Has anyone tried cardboard as insulation?

Hey guys, as the title says; has anyone tried cardboard as insulation?

According to this information cardboard has a pretty impressive R-value!

Phenolic rigid panel                                                  R-4–R-5
Urea-formaldehyde foam                                          R-4–R-4.6
High-density fiberglass batts                                   R-3.6–R-5
Extruded expanded polystyrene (XPS) low-density   R-3.6–R-4.7
Icynene loose-fill (pour fill)                                        R-4
Molded expanded polystyrene (EPS) high-density      R-4.2
Home Foam                                                              R-3.9
Rice hulls                                                                   R-3.0
Fiberglass batts                                                       R-3.1–R-4.3
Cotton batts (Blue Jean insulation)                            R-3.7
Molded expanded polystyrene (EPS) low-density        R-3.85
Icynene spray                                                            R-3.6
Open-cell polyurethane spray foam                           R-3.6
Cardboard                                                              R-3–R-4
Rock and slag wool batts                                          R-3–R-3.85
Cellulose loose-fill                                                    R-3–R-3.8
Cellulose wet-spray                                                    R-3–R-3.8
Rock and slag wool loose-fill                                        R-2.5–R-3.7
Fiberglass loose-fill                                                    R-2.5–R-3.7
Polyethylene foam                                                    R-3


I was planning on building with wood, using cardboard in the frame cavities and sealing the inside and the outside of the frame with plastic sheeting before applying the interior and exterior siding. Do you think that would work well?

If its cold enough for condensation to occur, cardboard will be soak in water in no time. BUT you have one option. That is to seal the cardboard from outside air completely. a pinhole will render it useless. Once cardboard is wet it losses its insulating property. Condensation occurs when hot air in the house meets cool surface or cooled air.

I could be wrong; But I believe you want a breathable outer barrier like "Tyvek" then on the inside you want plastic but it must be sealed to all outlets or transfer of heat and moisture will occur.

Homeless folks have been using cardboard for years.  Old homes around here built in the 1950's, commonly had cardboard tacked inside and then sawdust or wood chips to fill it up.

Certainly not the best solution but for dirt poor folks anything was better than nothing.

Would I use it today in a new home?  No way...

On a big scale ->

The issue with cardboard is that it is a great substrate for mould to grow on.  If you sandwich it between two layers of plastic, you'll create a sealed, moist environment that will cause rot and mould, so you would want Tyvek on the outside as Thomas says.  Cellulose insulation is treated with an anti-mould solution.

If you're in a warm environment, you could leave off the vapour barrier on the inside and use a dehumidifier to keep it dry, but that'll use a lot of energy.  Failing that, use tyvek on the outside, plastic on the inside, make sure all corrugated cardboard runs horizontal and keep it dry.  

I love the idea, but it's a lot of work to build even a small building, so I'm not sure I'd want to take the chance that the use of cardboard for insulation would cause problems down the road.  If it does, you'd need to rip it all out and you'll probably have rot and mould at that point.  

Personally, I wouldn't use it.  I'm a civil engineer with a background in building, building science and mould and I'm not sure I could make it work with any certainty.

julian Gerona wrote:If its cold enough for condensation to occur, cardboard will be soak in water in no time. BUT you have one option. That is to seal the cardboard from outside air completely. a pinhole will render it useless. Once cardboard is wet it losses its insulating property. Condensation occurs when hot air in the house meets cool surface or cooled air.

Sorry need to correct this one. Outside should be inside. That is as long as no air from inside the house comes in contact with cardboard you will fine. You can use breathable outer barrier but a completely sealed cardboard space is better in insulating as you can create a partial vacuum although more difficult to construct.

What about corrugated plastic? I took a bunch of corrugated plastic boxes from a store that was tossing them flattened them back out and used in the windows of my r.v,its helped a lot plus its water proof,and still insulates well,so mold shouldn't be a problem

The mice and squirrels will love your cardboard walls.
Lots of insects too.

Others have mentioned moisture and mold, I also wonder about resistance to fire and pests.

I don't think cardboard is a "good" insulator in reality even if its R-value looks good on paper. I've had similar thoughts about bread actually!

Treating the cardboard with borax could address the mold, insect and rodent issue.

Using corrugated plastic that you remove from the waste stream sounds like a double win. It would likely not be the best you could do per inch, but if you have the space I would say go for it. It would not be fireproof, but neither are some other materials. Protection would be called for and common sheetrock should help.

I think layers of corragated plastic would hold moisture,and thus support growth of mold and mildew.
For me its too useful to stick in a wall:

As long as the inner layer of corrugated plastic was unbroken, I think it would keep moisture from getting into the wall assembly.

I agree that if you have a use for roofing or enclosure panels, those would be a higher use than just insulation.

I've built furniture with cardboard, but never thought about using it as insulation. Hmmmm. This is why I love cardboard! But I love this idea even more: growing your own insulation! Have you heard of mycelium insulation? It's made from the fungal roots of mushrooms. It has the same R-value as fibreglass, and is much more fireproof than cardboard.

Yes, essential to seal the cardboard from the risk of condensation, unless you have a ventilation gap etc.

On a related note, A few years ago, I was interested in making insulated panels to build metal frame & mgo board Hexayurts from 'Ecobricks'  so experimented with 'Making an Insulated Eco Brick by Filling Empty Plastic Bottles with Rolled Cardboard or Plastic & Evacuating Air (VIB))' which was tough as I could not make a strong enough matrix out of the cardboard filler to resist the outside air pressure.

I then moved onto helping the bricks with filling bottles with smaller bottles / cardboard & with expanding foam - Pics at my blog link)    https://cabinznet.blogspot.com/2018/09/making-better-insulated-eco-brick-by.html

THis lead later to a more adventurous bottles and cardboard insulated panels in  a 3d Weave fabric   https://cabinznet.blogspot.com/2022/07/more-thoughts-on-reusing-plastic.html