This is for a house in Portugal, where most houses have no insulation. I want to insulate the house (it is quite cold in the winter). The plan is to lay down some isolation material on the floor of the attic. The attic is completely isolated from the rest of the house. The only way to get to it is from the roof. The floor is very thick concrete (80 cm of concrete).

The range of insulation product is fairly limited, because very few people isolate their house in Portugal. I basically have the following choices:

  • glass wool
  • rock wool
  • synthetic material consisting of several layers of aluminium, polyester and polyethylene

Glass wool is out because rock wool appears superior in all respects. Rock wool is great because it is fire retardant, but I worry that it might cause allergies to people living in the house (I am not worried about the allergy during installation, only for the people using the house). The synthetic material seems great, but it is very flammable. Insulation quality is not a criterion because it is comparable.

So I basically have to decide on allergy versus fire retardant properties.

Given the very thick slab of concrete, is it reasonable to not worry about the flammable properties of the synthetic product? Maybe I can consider that the concrete will be protection enough? (it is a country where fires are common, due to high temperatures).

For allergies with rock wool, can anyone speak to that risk? Does the fact that the attic is completely closed from the rest of the house, with no window prevent the risk of allergy?

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    I don't understand disregarding fiberglass because there's something better. It's the most commonly installed attic insulation in my northern climate (to R-50 or better).
    – isherwood
    Feb 17, 2021 at 22:20
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    I suggest limiting your question to that of allergens escaping the attic. "Speaking to risk" is a subjective discussion and not suited for DIYSE.
    – isherwood
    Feb 17, 2021 at 22:22
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    Doesn't affect my answer, but are you sure about 80 cm? That is 31 inches - we're talking bomb shelter or bank vault thickness. And an online calculator says that a 200 square foot (which would be a pretty small house) 80 cm thick concrete slab would weigh 19.5 tons! That is a lot. Maybe 8 cm? Feb 18, 2021 at 3:37
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    I am not entirely sure, no. I know for sure the walls are 80 cm thick, and I assumed the floor therefore would be as thick. Is that an incorrect inference ?
    – DevShark
    Feb 18, 2021 at 7:16
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    @FreeMan There is a lot of misinformation, partial information, urban myths, etc. out there about so many things. Has been for a long time - the internet has only accelerated it. I think it is to the credit of the OP that they asked specific questions in a reasonably reliable site rather than relying on hearsay. I'm sure they're not the only one out there unclear about the difference between allergies and irritants and so forth. Feb 18, 2021 at 15:06

1 Answer 1


TL;DR Don't worry, be happy, use the rock wool.

I am NOT an expert on insulation, so if anyone has better/more complete information, I will defer to them.

As a general rule, allergies have to do with specific organic materials - e.g., pollen - that cause an actual allergic reaction. This is a specific medical term and, as I understand it, allergies are not normally to minerals, synthetic fibers, etc. In other words, I doubt (but I could be wrong) that rock wool can actually cause an allergic reaction.

On the other hand, there are other types of irritants which can cause rashes (contact dermatitis), lung problems (asbestos being the classic example) and other symptoms, many of them quite serious, that are similar to allergies in certain respects. But they are not allergies.

My gut feeling is that none of these 3 choices (fiberglass, rock wool, synthetic insulation) will cause a true allergy, with the possible exception of outgassing of some chemicals used in production of the material. Such outgassing would be, in my opinion, a total non-issue for residents of a building if the materials are installed in a non-accessed attic. Any outgassing would escape through attic vents and dissipate into the atmosphere at very low levels.

Similarly, any particulate matter that comes out from any of these materials would be essentially indistinguishable from ordinary dust and other particles in the air from a variety of sources - soil, automobile exhaust, fireplaces, etc. In addition, properly installed insulation should have almost no loose particulate matter except as part of the initial installation. That initial installation should include cleanup of any loose particles (e.g., with a shop vacuum). The only exception would be if you used loose-fill (e.g., blown in) insulation, which would not be a good idea with these concerns, and in fact is probably best left to totally concealed spaces such as post-construction fill in external walls.

Fire is an issue. Attics get hot. Sometimes very hot. If (I haven't checked the specs) rock wool is significantly more fire retardant than other materials, use the rock wool. While the concrete should provide a lot of protection - hopefully enough for everyone to exit the building safely in the event of an attic fire - there could still be extensive (and expensive) damage. Best to avoid adding fuel to the fire.

But probably every one of these items has a Prop. 65 warning.

  • 2
    According to the State of California, everything is bad for you, so please stop eating, drinking, and breathing - they'll all kill you
    – FreeMan
    Feb 18, 2021 at 13:08
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    Lol, probably true. Life is a lethal sexually transmitted disease after all. Terrible idea to perpetuate it.
    – DevShark
    Feb 18, 2021 at 18:19

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