In rental property renovation, installing inner sash windows to improve insulation and soundproofing is a cost-effective measure with high return on investment. Without requiring major construction work, comfort can be significantly improved simply by installing them on the inside of existing windows.
This article comprehensively covers the mechanisms and types of inner sash windows, their advantages and disadvantages, cost estimates, DIY installation methods, retrofit considerations, and available subsidy programs.
What is an Inner Sash Window?
An inner sash is an additional window sash installed on the inside of an existing window. Also called double windows or secondary glazing, the air layer created between the inner and outer windows acts as a mechanism to prevent heat transfer.
Difference from Double-Pane Glass
While inner sash refers to the entire window system including the glass and sash frame, double-pane glass refers to the glass itself (two panes with space between them) fitted into the sash frame. Both are effective for insulation and condensation prevention, but they are completely different solutions.
Types of Inner Sash
Characteristics vary depending on the sash material.
Aluminum sash is lightweight, rust-resistant, and affordable, but has high thermal conductivity and is prone to condensation. Vinyl sash has thermal conductivity about 1/1000th that of aluminum, resists condensation, and suppresses mold and mite growth, though it is vulnerable to UV rays. Wood sash offers high insulation and warm design aesthetics but requires regular maintenance. Aluminum-vinyl composite sash uses aluminum on the exterior and vinyl on the interior, combining weather resistance with insulation, though at a higher price point.
What Are the Benefits of Installing Inner Sash?
Installing inner sash offers many benefits beyond insulation and condensation prevention, including security, soundproofing, energy savings, and improved aesthetics.
Improved Insulation
The air layer between the outer and inner windows prevents outdoor temperature changes from transferring indoors, improving heating and cooling efficiency.
Condensation Prevention
Condensation caused by warm indoor air touching cold window glass can be significantly reduced by installing inner sash. Since leaving condensation unaddressed leads to mold and mite growth and health risks, this is a particularly important measure in rental property management.
Enhanced Security
Double windows increase the time required to break through and enter, acting as a crime deterrent. The effect can be further enhanced by combining with security glass.
Soundproofing
Improved indoor airtightness reduces external noise and prevents sound leakage from inside. This is especially effective for properties with families with children or student tenants.
Energy Efficiency
By reducing the impact of outdoor temperatures on the interior, the load on air conditioners and heating equipment decreases, leading to reduced utility costs.
Improved Aesthetics
Wood sash in particular can be easily matched to various interior styles, creating a cohesive look in the renovated space.
What Are the Disadvantages of Inner Sash?
Understanding the disadvantages before installation will help maximize tenant satisfaction.
Cost Considerations
In addition to construction costs, new window frames and glass are required, and choosing double-pane glass further increases costs. When installing in all units of a multi-unit building, carefully consider budget and scope, and explore utilizing national and local government subsidies.
Extra Effort Opening and Closing Windows
For frequently used windows such as those accessing balconies, the need to open and close windows twice creates additional effort.
Increased Cleaning Effort
The doubling of window glass, frames, and tracks increases cleaning burden. Choosing sash materials that resist condensation can reduce this effort.
Some Windows Cannot Be Fitted
Installation is not possible on double-hung windows or skylights. If window frame space is insufficient, installation of "extension frames" (fukashi waku) becomes necessary.
How Much Does Inner Sash Installation Cost?
The cost range combining product price and installation work is approximately 40,000-150,000 yen per window. High-grade options can cost close to 300,000 yen.
Variations by Grade, Size, and Quantity
Typically, mid-height windows (waist-high windows) in living rooms and Japanese-style rooms measure 1,800mm wide × 900-1,200mm high, while balcony sliding doors measure 1,800mm wide × 1,800-2,000mm high. Window glass options include "single-pane glass," "double-pane glass," and "high-insulation double-pane glass," with high-insulation being the most expensive. Installing multiple locations simultaneously makes price negotiation easier.
Can Inner Sash Be Installed DIY?
Simple inner sash can be installed DIY using polycarbonate panels and tracks. What would cost tens of thousands of yen through contractors can be done for about 10,000 yen.
The basic steps are: measuring the window frame, installing tracks, cutting polycarbonate panels, attaching covers, and inserting into tracks. Pay attention to dimensional accuracy to ensure airtightness, and work carefully to avoid gaps.
What Should You Consider When Retrofitting Inner Sash?
Keep these considerations in mind for satisfactory renovation results.
Some Windows Cannot Be Fitted
Windows with air conditioners or ventilation fans, double-hung windows, inward-opening or tilt-in windows, skylights, and louver windows may be difficult to fit. Consult with a renovation company if uncertain.
Confirm Post-Installation Appearance in Advance
Additional windows may make rooms feel smaller or the extra effort of opening and closing may become bothersome. Simulate usability in advance.
Install on All Windows for Maximum Insulation
Installing on just one window while heat escapes through others limits effectiveness. To maximize insulation, inner sash should be installed on all windows in a room.
Utilize Subsidy Programs
Since inner sash installation contributes to energy savings, national and local government subsidy programs may be available. Check the type of subsidy you want to use and consult with registered contractors.
Frequently Asked Questions
Which is more effective: inner sash or double-pane glass?
Inner sash is more effective for maximizing insulation. The large air layer created between outer and inner windows provides superior insulation performance. Another advantage is that it can be retrofitted without replacing existing windows. Double-pane glass requires replacing the glass within the sash, which can involve more extensive construction.
Can inner sash be installed in rental apartments?
In rental apartments, windows are considered common areas, so replacing outer windows requires management association approval. However, since inner sash is installed on the interior side as exclusive-use area work, installation is possible in many apartments. Check management regulations in advance.
How can I reduce inner sash installation costs?
Methods to reduce costs include utilizing national and local government subsidies, negotiating discounts for simultaneous installation at multiple locations, and DIY installation of simple inner sash. Choosing single-pane glass also reduces costs but decreases insulation effectiveness.
What is the lifespan of inner sash?
Depending on materials used and maintenance conditions, they typically last 20-30 years. Wood sash requires regular painting maintenance, while vinyl sash and aluminum-vinyl composite sash require relatively less maintenance.
