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Old 03-04-13, 06:09 AM   #1
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Default DIY Multi-layer glazing U-value?

Proposal: I'm developing a plan involving adding additional air gap layers to my current windows.

Assumptions/Existing knowledge: Sealed air gaps are convection breaks and separated air gaps of an appropriate separation distance add additional insulating value.

Resolution expected: Conductive and convective Heating and cooling load reduction. Air sealing of any gaps in gasketing or along sliding rails or casement joints(depending on the window).

Identified problems that will be fixed beyond expectations above: At least 4 windows have failed seals, three internal-facing seal leaks and one with an external facing seal leak as evidenced by winter moisture and summer moisture seen inside the area that should be sealed. I expect that this is reducing the U-value, possibly considerably.

Current air sealing situation and air sealing expected result: All window frame outer perimeters have been air sealed prior to planning this project. This internal added unit should seal all remaining window air transfer.

Current heating load situation(measured): 24563BTU/hr over 78 degrees F interior/exterior differential as of January 21 2013 performing a 9 hour no-sun load test. 314.9BTUhr/deg F. 2100 sq ft. ASHRAE design difference for my region = 81 degrees F.

Plan: Build a removable and low-cost internal facing rigid framed glazing/glazing equivalent, with reasonable 'it fits in' value, multi-layered glazing panel to add additional insulation with the unit gasketed and designed for easy removal and reinstallation.

Similar to: Cereal Windows PassiveHaus-compliant models (South Park reference in there). Also similar to the Topher Belknap insulated panel approach.

Expected cost: 10's of US$.

Question: What is a layer of air worth in insulating value? Is there a rule of thumb that 3/8" or 1/2" air gap is worth a certain value? Assuming worst case that a single pane of glass is 1 U and a double pane is .5 U(or low as .3), what should I figure? Would an inch total of 1/4" air gaps be better than an inch total of 1/3" air gaps. I figure that 1/2" is the thickest I would want per layer but I don't want an 2 inch thick assembly either. Is there a law of diminishing returns to diving the U-value, aka, would 4 layers not be .25 of a single layer of the same material? I'm basically trying to decide how thick of a unit to make and how many air gap layers to implement. I'm trying to aggressively drop my heating load and my glass is frigid to the touch and seems like an obvious place to target for heating load reduction.


Last edited by MN Renovator; 03-04-13 at 06:12 AM.. Reason: Added credit to Topher Belknap for his similar project.
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Old 03-04-13, 03:41 PM   #2
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I think that ~5/8" - 3/4" is the maximum R value? More than that just allows convection to start up within the space. And with air in there, it is about R2, if I remember correctly. In commercial windows they use argon to increase it to R2.5-3; though I may be off a bit.
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Old 03-04-13, 05:54 PM   #3
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I made panels just like you are saying when we lived in Michigan (~30 years ago). I made a wooden frame and covered both sides with heat shrink plastic and used cheap foam gasket on the sides to "press fit" it into the window frame.

The windows were already double pane, but the improvement was remarkable.

Since my first attempt, I learned that a very thin layer of air is just as good as a larger layer as the thin air layer attenuates convective flow. Convective flow is what kills the insulation value.

So, I tried building very thin panels with only 1/4 inch (4-5 mm) spacing. My problem was that the frame was not strong enough and would warp when I heat shrank the plastic. Thus it would not seal well in the window.

Argon has pros and cons. It eventually leaks out - even in expensive Pella and Marvin windows. The increase in R value with argon is real. A friend sealed his frames really well and filled each with dry nitrogen. I knew that nitrogen worked as in there was absolutely no frost at the bottom of the insert on sub zero mornings. But after a few months, frost began to appear (as the dry nitrogen leached out).

I was amazed how quickly these frame inserts could be built. I used a simple miter box, some hard wood base materials (got broken ones from hardware store they were throwing out), a staple gun, a roill of foam gasket material and the film (the most expensive part).

It took a bit of experimentation to get the measurements down as the foam must compress just a certain amount. Too much and it doesn't fit in. Too loose and the panels literally blow out in a wind. On some windows I had to use foil tape to keep the panels in .

In our bedroom, we actually used "bubble wrap" inside the frame and it outperformed everything.

Interestingly, the windows worked very well in summer as well.

Let us know how this works for you.

consulting on geothermal heating/cooling & rational energy use since 1990
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Old 03-12-13, 08:10 PM   #4
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My first house came with wood single pane windows. I wanted to make improvements without spending any money, so added extra panes. I carved out rabbets in the frames and set in a second pane of glass.

I found out real quick that the inside paint film needed lower permeability than the outside paint film after the space between the panes collected puddles in winter. Sealed the inside wood surface, and that space dried up.

I also found that, no matter how much effort I put into cleaning the glass, the result ended up looking at least a little bit foggy.

I tried putting four panes of glass into one double hung window. The air space was about 1/4" in that window. One of my sisters put a window quilt on the inside and the wood frame got condensation while the glass stayed dry. Then that heavy window got away from her and smashed her finger.

More air spaces will reduce heat loss even if the air gap is not optimal. And smashing your sister's finger is not good.
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Old 03-13-13, 10:15 AM   #5
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Thanks for the advice. My plan is pretty much stevehull is suggesting with, oddly enough, matching the 1/4" gap with 4 layers. I wasn't sure if going with 3 layers with 1/3" gap would be any better but it seems that if my frame isn't strong enough that it might make more sense. I'm also not exactly certain how I will build this with 4 layers. It seems to me that 2 layers is easy by building a frame and putting a layer of plastic on each side of the frame and putting it in place. I might try making two of these, total of 4 heat shrunk layers and spacing them appropriately to make 4 1/4" gaps(1st gap between a layer of plastic and the existing window with another layer between each plastic layer). If I include the double pane window that would be a total of 5 gaps.

I'm going to have to do some research on what woods are the least flexible and available in the appropriate sizes when shopping for lumber. Aesthetics as far as the wood goes aren't too important for me so I'm not picky on the color. The existing trim is oak. Thinking about it, I might just look through the millwork aisles for pieces that I could use since most of that is suitable for nailing and is the appropriate thickness for decent strength and looks good.

We'll see once I get a chance to take a look, maybe this weekend, hopefully I won't be as busy as last weekend.
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Old 04-15-13, 04:11 PM   #6
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Any updates on this? Sounds very interesting and I'm sure I'm not the only one who would like to see pictures.
Current project -
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Old 04-15-13, 05:18 PM   #7
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pictures would be great!
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Old 04-15-13, 05:54 PM   #8
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No updates. I had a tenant move in and they brought the full furnishings of the place they were already living into my furnished house. Need to get rid of couches and I have no workspace to build the frames. I have gotten a bit lost looking for what to use as wood frames. I was going to use whatever was cheap and dimensionally large enough to hold the stretched plastic but found out that the wood expense is higher than I'd like. Seems the best deal might be 1"x2" dimensional(3/4"x1.5" actual) 8 foot furring strips for 75 cents each. The trouble is that wood is racked terribly at the local lumber shops so not much of it is straight but I should be able to cherry pick for the good stuff.

I'm also not sure if I'm going to have the tenant wanting to open the windows or not, I prefer to leave them shut once our high dew point summer starts and opening the windows adds tons of moisture to the house so if she opens them, I might see an increase in cooling costs versus leaving them shut as the a/c tries to remove all that latent cooling load. Since the winter season is supposed to be over, I might focus my efforts on 90% solar blocking screen at this point to reduce the summer cooling load and return to this in the fall. This is also put on hold a bit more because I don't currently have access to a table saw, the person who has one currently has it tucked away in the shed for the winter so he can use the garage to keep the snow off the cars.
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Old 04-26-13, 01:23 AM   #9
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Building interior window insulation panels

All questions answered

I agree, sun block for the summer sounds like a good idea for the season.
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Old 04-26-13, 07:07 AM   #10
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Same link that I've posted in other threads and I thought I had it in this thread too. The problem is that doesn't answer all of my questions and actually brought out more. The wood is too large for having more than two layers of plastic. I'd like for 3 or 4 with the minimum depth possible. Furring strips seem to be a better option and the pre-primed ones that arttec used are not available for a good price at the places I've looked.

..but yes, his project is essentially what I'm looking to do but with improvements.

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