A house becomes too cold for health when the indoor temperature drops below 64°F (18°C). That’s the threshold the World Health Organization identifies as the minimum safe indoor temperature for general populations in cold climates. Below that point, the risks to your cardiovascular system, lungs, and overall well-being start climbing in measurable ways. For infants, elderly adults, and people with chronic conditions, the danger zone starts even higher.
The 64°F Health Threshold
The WHO’s guideline of 18°C (about 64°F) isn’t arbitrary. Two large cohort studies from Scotland found that people living in homes heated below that temperature had a greater risk of high blood pressure. When indoor temperatures dropped below 61°F (16°C), the odds of high blood pressure nearly quintupled. A separate UK study found that for every 1°F drop in room temperature, both systolic and diastolic blood pressure crept upward.
Cold indoor air inflames the lungs and constricts blood vessels. That combination increases the risk of asthma attacks, worsening COPD symptoms, respiratory infections, heart disease, and stroke. Cold homes have also been linked to poorer mental health, likely a mix of physical discomfort, poor sleep, and the stress of living in conditions your body constantly fights against.
For otherwise healthy adults, keeping your home at or above 64°F during waking hours protects against these effects. The U.S. Department of Energy recommends setting your thermostat to 68–70°F while you’re awake and lowering it while you sleep or leave the house. Dropping the temperature 7–10°F for eight hours a day can cut your annual heating and cooling costs by roughly 10%. So there’s real room to save money without crossing into unsafe territory, as long as you know where the floor is.
Why Babies and Older Adults Need Warmer Rooms
Infants can’t regulate their body temperature the way adults can. The Lullaby Trust recommends keeping a baby’s room between 61°F and 68°F (16–20°C), with the emphasis on not letting the room get too hot. Overheating is a known risk factor for sudden infant death syndrome. If you leave the heat on overnight in a nursery, keep it at or below 68°F. Check your baby’s chest or the back of their neck regularly; hands and feet will naturally feel cooler, so those aren’t reliable indicators.
Older adults face the opposite problem. They lose body heat faster, often have thinner skin and less muscle mass for insulation, and are more likely to have cardiovascular or respiratory conditions that cold air worsens. For anyone over 65, or anyone managing a chronic heart or lung condition, indoor temperatures below 64°F pose a real and compounding risk.
What Cold Does to Your Pipes
Your house itself suffers in the cold, too. The most expensive risk is frozen pipes. When water freezes inside a pipe, it expands, and the resulting pressure can split copper, PVC, or galvanized steel. A single burst pipe can cause thousands of dollars in water damage.
The exact temperature at which pipes freeze depends on where they’re located, what they’re made of, and how well they’re insulated. Pipes running through exterior walls, unheated garages, or crawl spaces are the most vulnerable. There’s no single magic thermostat number that guarantees safety, but most plumbing professionals recommend never letting your thermostat drop below 55°F (13°C), even when you’re away from home for extended periods. If your pipes run through particularly exposed areas, you may need to keep it higher or take additional precautions like letting faucets drip during extreme cold snaps.
Cold Homes and Mold Growth
A house that’s too cold doesn’t just feel uncomfortable. It creates the conditions for mold. When warm, humid air from cooking, showering, or even breathing meets a cold wall or window surface, condensation forms. That moisture feeds mold growth, particularly in cold corners, behind furniture pushed against exterior walls, and around window frames. The EPA recommends keeping indoor relative humidity between 30% and 50% to prevent mold. In a cold house, hitting that target becomes much harder because cold surfaces act as condensation magnets. Keeping your home at a reasonable temperature is one of the simplest ways to keep walls dry and mold-free.
Safe Temperatures for Pets
Dogs and cats are more cold-tolerant than many people assume, but there are limits. USDA regulations for animal care require that indoor temperatures for dogs never fall below 45°F (7°C) for more than four consecutive hours. For dogs that are small, short-haired, elderly, young, sick, or simply not used to cold, the minimum is 50°F (10°C). Below that, dogs need supplemental warmth like dry bedding or a heated resting area.
Cats have similar tolerances. Most healthy adult cats handle temperatures down to about 50°F without trouble, but kittens, senior cats, and hairless breeds need warmer environments. If you’re lowering your thermostat while you’re out, make sure your pets have a warm spot with blankets or bedding away from drafts.
Practical Temperature Ranges
- 68–70°F (20–21°C): The Department of Energy’s recommended range while you’re home and awake. Comfortable for most people and well within the safe zone.
- 64°F (18°C): The WHO’s minimum for healthy adults. Fine for sleeping, especially under warm bedding, but not ideal for prolonged daytime activity if you have any health vulnerabilities.
- 60–61°F (16°C): The lower boundary for infant sleep rooms. For adults, blood pressure risks begin increasing noticeably at this level.
- 55°F (13°C): A common minimum to protect pipes and prevent structural moisture issues when you’re away from home. Too cold for extended living.
- Below 50°F (10°C): Unsafe for vulnerable pets, young children, and elderly adults. At 48°F (9°C), studies on children with asthma showed measurable declines in lung function with every additional degree of cooling.
The short answer: keep your home at 64°F or above when anyone is living in it. If you’re healthy and bundling up, brief dips below that won’t cause immediate harm, but sustained cold exposure in your own home is a genuine health risk that compounds over days and weeks. The savings from an aggressively low thermostat rarely outweigh the costs to your body, your pipes, or your walls.