Kneewall triangle in attic solutions/specifics
To start, I am in climate zone 4, non coastal.
I have a non-insulated vented roof, the vents are ridge and over fascia soffit gap all the way along the roof above gutter with shingle hanging over…
The attic floor has blown in cellulose on top of batts insulation. Same in the attic floor above the bonus rooms ceilings.
The knee-walls on most of it have batts that have 1/2 rigid foam insulation board over it on just the knee wall. It is all finished attic space. The foam is not sealed or even very tight, it is unfaced. There is a whole section that was added on later that only has the batts, no rigid foam and nothing sealed.
There are batts in the rafters that are above the room up to the ceiling.
The issues I want to attack.
1. Putting foam board on section that only has batts. I know I need to seal with foam first. Then what is the best type of foam board to use and how thick for the areas that have only batts? I have seen/read different reports on this, faced, unfaced, 1/2 inch unfaced due to moisture…. etc.
2. I assume I should foam seal all the spaces between the existing foam boards and tape correct? Should I pull it off first and foam any air leaks or is that a lot of work for not much return?
3. Is there any benefit to using foam board and enclosing the hvac unit that is in the attic? Or on that side run foam board from the ceiling and leave kneewall with just batts… There are well sealed double closet doors that give access to it, but the whole hvac is open to the entire attic. The unit and ducts seems well insulated…
4. On the attic roof by the knee walls, would a radiant barrier help? Could I put foam board against the roof rafters? Since there are ducts running through the knee wall space wanting to try and keep the temp reasonable…. Should I look into ventilating better? add a gable vent at the knee wall?
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Replies
1. Foam Board for Areas with Only Batts
When adding foam board to sections that currently have only batt insulation, it's crucial to choose a product that balances insulation value with moisture control. For climate zone 4, a minimum R-value of R-5 to R-10 for the foam board is recommended, which translates to about 1 to 2 inches of extruded polystyrene (XPS) or polyisocyanurate (polyiso). Between faced and unfaced foam boards:
Unfaced boards are generally preferred in attic applications to avoid trapping moisture, but the key is ensuring that the foam board is properly sealed at the edges and joints to create a continuous insulation layer.
Seal all seams, gaps, and penetrations with spray foam or a compatible sealant to minimize air leakage.
2. Sealing Existing Foam Boards
Yes, sealing all the spaces between existing foam boards and using tape for the seams is advisable. Before doing so, inspecting and sealing any air leaks behind the boards can significantly improve thermal performance and air quality. While it might seem labor-intensive, ensuring a tight air seal can provide substantial returns in energy efficiency and comfort, especially in extreme temperatures.
3. Insulating HVAC Unit in the Attic
Enclosing the HVAC unit with foam board insulation can help in maintaining the efficiency of the system, especially if the attic experiences significant temperature fluctuations. Insulating around the HVAC unit should be done carefully to ensure accessibility for maintenance and repairs. If the unit and ducts are already well-insulated and sealed, additional enclosure might not be necessary, but ensuring that the surrounding area is properly insulated and air-sealed (including the knee walls) will help in maintaining system efficiency.
4. Radiant Barrier and Roof Insulation
Radiant Barrier: Installing a radiant barrier can be beneficial in reflecting heat and reducing temperature gains in the attic, especially if your attic gets a lot of sun exposure. It's most effective when installed with an air gap between the roofing material and the barrier.
Foam Board Against Roof Rafters: Placing foam board insulation against the roof rafters is an effective way to insulate the attic space, especially in areas with ductwork. Ensure that the foam board is properly sealed and consider the overall attic ventilation to prevent moisture issues.
Ventilation: Proper ventilation is crucial, especially if you decide to insulate closely to the roof. Adding a gable vent at the knee wall can help improve cross-ventilation, but the specific ventilation strategy should be tailored to your attic's design and local building codes.
Additional Recommendations
Air Sealing: Before adding insulation, focus on air sealing to address any leaks. This is critical for preventing conditioned air from escaping and unconditioned air from entering, which can lead to inefficiencies and moisture problems.
Consult a Professional: Given the complexity of building science and the potential for unintended consequences like moisture issues, consulting with a building performance specialist or an energy auditor can provide tailored advice and ensure that the improvements align with best practices and local codes.
By addressing these areas thoughtfully, you can significantly improve the comfort, efficiency, and durability of your home.
Thanks a bunch for the reply. I had an energy audit done and they never were a joke to be honest, basically suggested sealing some attic hatches
Then I had a contractor and he didn't seem to think insulation was very important and I needed ventilation. Then 2 insulation guys, neither mentioned addressing the walls or air sealing. But I did hire one of them to blow in cellulose. The other insulation guy was all about adding solar powered roof fans.
Would you suggest insulating the roof of the kneewall instead of the kneewall in the area that the HVAC is in? If so what would you suggest to use as foam board? Faced? Unfaced? If I did this is it ok leaving batts in the wall and cellulose on floor?
Given the mixed advice you've received and the specific concerns you've outlined, focusing on effective insulation and air sealing strategies is crucial for improving your home's energy efficiency and comfort. Here's a tailored approach based on your situation:
Insulating the Roof of the Knee Wall Area:
Approach: Insulating the roof of the knee wall area, especially around the HVAC unit, can indeed be more effective than just insulating the knee walls. This strategy helps maintain a more consistent temperature in the attic space and reduces the load on your HVAC system.
Material Choice: For the insulation material, using rigid foam board is advisable. In your climate zone and application, unfaced extruded polystyrene (XPS) or polyisocyanurate (polyiso) would be suitable choices due to their high R-value per inch and moisture resistance. The choice between faced and unfaced depends on the specific moisture dynamics of your attic; however, unfaced is generally preferred to avoid trapping moisture. Ensure that whatever product you choose is compatible with your attic's moisture management strategy.
Installation: When insulating the roof, ensure the foam board is properly sealed at the edges and joints to create a continuous insulation layer. Seal all seams, gaps, and penetrations with spray foam or a compatible sealant to minimize air leakage. It's essential to maintain an air gap between the insulation and the roof sheathing to allow for ventilation and moisture management.
Leaving Batts in the Wall and Cellulose on Floor:
Batts in Knee Walls: If you insulate the roof of the knee wall area, leaving existing batts in the knee wall is acceptable as long as they are in good condition and not causing any moisture issues. However, inspect these batts to ensure they're not compressed, wet, or moldy, as this would compromise their insulative value and potentially lead to moisture problems.
Cellulose on Attic Floor: Blown-in cellulose on the attic floor is effective for increasing the overall insulation of your attic. Ensure that it's evenly distributed and that there are no gaps. It's also important to air seal the attic floor before adding insulation to prevent warm air from the living spaces below from migrating into the attic and causing condensation issues.
Final Recommendations:
Professional Consultation: Given the complexity of your project and the conflicting advice you've received, it might be beneficial to consult with a building science professional or a different energy auditor who can provide a comprehensive assessment tailored to your home's specific needs and challenges.
Ventilation: Proper ventilation remains crucial, especially if you're insulating the roof of the knee wall area. Ensure that your attic has adequate intake and exhaust ventilation to manage moisture effectively. Solar-powered roof fans can help in some cases, but they should be part of a broader ventilation strategy that considers the attic's overall airflow.
By focusing on targeted insulation improvements, air sealing, and proper ventilation, you can enhance your home's energy efficiency and comfort while addressing the specific challenges of your attic and knee wall areas.