Tag Archives: windows

How to Improve Energy Efficiency in Historic Buildings

As historic architects we find window replacement projects to be particularly challenging — removing original materials from a structure can fundamentally change the design aesthetic. Our built environment must evolve to support more sustainable living, but finding the best way to achieve this goal for historic structures, while minimizing any aesthetic impacts, is an ongoing challenge.

When looking to improve energy performance the first inclination is often to replace the component with the lowest thermal resistance – the windows. Single pane historic windows provide minimal thermal resistance and contribute to heat loss through the building envelope. But is window replacement really the best option for reducing the carbon footprint of a historic building – how does it compare to other strategies?

energy-analysis-West ElevationPMA recently performed an energy analysis study to answer that question. The project was to provide quantitative data on the energy savings associated with window replacement versus insulating exterior walls. We choose to study a structure on the brink of historic status – a 1960’s multi-story residential structure with large character defining view windows. The structure is composed of concrete walls, beams, floors, and columns with single pane aluminum windows. The existing building has approximately 36% glazing and no insulation.

The analysis we performed compared seven retrofit strategies ranging from minimal code compliance to super insulated walls and windows. Details on the specific constructions, r-values, and glazing properties are outlined below.

Construction Types Chart A wide range of constructions were chosen in order to see the full range of possible results. Future studies may focus on more refined material choices and a narrower set of parameters. The analysis was run in Autodesk Green Building Studio which is an excellent tool to perform basic energy models. While GBS does not allow for complex simulations it can quickly and accurately compare a variety of different design alternatives.

We chose to look at four different indicators to compare the results:
• Energy Use Intensity (EUI) indicates how much energy is used per square foot per year and is a very common way of comparing how different buildings perform.
• The quantity of electricity used per year indicates how much energy is used on cooling loads, heating loads, interior loads, and lights.
• The quantity of fuel used per year indicates primarily energy used for heating.
• The annual peak demand indicates the maximum amount of energy used at any single time over the course of a year.

We assessed the data in terms of percentage improvement over the Existing scenario. The charts below provide a comparison of the seven different retrofits.

Results Chart

The Results
What is intriguing in the results is the large difference in performance within the glazing retrofits options between the Double Pane LoE Glazing and the Triple Pane Glazing. While the Double Pane Glazing provides a notable improvement to the building’s energy performance it is still surpassed by all of the other retrofits. Conversely the Triple Pane Glazing far out performs all of the insulation retrofit strategies. The range between the two glazing retrofits indicates that new windows have the potential to have a substantial impact on energy performance. Unfortunately triple pane glazing is typically cost prohibitive and the LoE coatings applied to achieve maximum efficiency are incongruent with historic buildings. As technologies change and improve it is possible that these obstacles will be overcome – potentially making window replacement for energy efficiency purposes a more viable option.

window-detailWith current technologies the results indicate that adding insulation to a building has the most cost effective impact on energy performance. Installing new insulation is typically less expensive than window replacement and the results of this study show that Code Compliant (R-~7) insulation can have a significant impact on overall energy usage, outperforming Double Pane window replacement. Interestingly, the results also indicate that a High Insulation (R-25) retrofit performs better than a Combined Retrofit with Code Compliant Insulation (R-~7) and Double Pane Glass.

The results clearly indicate that adding insulation is an excellent way to improve energy performance without impacting the exterior façade of a historic building. Like any retrofit, insulation poses its own challenges: can it be installed on the interior without affecting historic finishes? Will changes in the temperature of the wall cause deterioration?, etc. Conversely, there are instances where window replacement is the right choice (when the existing windows have reached the end of their lifespan) and in this instance choosing a double pane glazing option can improve energy performance. In most cases, if you are looking to improve the energy performance of your building – it is more effective to explore insulation retrofit options rather than window replacement.

Written by Halla Hoffer, Architect I

Steps to Replacing Historic Wood Windows

QAHSC-landmarks-review-pmapdxOur first choice, and ethical preference, is to retain historic wood windows. Repaired and maintained wood windows constructed of old growth lumber will outlast any modern alternative. We advocate strongly for a process and philosophy that seriously evaluates retaining original material. The best approach compares long-term costs, embodied energy, and cultural importance relative to the same criteria for new replacement material.

But what do you do when the comparative process favors new material and replacement becomes the option of choice? And how do you gain jurisdictional and historic approval for removing character defining features from a historic property? Correct research, documentation, presentation, and material selection are the key factors to successfully replace historic wood windows.

Lack of maintenance is rarely accepted as a justifiable rationale for window replacement. Arguments for window replacement based on peeling paint, surface tracking of the wood, and/or glazing putty failure are typically countered with comments that benign neglect is a conscious act and straight forward maintenance will reverse the deterioration and deficiencies noted. A better strategy is to base replacement rational on existing significant deficiencies that require financial investment and resource allocation to repair the deficiencies.

QAHSC-landmarks-review-windows-pmapdxMost existing, older properties have had more than one owner. Research into original design documents, major rehabilitation projects, building permit requests, and other documents provide insight into processes that might have replaced original material. The removal and replacement of non-original material is justifiable and acceptable rationale.

Documentation by means of an on-site, window by window survey is the only method that will yield quantifiable data regarding the physical condition of existing wood windows. The resulting comparative data is critical for structuring an argument in favor of replacement. The field observations also provide invaluable information pertaining to the means and methods of construction and conversely deconstructing, or removing, the windows. Understanding wood window construction is important to understanding how wood window fail. Source documents like the Association of Preservation Technology’s Window Rehabilitation Guide for Historic Buildings (1997) and the National Park Service Preservation Briefs: 9, The Repair of Historic Wooden Windows provide exploded diagrams of both wood window construction and typical failure locations. These locations generally include the sash mortise and tenon joints, the exterior stops, and horizontal rails. The field assessment will need to document the quantity, location, and extent of any failed components.

QAHSC-landmarks-review-pmapdxAfter a thorough evaluation and understanding of the existing wood windows, the next decision is to choose a replacement product. In-kind replacement,(i.e. wood window for wood window; true divided lites for true divided lites, matching pane divisions, etc.) is preferred. When the replacement window is virtually identical to the historic window, it is hard to say no. Absent exact replacement, the visual qualities exhibited by the cross section profiles, the sash height and width, and the proportion of wood to glazing, are the most important attributes to match. Appearance from the exterior will trump appearance from the interior during a historic review approval process.

How the research findings, existing conditions, and replacement products are presented is fundamental to a successful request to replace historic wood windows. Agencies and commissions with jurisdictional review and approval authority require clear, methodical, and linear processes to understand the research, findings, and selection process. Collating the field data using charts and graphs, including graphic representation of previously altered windows, and defining the quantity of failed components will assist a decision in favor or replacement.

QAHSC-window-flashing-pmapdxWhen an opportunity to retain original fabric/windows is available, the opportunity should be incorporated into the work. Even retaining as little as 20% of historic fabric will increase the likelihood of approval for replacement of the remaining components. The retention of historic fabric also allows successive generations to better understand the history and changes of an existing property.

Written by Peter Meijer AIA, NCARB, Principal.