How can I block cold air from entering the crawl space under my furnace?
Asked by Dré Tempel 
2 months ago
Last Updated: October 21, 2024
Hey, so I’ve got this area right beneath my furnace that seems to be letting in a ton of cold air. It’s right above the crawl space under the horizontal boards. Any ideas on how I can block some of that chilly air from getting in?
To prevent cold air from getting into the furnace area, consider using insulation or sealing gaps with weatherstripping. It’s important to inspect for any openings or cracks and seal them using the right materials.
Make sure to seal and insulate the area properly.
Apply spray foam to the joints before adding insulation. Alternatively, just opt for insulation.
It seems like the furnace needs combustion air and fresh air makeup. Add the necessary combustion air makeup inlet and fresh air makeup to your heating system.
I’m relieved I decided to inquire.
It’s important to prevent your water lines from freezing. Remember, you still need some fresh air flow underneath to lower the humidity and avoid mold and mildew. These older buildings were constructed without proper vapor barriers, leading to air leaks due to negative air pressure from the heating system, creating drafts. Retrofitting a modern pressurized heating system can exacerbate the issue by forcing interior air out through gaps in the walls, causing condensation and damp spots. Luckily, there are vapor barrier rated paints and window frames that need sealing, along with other exterior leaks in the building envelope. To combat this, adding a bit of fresh air to the cold air returns can help reduce pressurization. The furnace should have enough combustion air to operate effectively.
Is your furnace under this or is this under your furnace? Adjust your approach as the furnace may be pulling air from or into this space.
Hey Michael, my furnace is right above this area. I wasn’t sure if the furnace needs the air from here or if I should leave it alone.
The approach may vary depending on the type of furnace you have. If it pulls air from the outside, you can insulate the surrounding area without any concerns. However, if it draws air from the interior space, it might be wise to consult an HVAC professional for advice.
Spray foam expanding will block those cut outs. Hedge your bets and use the version that blocks pests.