The Acanthus Group
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Historic Window Restoration

Perhaps no other subject regarding the restoration of a building generates as much debate as to whether to keep or replace the original windows. Most everyone agrees that they love the appearance and character of the original windows. However, the perceived lack of energy efficiency causes many people to discard the originals for new replacement windows to do so.

Window Restoration Process

old sash
Removal of damaged sash
old sash 2
Windows back at shop
damaged sash
Damaged sash
Putty, points, panes and paint removed
Sash prep
Primed, panes returned, re-glazed
finshed sash
Finish painted and re-installation

The Acanthus Group has spent many years working on and studying historic windows. In so doing, we have developed a system for restoring windows and have some compelling reasons why it makes sense to.

In regard to energy efficiency, only 10–12 percent of energy loss is through the windows. The real weak spot for energy loss is the roof and sills of a structure. Looking at the windows specifically, a single pane window has an insulation value of R1. Modern replacement windows with double paned glass offer only a value of R3.

Unlike old wooden windows, replacement windows cannot be maintained to extend their lifespan. Once replacement windows fail the only option is to buy...replacement windows. You may believe that yes, failure will occur but only later after you have moved on to another house or commercial building. The surprise is that in our experience we have seen high end replacement windows fail after only several years.

Our Approach

The Acanthus Group strongly encourages clients to restore their historic windows. From an aesthetic point of view, nothing else will match the character and charm of the original window. We do agree that energy efficiency is something to consider. We advocate the use of a storm window as a means of reaching a higher R value. We believe a historic window combined with a storm window yields the same energy efficiency as replacement windows. Many of our past clients have found success with an “invisible” - type storm window. Designed to mount inside the window frame rather than on it, the invisible storm does not detract from the appearance of the window.

The actual restoration process involves removing the window sash from the opening and taking it to the workshop where work can continue. The window openings are generally secured with plywood until the sash can be reinstalled.

At the workshop, the glazing putty and points are removed from around the glass and the glass pane(s) are labeled and removed. We then remove the paint from the exterior of the sash and clean the rabbets. At this point, carpentry repairs can be executed. If deterioration is fairly minor, repairs can be performed using epoxy consolidants and fillers. Occasionally, the damage is severe enough that new wooden elements are milled to match the originals and inserted into the sash.

Following the carpentry repairs, the sash is cleaned and the exterior face is painted with oil primer. The glass panes are returned to their openings and new glazing points and putty are applied. Whenever possible, we like to replace broken or missing panes with antiuqe glass to preserve the character of the sash. After the glazing putty has sufficiently cured, the putty is primed with oil paint and the exterior face is given two (2) finish coats of oil paint. The sash is then delivered back to the project site, the ropes or chains are reattached, if neccessary, and the sash is installed.

Frequently Asked Questions

How much does it cost to restore a window sash?
This is almost impossible to answer without seeing specific sash and assessing the condition of the window. Also, clients can control how much is done to the sash. For instance, some clients prefer to fix the upper sash in place rather than hanging it from the weights as a means of controlling costs. The short answer is that restoring the window sash costs less than what a high end replacement window would cost.

Do you take all of the windows at once?
No, hardly ever. Most clients want us to do the windows in stages so as not to cut off all natural light to the building. We usually take several windows at one time, restore them, place them back in their openings, and then remove the next several windows.

How long are the windows in the workshop?
It takes roughly three (3) weeks before the sash can be returned to the site. This is largely dependent on the cure time for the glazing putty.

Can you replace irregularly shaped glass panes that have been broken?
Yes. In fact, we have sourced glass for arched windows, bowed or curved windows, and pie shaped panes common in fan lights.

The wood on the interior faces of my sash was originally stained and varnished to match the rest of my woodwork. Unfortunately, the original stain and varnish have been painted over. Can my window sash be refinished to match the rest of my interior millwork?
The window sash can be refinished. We have considerable experience at removing layers of paint and then refinishing the wood to match the surrounding millwork.


The following are links to articles that discuss old windows and their energy efficiency: