This project involved installing a mini-split system for a small house we built on my property. A few years ago, we needed additional space for a guest bedroom and an office, so we decided to construct this space. Being involved in the HVAC and energy efficiency industry, I had a clear vision for how I wanted to build and condition the house from the start.

Building Structure and Efficiency

Since this was new construction, I had the freedom to build the structure exactly as I wanted, from the foundation up. In our area, solid rock is just 20 to 24 inches below the surface, so I chose a pier and beam foundation. For insulation, I installed fiberglass batt insulation in the floors, supported by a net. The walls and roof were insulated with blown-in cellulose. Additionally, I used a polyseal technique, applying spray foam to seal edges and exterior penetrations, ensuring a solid thermal envelope.

For the exterior, I opted for Zip sheathing, which was taped and sealed at all seams. I also added a continuous layer of 1-inch foam board insulation all around, giving the walls an effective R-16 insulation value. The roof is metal and shaded by trees in the summer, so I didn’t feel the need to add extra insulation there. The windows were custom-built on-site using locally sourced low-e glass. They are fixed in place and non-operable, except for a small bathroom window, which I purposely designed to open for ventilation.

Heating and Cooling Capacity

The structure is about 400 square feet, and because it was built to be airtight and well-insulated, I could have likely used a 12k or even a 9k BTU/hr mini-split unit. However, I prefer to keep the temperature around 70°F, so I opted for a 15k BTU/hr Mitsubishi wall-mounted unit. It performs exceptionally well, keeping the interior at 68°F even when the outside temperature drops to 15°F. On the other hand, when it’s 105°F outside, it effortlessly maintains 71°F indoors.

System Placement and Layout

For system placement, I aimed to maximize airflow throughout the space while considering the layout. I always try to avoid using condensate pumps where possible—not because they’re inherently bad, but they introduce an additional point of potential failure. In this case, I was able to avoid using one by pre-planning the drain line routing. Since the bathroom is adjacent to the wall where I installed the unit, I ran the drain line under the floor to tie into the bathroom drain.

Maintaining Even Temperature

One of my initial concerns was whether the bathroom would maintain the same temperature as the rest of the space. To mitigate this, I installed a modulating exhaust fan. However, I discovered that the bathroom stays at a consistent temperature with the rest of the space as long as the door is left open. Even when the door is closed, it quickly catches up in temperature once opened. The mini-split unit does an excellent job of distributing air evenly, thanks to its 30 to 40-foot airflow throw, especially when set to oscillate up and down. Another key feature of mini-splits is their continuously running fan, which helps maintain a consistent airflow and temperature balance throughout the space. This is true whether it’s 20°F outside or 100°F, although I still leave the bathroom window slightly cracked to allow for fresh air exchange because of how tightly the space was built.

Lessons Learned

This project taught me a few things and reinforced some of my existing beliefs about using mini-splits in small or unique spaces:

1. Energy Efficiency: These units are incredibly efficient. The power consumption is minimal, making them an excellent choice for running on backup generators during outages. My mini-split draws only about 500 watts when running, and the amperage is well below a few amps, ideal for low-power situations.
2. Airflow and Temperature Distribution: Mini-splits do a fantastic job of distributing air and maintaining temperature consistency, especially in well-built spaces. Even in poorly insulated environments, they perform surprisingly well.
3. Quiet Operation: The unit is so quiet that you hardly notice it running. The only time I hear anything is when the louvers adjust, or if I turn the fan to high speed. Even then, it’s more like the sound of a regular fan, and the outdoor unit is virtually silent even when standing right next to it.
4. Ventilation Needs: Because the space is so airtight, I have to keep the bathroom window slightly cracked to allow for ventilation. Eventually, I plan to install an ERV (Energy Recovery Ventilator) to eliminate the need for this, but it’s low on my current list of priorities.

In Conclusion

Building this small guest house and installing the mini-split system was a fun project for me. It allowed me to use what I know about energy efficiency and HVAC, and proved to me how well mini-splits work in small, insulated spaces. The system keeps the temperature comfortable, runs quietly, and uses very little electricity, which is perfect for a backup generator. While I still plan to add better ventilation with an ERV, I’m really happy with how everything turned out. This project showed me that with good planning, even a small space can be comfortable, energy-efficient, and easy to manage.