Why do I have Insulation in internal wall between two rooms?

Question

I have been carrying out a bit of DIY in my bathroom, in a new wall which was put up with my loft conversion which happened of the winter. Inside this new wall, I've found a significant quantity of insulation (rockwall or something of the like).

Clearly this was put in for a reason. But this is in a wall separating my bathroom from my internal staircase to the loft, so there is unlikely to be much of a thermal gradient between these two rooms.

Can anyone tell me why this was put in? Thermal insulation seems irrelevant, unless maybe it's to increase the fire redundancy of the partition wall. Only other idea I had was sound insulation, but wouldn't expect it to have that much effect.

I'm in the UK in case that helps anyone.

Answer 1

It was probably installed to reduce sound transmission. It's common to see materials that are more dense than fiberglass used as acoustic insulation, but fiberglass is used as well.

The sounds of water running and being flushed, drawers being closed, and other... human activities are often something folks wish to reduce in areas designed for serenity, such as a loft.

Answer 2

Around the bathroom, it is most likely for noise isolation. Usually the material used in this application is different from standard fibreglass batts. I've used Roxul AFB in this way before. It is a rock based material, and the structure of it deadens transmission of sound. We didn't do actual acoustic tests, but anecdotally the stuff really works well to keep noise down.

It might also be worth pointing out you may find this material in other locations, as it is usually also a fire-retardant. We put it around kitchens, or in walls separating a garage from the main housing space. Also in every wall in commercial/office space, but I doubt that applies here.

Answer 3

Thermal insulation between bathroom and the rest of the house is very relevant.

Bathroom is a room which is expected to be sometimes used naked, contrary to the rest of the house. Which means that the temperature in bathroom is expected to be kept at a higher value than the rest of the house.

For example, Polish building code in requirements for heating systems: in "rooms meant for constant occupancy of humans without wearing overcoats" (eg: a living room) - "ability to maintain temperature at least +20°C", while in "rooms meant for undressing or for occupancy of naked humans" (eg: locker rooms or bathrooms) it's up to +24°C. That's 4°C warmer than the rest of the house. Note that the law doesn't tell you to actually keep it that warm, just that you could if you wish so. This example is not about requirements in a particular country, but about general expectations for a bathroom to "be able to be warmer than the rest of the house".

So, it's only reasonable to insulate a bathroom against the rest of the house, to avoid heating up adjoining rooms. Also, the bathroom heats up (eg from a long, hot shower) even if you don't meant it too. Having cold spots on walls would increase condensation there, leading to increased water damage.

Rockwool is not a sound insulation per se, but the empty space between drywall acts like a resonator - so filling it up with anything yields noticeable improvement. Filling it up with sand or concrete would insulate sounds even better, but the thermal performance would suffer. And would require tougher structure to cope with the extra weight.

Answer 4

Insulating internal walls is also useful for heat loss. Closing the door and heating vents in an unused space, like a guest bedroom, will cause that room to be minimally heated and cut the cu.ft. of space heated, and thus the heating bill. Insulating the surrounding walls will reduce the heat loss from the fully heated space to the minimally heated space. I built a house that way and it works quite well.