How to insulate a flat roof covered with tar paper?

Hi, there is something like polystyrene, you can put it directly on the existing roofing felt, and on this styrofoam, the undercoat is also heated and then the top felt.

A colleague Jacek asked about thermal modernization with wool. But Peter is a good suggestion. It is possible to perform thermal modernization with the use of polystyrene. It will be possible to lay it only after assessing the technical condition of the existing covering, because if it is badly “worn”, it has leaks, bubbles, etc., it may turn out that the old covering needs to be torn off. A good roofer will be able to judge this.

In any case, polystyrene is also a good solution for flat roofs, max slope up to 20%, if the cover of the old roofing felt is ok, then you can put it directly on the old roofing (you will save on waste disposal), remember about appropriate dowel to the ground.

On top of it, or immediately, a surface heat-sealable roofing felt, or a primer + topcoat. There are many producers on the market, so you can choose the color.

first and foremost - make sure you do a cavity against condensation (space between the roof and the insulation structure), use several different materials, not one

It all depends on your budget and what end result you are interested in. Styrofoam will certainly be much cheaper than wool. Styrofoam doesn't breathe and wool does. You can also think about the possibility of making a roof with a membrane in the future in order to make the roof of the terrace. But it's an additional hint about the possibilities, maybe you don't need it at all. The cheapest polystyrene is important to choose the right thickness so that the dew point is in the polystyrene and pay attention to the hardness of the polystyrene offered, because it may cause discrepancies in the price. And the apparent economy may cause you to make holes when walking on the roof. And do not skimp on the top paper must be good.

In order to insulate a flat roof covered with roofing felt with rock wool, it is important to follow the appropriate technological steps. First, the roof must be thoroughly cleaned of any residue and primed. Then, a layer of rock wool insulation can be glued to the existing covering, using the appropriate adhesive for this purpose. After insulating, it is possible to lay thermal roofing felt on the roof again. It is also important to ensure proper bonding of the insulating layers and correct sealing of the joints to prevent leaks and insulate the roof as best as possible. It is not always necessary to remove the entire roofing felt, but the condition of the existing covering and any damage should be carefully assessed before starting repair work.