you turn over. Look for a full-length zip with a thick draught tube, and consider a two-way zip so you can vent at the foot on milder nights. A centre zip can be easier to manage in a confined bothy space. Also, check the shell fabric: a lightweight, water-resistant outer with a durable water repellent finish helps shed condensation, but don’t expect it to keep you dry in a downpour.

Warmth-to-weight ratios and temperature ratings

Warmth-to-weight ratio is the holy grail for wild campers. Down wins on this metric, but only if it stays dry. Synthetic bags are heavier for the same warmth, yet they keep working when damp. When comparing bags, look at the EN/ISO 23537 temperature ratings. The comfort rating is the temperature at which an average woman can sleep comfortably; the lower limit is for an average man. For three-season UK use – spring to autumn – a comfort rating around 0°C to 5°C is versatile. In bothies, you might get away with a slightly lighter bag because you’re sheltered, but stone floors and no heating mean you still need decent insulation. Upland tarns often sit in frost hollows, so expect colder temperatures than the valley forecast. Remember that ratings assume you’re wearing a base layer and sleeping on a mat with a suitable R-value. A warm bag on a cold mat is a recipe for a miserable night. Also, consider the shell: a lighter fabric saves weight but is less durable and more prone to damp penetration.

Practical wisdom for damp nights in bothies and by tarns

Even the best sleeping bag needs help in British damp. Follow these time-tested habits to keep your insulation working.

• Pack it waterproof. Always use a dry bag or waterproof stuff sack inside your rucksack. Moisture from wet gear will find a way in otherwise.
• Add a bivvy or cover. A bivvy bag protects against condensation dripping from bothy ceilings and tent walls, and adds a few degrees of warmth.
• Use a liner. A silk or fleece liner adds up to 5°C and keeps your bag clean, so you wash it less often.
• Air your bag daily. In bothies, hang it up during the day, but keep down bags away from open fires. Shake out any dampness.
• Don’t breathe into it. Use a buff or balaclava to cover your mouth and nose. Your breath is a major source of internal moisture.
• Choose the right mat. By a tarn, use a mat with a high R-value and a groundsheet to block rising damp. A warm bag on a cold mat is useless.
• Ventilate your shelter. Even in a bothy, crack a window or vent to reduce condensation. It feels wrong, but it keeps your bag drier.

If your bag does get damp, shake it out and dry it gently – never direct heat. Store it loosely at home. A dry sleeping bag is a warm sleeping bag.