Passive heating takes advantage of incidental heat sources. Typically, this means the sun or bodies (e.g., the bodies of you, your household, and your pets). However, portable water containers can also be an excellent method of passive heating, especially when used to supplement active heating like a Mr. Heater Buddy.
To learn about keeping your body, itself, warm, try this article. But just like your body can heat up your coat or your blankets, it can also heat up a space. Obviously, the more bodies, the more heat and the smaller the space, the more perceptible the difference.
One of the most important factors to try and control is the space itself. Ideally, the space will be just barely big enough to be comfortable for all the bodies and gear that need to stay warm. If possible, you’ll want it to have access to sunlight during the day, and the ability to control a small amount of ventilation.
Even if you’re inside your house or cabin, I would recommend setting up a camping tent (or a makeshift shelter from blankets, tarps, etc.) The reduction in space can make a huge difference and most tents come with some sort of ventilation mechanism.
Ventilation is very important for three reasons:
1. Letting oxygen in
2. Letting non-oxygenated gases out
3. Letting moisture out
If you want to stay warm you must stay dry… and bodies are not dry. A few bodies respiring in an improperly vented space can easily result in enough moisture to ruin your night. (Depending on the structure, this can be compounded when using propane as an active heating source… like a Mr. Heater Buddy.)
Wool is a great material to have in your space for this reason. Not only can it keep your body warm, but to a small degree it can also passively regulate moisture in the air.
While you do want ventilation, remember that air is your primary insulation. You do not necessarily want a breeze blowing out all the air that your bodies just warmed. Again, think of a tent when trying to determine your ratio of trapped air to ventilation. Be mindful and adjust as necessary.
Also, remember that you want to be heating the air with your body. While that may seem obvious, we often end up warming the ground. Once the air is somewhat trapped, you’ll want to make sure you’re not losing heat to the ground. Extra rugs, blanket, or towels can help keep your warmth in your space. Once you are insulated from the ground, it’s usually beneficial to insulate the rest of your space as long as you don’t impede the desired airflow.
If things are really cold, you will want to bring at least some of your portable water storage into your space (room permitting and assuming it’s leakproof). This can help prevent your water from freezing and that water will actually store the heat longer the air will… passively warming your space.
When it comes to passive heating, windows can be your ally and your opponent. If sunlight is coming in, use dark surfaces (like towels or blankets) to help soak up that heat. If applicable, having your water containers on those dark towels might be a good idea.
Whenever sunlight is not coming through a window, the window should be insulated as effectively as possible (think curtains, blankets, etc.) If you’ve planned ahead, clear bubble wrap (with the bigger bubbles) can do a pretty good job of insulating and still let sunlight through.
A more extreme option when you don’t need the light is to spray paint cardboard black and tape it to the wall on the outside of the window sill (leaving a gap between the cardboard and the window pane). Make sure there’s a gap at the top of the cardboard for the heated air to rise and flow into the room.
Also, focus on reducing drafts coming through leaks in windows or doors. There are many professional options, but rags, towels, and even Play-Doh can be pretty effective in a pinch. Doorways without doors can be blocked off with Visqueen, tarps, painter’s plastic, or even a patchwork of taped garbage bags. Duct tape is ideal for attaching these to the wall, unless you care about how your walls will look afterwords, in which case you will probably want to go with blue painter’s tape. (Again, remember you will need some airflow.)
Even if you have some sort of heater, taking advantage of passive heating can greatly multiply the efficacy of any other heat sources you have available.