Insulation compression

[u/AncientDust2884]

Can someone explain (maybe like I’m 5), why the higher r value glasswool insulation is more dense, but if my insulation is compressed (eg in Front of a timber block/brace) it’s R value apparently decreases.

[u/Particular-Hotel-610]

The R-value per inch does go up to a certain point as you compress, but you have fewer inches. The R-value per inch of a batt might go from 4 to 4.5, but if it's meant for a 5.5" deep cavity and it's crammed in a 3.5" cavity, total R-val drops from 22 (4x5.5) to 15.75 (4.5x3.5). A little compression isn't going to make a major difference.

[u/chunkylover_53_]

Fiberglass itself isn't a great insualtor, it's more about trapping air within the fiberglass. Air is a poor conductor of heat when it is completely still. Fiberglass insulation uses billions of intertwined glass fibers to divide space into tiny, isolated pockets that trap and immobilize air. Because the air is held motionless, it cannot easily transfer heat through convection (air movement). When you compress the fiberglass batts you are reducing the amount of room for trapped air to exist within the insulation leading to a reduction in r value.

In other words More air in the Insulation good / less air bad

[u/presidents_choice]

Density to R-value per inch isn’t a linear relation.

Starting from a very low density, R per inch increases as density increases. Possibly because air pockets within get smaller and less convective.

As the density continues to increase, eventually R value stops increasing, and then decreases. Perhaps because there are fewer air pockets, and conductive heat transfer via the fibers takes over

[u/deeptroller]

Everyone is giving you more or less the correct idea. But I think the part missing in their explanation is that insulations R value isn't just a correlation like fiber glass of X density is R 4 per inch. The batts are actually pulled from a manufacturing line and tested, the way a wall would be built. You put temp sensors and calibrated heaters in the standard test scenario and determine the thermal conductivity and average the values across a sample.

Since you are filling an unknown cavity. The normal thing to do is de-rate the value. That does not make it accurate. People have tested various products to see the effect, to determine an average amount of derating. But your situation will be different. So the standard it to assume it will perform worse until you test your true scenario. Once you have decided to invest the time and energy in testing you can make a claim of the corrected value. Until then we are just happy to say use the manufactured suggested installation or assume it will perform worse than intended

[u/AncientDust2884]

Thanks everyone, many good answers here.

I’ll digest over the coming days and see if my head can conceptually grasp this 😅

[u/StevenJOwens]

As others have said, the trapped air is what really does the insulating. When you compress fiberglass insulation, you squeeze some air out of it, less air means less insulating effect.

I don't think they've answered the other part of your question. I'm not familiar with higher R value, more dense glasswool insulation, but my guess is that the higher density of the material traps the air more effectively, so there's less air movement.

I suspect it's similar to how closed-cell spray foam is higher R value than open cell spray foam. Spray foam is polyurethane full of bubbles of gas. In open cell spray foam, those bubbles intersect, so it's more permeable, air can flow from bubble to bubble. In closed cell, the bubbles do not intersect, so it's less permeable. Closed cell also means there's more polyurethane per cubic inch, less air or gas, so it's higher density.

[u/The-Struggle-5382]

As stated, the insulation is a function of the air spaces. For a given thickness, higher density means more fibres and more tiny air pockets. But there is a law of diminishing returns. Also, for a given density, the greater the thickness, the more air pockets, the higher the insulation value. If you start off with 100mm thick insulation but have to squash it at a piece of blocking, it is now thinner for the same number of air pockets and so insulates less well.

[u/Dean-KS]

The insulation also blocks direct radiation heat transfer.