Interior Storm Windows

[Joe Duehmig]

This is my first post here so I hope I'm posting to the correct forum. I am in the design phase for my project of building interior storm windows for my 1895 Victorian home. My plan is to make these storms very similar to the original exterior storms, but without the weep holes. I'm planning to use modern low-E glass. The windows will be between 60"-72" high and around 32" wide, with a single mutton. At this point a good finished pine is the wood of choice due to its cost and availability. This will also help match the grain with the existing old growth pine trim in the house.

My question now is how thick should I make the rails and styles? I have a Freud router bit set for making just this type of window but it requires a 1 3/8" stock. This would result in the new windows protruding from the window frame and detracting from the esthetics. It would also make the storms quite heavy and unwieldy. I think I can make due with a combination of other router bits for the copes and rabbets, but how thin can I go with the stock? Obviously I need enough thickness to produce a stable frame without so much flex that the glass would break or the glaze would move. But I also want to minimize weight and ensure a good overall look.

Any thoughts are greatly appreciated.

Thanks,
Joe

[Mike Wilkins]

You did not state the thickness of the glass, so that will have some bearing on the thickness of the rails/stiles. You may be able to use standard single thickness glass optimized for the size you are considering, which looks large. Since you are just creating an air barrier between the inside and outside, insulated class may not be needed.

I am about to start on the construction of some exterior storm windows for my shop, and plan to use single thickness glass.

You could also check out some of the interior storm window makers, listed in Old House Journal magazine. I sometimes get a copy just for the interesting reading on old houses. Their sites could possibly give you some ideas.

Good luck and watch those fingers.

[Mel Fulks]

Interior storm windows give a real different look from exterior. There is no disguising them,I mention that because you are concerned about design. To me the only reason to use interior type is be able to use cheaper non weather resistant material.

[Joe Duehmig]

Mike,
Good thought. I was going under the assumption that a standard pane of glass was 1/8" but I should confirm that. I have looked at some of the manufactured interior windows and I'm using some of their ideas in my design, but I have a lot of large windows and $400-$600 each is way out of my budget.

Mel,
Two things are driving the decision to go with interior storms: the look of the exterior and the effort to install/remove 60 pound windows on the second and third floors. If you dig into the architectural preservation sites you find lots of good information on the loss of 'texture' and depth on a house when you use flush exterior storms. Bringing the storms inside makes install/removal a breeze and improves the look of the exterior. I understand this removes any protection of the windows themselves from the elements, but all my windows need to be removed/stripped/reglazed, etc. so they will be getting new paint over the next few years anyway.

Thanks for your comments.
Joe

[Shawn Pixley]

Joe, as an architect who once specialized in historic preservation, i sympathize with the aesthetic elements you note. On the practical side however, you should consider on which side of which plane the dew point will exist (it would also help to know of your location). With exterior storms, the water condenses outside of your main window glass and weeps to the exterior. With internal, it might very well condense between the existing windows and the interior storm and damage your sills, casing, etc...

Something to consider.

[Joe Duehmig]

Shawn,
Thanks for the reply. I hadn't considered condensation on the interior side. I live in Wisconsin. My heating system is an under-floor hydronic system. I typically don't use any humidifier during the winter, and I'm currently showing about 20% relative humidity. Do you think this will be a concern?
Thanks,
Joe

[Mel Fulks]

I certainly understand the appearance considerations. I've seen interior "storms" that were basically just a big piece of plexiglass, since some people never open windows they find it an easy way just to provide more air seal.

[vince dale]

I also would be concerned with moisture between the glass. If you plan to use LowE glass, any moisture will stain the coating and you won't like the results. I would use just clear glass. And if I remember a sealed unit losses R value beyon 1/2".

Is it not possible to build storm frames that blend well with the exterior.

Joe, as an architect who once specialized in historic preservation, i sympathize with the aesthetic elements you note. On the practical side however, you should consider on which side of which plane the dew point will exist (it would also help to know of your location). With exterior storms, the water condenses outside of your main window glass and weeps to the exterior. With internal, it might very well condense between the existing windows and the interior storm and damage your sills, casing, etc...

Something to consider.

[Joe Duehmig]

Vince,
The root problem isn't really the appearance of the exterior, although that is a factor in the overall design goals. The real problem is installing the exterior storms in the fall and removing them in the spring. I have 8 large windows on the second floor and 7 on the 3rd floor. Last spring I only removed one storm from the second floor because it was always too windy when I had an opportunity to remove anymore. (Trying to handle a 50# window one-handed while standing at the top of a 28' ladder in 20mph winds is not fun.) And I don't even have a ladder long enough to reach the 3rd floor. So in essence I have 15 windows in my house that can't be opened because of ever-present storms. The first floor is similar because the windows are so large it is a two-man job to get them on and off. The real advantage of the interior storms is the relative ease of installation and removal.

Having said that, the comments here about condensation need to be taken seriously. The first window I plan to build is for a second floor bedroom. Since those windows are painted I can judge the effect of any condensation without fear of harming any stained trim like I have on the 1st floor. It may be that I need to engineer a type of drip pan or wicking system to prevent wood damage.

Thanks again for everyone's comments. Keep 'em coming if you have more thoughts!
Joe

[Shawn Pixley]

Joe I think it could be. Right now, the interior temperature and the interior glass temp is high enough that the water doesn't condense out (dew point). With interior storms, if the existing interior glass will be colder if at or below the dew point then it will condense their. There are calculations that can allow you to define where the dew point will be in the insulative structures.

I see the problem of installing them and taking them down. Could the storms be placed like exterior shutters and then only rotated to the proper location from inside the house?

[Mel Fulks]

Agree it could be,but 20 percent RH is pretty low and has its own problems.

[Joe Duehmig]

Shawn,
I'll have to look up the condensation formulas and see what I come up with. I'll probably proceed with building two windows this year, one for the 1st floor and one for the 2nd floor, and see how things go.

I looked at the FAQ's for the Indow Window website, and here is their answer about condensation with their products:

• Indow inserts are edged with our patented silicone Compression Tubing which forms a near airtight seal around
the inside of the window opening.
• Existing windows will leak significantly more exterior air into the cavity than the Indow insert will leak interior air.
• Air within the pocket is comprised mainly of exterior air that warms in the cavity, rather than interior air that is
cooled on the surface of the Indow inserts.
• The air pocket between the existing window and the Indow insert buffers the acrylic glazing from the outside
temperatures, allowing a more neutral temperature to be maintained within the air pocket.
• In addition to the air pocket, Indow window inserts are made of a high-grade acrylic, which is a far superior temperature
insulator than glass.
Given this I'll have to really focus on getting an air-tight seal!
Thanks,
Joe

[Shawn Pixley]

Shawn,
I'll have to look up the condensation formulas and see what I come up with. I'll probably proceed with building two windows this year, one for the 1st floor and one for the 2nd floor, and see how things go.

I looked at the FAQ's for the Indow Window website, and here is their answer about condensation with their products:

• Indow inserts are edged with our patented silicone Compression Tubing which forms a near airtight seal around
the inside of the window opening.
• Existing windows will leak significantly more exterior air into the cavity than the Indow insert will leak interior air.
• Air within the pocket is comprised mainly of exterior air that warms in the cavity, rather than interior air that is
cooled on the surface of the Indow inserts.
• The air pocket between the existing window and the Indow insert buffers the acrylic glazing from the outside
temperatures, allowing a more neutral temperature to be maintained within the air pocket.
• In addition to the air pocket, Indow window inserts are made of a high-grade acrylic, which is a far superior temperature
insulator than glass.
Given this I'll have to really focus on getting an air-tight seal!
Thanks,
Joe

It is true that with a good seal, the majority of the air between the inner storm and the primary windows will be outside air if the issue with the windows is lack of sealing. If the exterior windows actually seal well but are single glazed where you lose a lot of heat through conductive loss, that exterior air with low RH might not be so true. Can you verify that that is the problem and if so, could you not fix the weatherstripping?

Acrylic has less conductive loss compared to glass as well per the Mftr's literature. I don't think however that the condensation is likely to be on the face of the interior storm (as referenced in your note). More likely IMO is it to be on the inner face of the primary window. Your money and your decision. I would encourage you to think of the window opening as an assembly of multiple components. Looking at only the literature from one component can lead to false interpretations / conclusions.

Back when I lived in the midwest, I rented a house with single glazed double hung wood windows (~1910, four square house). We used a shrink to fit film for interior storms. They worked okay. You may want to try those before investing in interior storms.