What IGU specs do you recommend for storm windows in Wash. State?

fromhollywoodSeptember 25, 2009

I have been reading the postings here and learning a great deal (especially from the Oberon postings which are very informative--you should write a book).

I recently bought a 1920's Craftsman house in the Pacific Northwest, about ten miles inland from the coast, near the Canadian border. Luckily, most of the original wood windows are still in-place (I don't care for new windows). I can't find much local, good glass advice.

I am planning on building some storm windows myself for winter use (and occasional hot days in the summers). I plan on using 1-1/4" square cedar for the frames (primed & painted) and routing out a dado channel for double-pane IGUs. My plan is to use silicone to thoroughly seal the glass in these channels and use a good neoprene foam tape all around the seal between the storms and the old windows. I know installation is critical.

I want to use double-pane, argon filled, LowE coated glass in these storm windows. I'm a little unclear though as to what the best specs would be for the glass? I also have no idea what to expect as to cost for the glass. Is there a rough ballpark per-square-foot estimate for the different types? I've searched online hoping to find a supplier of IGU's but I guess fragility and weight are the reasons you can't order an IGU online. I started at a big box and found they can order replacement double-panes (technically for one of their window products but I guess they will order even if you want it for something else) which were about $80. for a 55"x22" double-pane, argon with LowE (5/8" total thickness). I assume this is Cardinal glass that this company sells through the big box though they can't seem to tell me any more detailed specs than what I've written here.

Our winter temperatures are around 40's F (day) and 30's F (night) (though it can reach zero on rare occasions). The summer temps here are rarely hot though this year it got to 100 F on a few days. Currently, on very hot days, this house stays comfortably cool without air conditioning but during winter, it can become unbearably cold and cost hundreds in propane bills each month. I assume that means a "high" SHGC coating would be better so that I could get some passive solar benefit in the winter (even though the house may get a little warmer in the summer)? How do you communicate that you want a "high" SHGC or do the other specified variables determine the SHGC? What thickness of glass should I look for? What type of edge-spacer should I specify or is that not too important? Should the LowE be soft or hard coat? Can all these things be specified if I buy from a glass supplier?

I've called some wholesale glass shops that I'd love to do business with but they won't talk to me and instead send me to the small glass stores--none of the small shops I've talked to seem to know much.

Thanks very much for any advice.

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If you want to buy an IG directly you will need to go to a retail glass shop - as you have already found out.

You could buy the full sash at the big box for $80 as you said, but then you would be paying for components that you really don't need.

Cardinal sells directly to window companies. While some of those companies may sell thru the big boxes, Cardinal does not do so directly. You cannot buy a Cardinal IG directly.

Smaller glass shops that retail their own IG's (versus ordering replacements from elsewhere) will generally use hard coats because they are much easier to handle than are softcoats. Since most hardcoats are high solar heat gain, this can work to your advantage if you want a higher SHGC.

I might hesitate to specify argon from a mom and pop shop that makes their own IG's. Keeping argon between the lites of an IG for an extended time does require a high degree of precision in the manufacture and fill of the unit. Some smaller shops who fabricate their own IG's simply don't deal with argon and some do.

I personally wouldn't worry too much about argon in your application. I would be satisfied with air in the space.

You might not have a choice on spacer. Typically, smaller shops are going to use a spacer that can be applied by hand. There are a number of different spacers that fit into that category. I wouldn't sweat the spacer, especially if you don't get argon.

I would specifiy double-strength glass(1/8" or 3mm glass). It is a bit stronger than single strength and a bit more durable for the DIY'er to work with and also for application as a storm window.

    Bookmark   September 26, 2009 at 9:50PM
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Thanks for your reply Oberon.

You answered a question you didn't know I had----I incorrectly thought the glass manufacturers made the IGUs that small shops sell. Thanks for making that clear.

I'm curious as to why you don't recommend argon for my use? If I find a good glass shop that I can trust, isn't that a small added-on cost with big results? Does argon not make sense for me given that I am not in a super-cold part of the country?

Does a high-SHGC make sense given that I don't mind some additional inside heat in the summer but would love any added heat I can get in the winter?

How do I specify to a glass shop that I want a high-SHGC? Do I just say "use a hard coat on side 3"?. Are SHGC and U-factor numbers even available for sheets of glass or is it just done for windows?

Thanks very much. It's starting to get cold here---29 F last night. My propane company loves me!


    Bookmark   September 28, 2009 at 4:39PM
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Hi Chris,

Some glass shops make their own IG's and some order them from IG manufacturers. So it could depend on the shop.

It isn't so much that I wasn't recommending argon, it was more that I was a bit concerned that if you were getting IG's manufactured by a mom and pop shop that (a) they may not offer argon or if they do, (b) their method of filling and sealing the IGU may or may not result in long term retention of the argon.

In your situation high SHGC does make sense.

If the IG is fabricated by a smaller shop it is very likely that they will be using a hardcoat LowE - hardcoats are primarily high SHGC.

If they order their IGU's from a IG fabricator then they should be able to specify a high SHGC from their supplier.

You can order the IGU with a hardcoat LowE to the exterior. You can then build the storm window with the LowE coating in the space between the existing window and the storm.

    Bookmark   September 29, 2009 at 7:55PM
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Thanks Oberon, I didn't know one could specify a coating on the "outside" of an IG unit. That's a good idea for storm windows.

My idea of using the big box to order replacement glass now won't work----they won't sell it unless I am specifically using it for one of their manufacturer's windows. I'm not---I'm making storm windows.

So, now the price of glass has doubled and tripled based on quotes from the mom & pop shops I've talked to (none of which seem to know much about coatings and IG).

So, I may be forced to use single-pane 1/4" glass. But, before I give up on IG, I want to poke around a bit more.

EfficientWindows.org (if I'm reading it correctly) says clear, single panes get 1.04 U-factor and double-pane clear is 0.50 That seems a big difference and that's without any coating. What they don't say is the thickness of the glass panes in either example. Is that likely to be 1/8"?


Their stated 0.50 U-factor is based on double-pane, clear with a 1/2" airspace. But again, they don't say the thickness of the glass.


My winter temps average 20's nighttime & 30's daytime. Since my old drafty home never gets warm in winter or hot in summer, what are the best thickness choices for an IG unit?

I was thinking 5/8" overall (two 1/8" lites with 3/8" airspace) but maybe another configuration is much better given performance & prices. Maybe 1/2" overall is good enough and much less expensive. What do you recommend?

I am gearing up for winter! I'm new to this area (I'm from Los Angeles where I also had an old, drafty home but never really noticed).

Thanks very much for all the great advice Oberon. You are a real asset to this forum.


    Bookmark   September 30, 2009 at 6:03PM
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Thanks Chris.

Glass thickness has little effect on overall energy performance. Note I didn't say that glass thickness has no effect, but the thickness difference is negligible in your application, so use 1/8" (also called double strength or 3mm) glass for your storms.

You also want to get as close to a 1/2" air space (not overall width) as possible. This is the optimum spacing for energy performance.

A 1/4" airspace is too thin, while 3/8" is acceptable, but it is not as good as 1/2".

One option is that you could build your own dual pane storms. This is not something that I would suggest for actual windows, but for storms it can be a workable solution.

Does that idea interest you?

    Bookmark   September 30, 2009 at 8:30PM
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Thanks for the very informative clarification on glass thickness and its effect on energy performance. Now I understand.

Make my own double panes? Now you're talking my language! I love do-it-yourself projects. I made my own casement windows once.

I had thought of taking a sheet of 1/8" glass and applying 1/2" thick neoprene self-adhesive foam insulation around the edges, put some silicone on-top of the neoprene and applying the second sheet of 1/8" glass. Might not prevent condensation though. You probably have a better idea.

I'd appreciate any suggestions on how to do this.

I wonder if the small glass shops sell single panes with hard-coat LowE. Even if I have to go without LowE, making my own dual-panes would still be a lot better than resorting to 1/4" single panes.


    Bookmark   September 30, 2009 at 9:12PM
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Follow-up from my last posting:

I've been researching IG units and now understand the basics of how they are made. It looks like you need expensive fabrication equipment to use aluminum spacer bars and hot melt sealants.

But, I also saw a "warm edge" foam product that incorporates desiccants and uses a corresponding silicone product as the sealant. It seems you can put these products together by hand. It doesn't look very difficult as long as you do it carefully. I'm thinking I could put the units together and then use some type of plastic or metal u-channel on the edges to finish them off.

But, I may be missing something and I'm not sure how to acquire the materials.

    Bookmark   October 2, 2009 at 2:27AM
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Although there are still many folks using aluminum spacers, most of the spacers on the market today would be considered low conductance or warm edge spacers. Aluminum doesn't fit into that category.

And while aluminum wouldn't be a horrible choice for your application, as you have seen, you really don't have the the equipment to use it anyway.

There are a couple of foam products available. You can find them at CRLaurence.com. CRL is the primary supplier of materials to the window and door industry in North America.

I would guess that pretty much every mom and pop shop in the North America buys supplies from them and the biggest companies use them as well. If there is a window or door accessory available, they have it. Heck, if it is for shower doors, jewelry cases, store display shelves, etc, etc, etc they are the primary supplier for all of those items as well.

And, they sell to consummers too.

Take a look at their catalog when you have a minute and you will see what they have for spacers and accessories.

If you were to use the foam spacer, you would not need to use anything else - no metal U channel or plastic to finish them off. The foam spacer with silicone fill wold be sufficient.

Another option, and the one that I had in mind, was building the storms so that you could take them apart as needed for cleaning if they happened to get a little moisture inside.

But, the foam spacer for a permanent IG is an option as well.

I would suggest looking into "professional" spacer systems on CRL site and see what you think. And then we can go from there.

    Bookmark   October 2, 2009 at 7:29AM
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Hi Oberon,

Thanks for the lead to CRL. I saw the Edgetech and Truseal non-metal foam spacers. It looks like either one of those foam seals and their silicone will cost around $2. per linear foot if those two things are all I need.

It didn't occur to me that you were thinking of my building double-pane storms that could be taken apart in case of condensation. I like that idea if you think I'd get comparable thermal performance to doing it with the foam IG seals. Did you have a specific idea in mind?

The U-channel idea I mentioned was to protect the edge of the glass so I could eliminate the need for wood frames. If I'm using two 1/8" lites + 1/2" space, it totals 3/4" thickness which is kind of thick to then add wood. I can't find a weatherproof 3/4" u-channel though. Do you know of anything that would be weatherproof and protect the edges of 3/4" IG? Maybe tape of some sort?

Thanks for all your help in this project Oberon.

    Bookmark   October 2, 2009 at 7:54PM
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I read the following posting on doityouself.com:

"IGU's also need to be surrounded by an air space, must sit on rubber setting blocks, and usually need to be shimmed around the perimeter with rubber setting blocks so as to cushion the glass and keep both panes from slipping past one another independantly. Believe it or not, that WILL happen with time if an IGU is left unsupported."

My new plan is to make frameless storm windows---no wood frames or sashes but just wrap the edges with a aluminum butyl rubber or foam tape (3" wide, UV, temp and water resistant and applied so it looks like a small sash even though it's just tape). By doing this, I can make the IGU as large as possible and simply the project as I don't have to make wood sashes. I'm thinking these thicker tapes will suffice in protecting the edges of the unit. The posting I copied above makes me concerned though. I was hoping to just put some neoprene weatherstripping on the wooden window stops and then set the IGU against that. Do I really need these "setting blocks" and air space on all sides? That posting makes it sound like I need to do this even if I do make wood frames.

    Bookmark   October 7, 2009 at 12:22PM
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I am curious with where this discussion ended up. Interested in doing something similar; Triple pane IGU as an interior storm behind historic double hung. Trying to learn why you couldn't use a framless IGU as fromhollywood was suggesting.

    Bookmark   October 16, 2012 at 9:32PM
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