DIY Multi-layer glazing U-value?

MN Renovator · 03-04-13, 05:09 AM

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.

Thoughts?

03-04-13, 05:09 AM	#1
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	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. Thoughts? Last edited by MN Renovator; 03-04-13 at 05:12 AM.. Reason: Added credit to Topher Belknap for his similar project.