The Cardboard Shutter

Based on how popular the simple Bubble Wrap insulation for windows is, I thought I would add this cardboard shutter as suggested in the book "Movable Insulation" by Langdon (a great book). 


The idea is cut out several pieces of cardboard to fit the window, and stack them together.  The edges can be taped with duct tape to hold the whole stack together.  Some spots of glue here and there between the layers might also help.  The book suggests applying a layer of thick aluminum foil to both faces of the cardboard panel -- the improves the thermal performance substantially, and also makes the shutter more fire resistant.


Ideally you should mount the cardboard shutter such that any gaps around the edges are small, and spaced a little ways from the window to leave an air gap. 


I've picked up large pieces of cardboard from our local kitchen appliance store -- they were quite happy to part with them.


In my climate (cold), the saving for this scheme on a 3 ft by 5 ft window would be:


Single Glazed:

Using the Insulation Upgrade Calculator,

    Current R value = 1 (single glazed window)

    New R value = 1 + (1 air films + 1 cardboard + 2 foil) = R5

    Propane fuel at $2.2 per gallon with an 80% efficient furnace


The R4 for the cardboard shutter is roughly based on:  The air films on either side of the cardboard add R1 (like any inside storm window) + the cardboard itself -- say half an inch at R2(?) per inch adds R1 + the R value associated with alum foil on the inside and outside adds R2.

The aluminum foil on the inside reflects IR heat from the room back into the room, and the aluminum foil coating on the outside acts as a low emissivity surface to reduce heat radiation to the outside.


Insulation Upgrade Calculator for Cardboard Shutter:


$66 in one year!!   And, 400 lbs of green house gas saving !!


The saving for a double glazed window is $27 with a 170 lb CO2 saving.


This should be about one week payback on the cost of the duct tape and aluminum foil!


So, even if you plan to do something fancier for "next winter", you can do this right now, save some money and greenhouse gas, and then recycle the cardboard when you go to fancier window treatments.


If you give this a try, please email me a picture -- Gary.



Note: I got one comment from Dave in the UK to the effect that if even a small gap is left between the window and the insulating shutter, that he got a lot of condensation.   This probably has a lot to do with climate and whether the existing window is double pane or not.  High indoor humidity and single glazing will aggravate the condensation problem.

In our dry climate and with double glazed windows, we rarely if ever experience condensation on the windows.

At any rate, if you do get a lot of condensation, then you should do something about it, as the condensation can cause damage over time.


Further Note:  Dave came through with a calculator to tell how likely you are to have a condensation problem with a loose fitting shutter.  It depends on outside temperature, inside humidity, and the R values of the window and shutter.  Very helpful for determining if you are likely to have a condensation problem or not -- Thank You Dave!




The "Movable Insulation" book by William Langdon is a real classic -- it has hundreds of ideas for using movable insulation in ways that dramatically improve thermal performance -- highly recommended.

It was published in 1980 by Rodale.  Used copies can be found on for about $5, it has also recently gone back into print via an internet publisher.






Gary 2/7/07

Updated July 20, 2009