Movable window insulation

We are contemplating purchasing new double pane vinyl windows for many of the operable windows of our super-insulated straw bale home. For the south side, however, with its large expanse of glass, we plan to make our own non-operable windows out of two tempered glass panes ($4 each for approximately 3'x6' size recycled patio door glass) with movable quilted insulation in between to be used at night.

We have successfully used roman shade style (fan-fold) window quilts in our existing home that are easily constructed with two layers of fabric surrounding a synthetic batting quilt. These are quickly raised and lowered by pulling or releasing a string, insulating the original single pane glass. We have been generally pleased with their performance, although lacking seals at the bottom and sides, they suffer from significant convective heat loss. The usual solution to prevent convection is to seal the quilt edges well using magnetic strips, velcro, or flaps that squeeze the quilt against the window frame. Unfortunately, this not only complicates the construction but also the retracting of the quilt.

We plan to use a variation of these retractable quilts in our new home. South facing glass will be mounted as 2 panes sealed on the sides and bottom, held in place by strips of wood screwed into the jamb and sill. A rolled quilt assembly will be mounted on the top that can be lowered in between the two panes. The roller is 1.5" PVC pipe capped at each end and held in place by two pipe straps. Because of the tight wood-glass seal at the bottom and sides which contains a pool of still air between the glass panes, convective losses around the quilt should be reduced to a minimum. Radiant and conduction losses will be significantly reduced by the high R-value quilt (typically many times more resistant to heat flow than the glass panes). We have constructed a sample window with an inexpensive 2"x4" lumber frame for less than $35 per 33"x74" window. (Lumber 4 8'x2"x4" @$2, quilting $13, straps $1, caps $1, glass $8, deck screws, hardware, and cord less than $3, recycled wood lathe, free.)

A variation on this method, for smaller windows, would consist of placing 4" foam pads against double or single pane windows at night or during cold periods to improve window performance. (We use many of these 2-3 foot square foam pads for seating cushions and have been searching for a way to put them to use when they are not needed. In effect, the window area would become their night-time storage area.)

Probably the best book I've found about movable insulation is called Thermal Shutters & Shades (subtitled Over 100 Schemes For Reducing Heat LossThrough Windows), by William A. Shurcliff, copyrighted in 1980 and published by Brick House Publishing Co, 3 Main St, Andover MA, 01810, USA. ISBN 0-931790-08-5.

It's a nice 240-pager, with chapter headings as follows:
Introduction
How heat is lost through windows
Economics of shutters and shades
Considerations governing design of shutters and shades
Main insulating materials
Outdoor transparent and translucent insulating devices
Outdoor opaque shutters that are not permanently attached
Outdoor opaque shutters attached by hinges
Outdoor opaque shutters that slide to right or left
Outdoor opaque shutters of other types
Between-glazing-sheets devices
Indoor transparent and translucent devices
Indoor opaque shutters that are not permanently attached
Indoor opaque shutters that slide
Indoor opaque shutters hinged at side
Indoor opaque shutters hinged at top
Indoor opaque one-sheet roll-up shades and roll-up shutters
Indoor opaque multi-sheet roll-up shades
Indoor devices of other types
Devices for special applications
Conclusions
appendixes:
How to measure the heat-loss
Seals: some experimental results
Flow of energy from a hotter flat surface to a nearby cooler flat surface
Trade names of shutters and shades
Trade names of insulating plates and sheets
Trade names of glazing materials
Manufacturers of special adhering reflecting films
Automatic actuators for windows, shutters, and shades
Organizations and individuals involved
Master index of persons
Annotated bibliography

Revised March 28, 2000