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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.

  • could it be an addition, so the wall was exterior? the older insulation type suggests the work was done some time ago. – dandavis Oct 16 '18 at 16:23
  • Sorry if I wasn't clear, this was a new internal wall, put up inside what was the bathroom, dividing the space into two; one bathroom one stair well. – Puffafish Oct 17 '18 at 7:27
  • There are UK fire safety rules for loft conversions, in terms of making sure that there's a safe way out of the house. We had to divide our open-plan living room to separate the hallway in order to comply -- it was that or have a ladder-type fire escape. I don't know whether this would come under the same thinking. – Roger Lipscombe Oct 17 '18 at 9:59
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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 is often something folks wish to reduce in areas designed for serenity, such as a loft.

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    When I built my latest home 20 years ago, I had the contractor install insulation in all the bedroom and bathroom walls to isolate and reduce sound transmission. – d.george Oct 16 '18 at 16:07
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    I guess you did but I was being a little more graphic LOL. – Ed Beal Oct 16 '18 at 16:35
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    My first house was a brand new one in 2002. The guest bathroom (and even the laundry room that backed up to it) was fully insulated even though it had no exterior walls – Machavity Oct 16 '18 at 20:06
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    It's regularly used for soundproofing, but sadly by people who don't actually understand soundproofing. Reducing sound transmission requires mass, and rockwool insulation is obviously not heavy. The real solution for soundproofing is multiple layers of heavier plasterboard, and make sure there are no acoustic paths from one side to the other. – Graham Oct 16 '18 at 20:17
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    Inexpensive solutions like rock wool do reduce high-pitched sounds like water and air noises and human speech quite a bit. It's the thumps and bumps that still get through unless acoustic isolation channel and/or mass are employed. – isherwood Oct 16 '18 at 20:26
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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.

  • Good to hear that the two main reasons I could think of (noise and fire retardancy) are sensible. – Puffafish Oct 17 '18 at 7:29
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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.

  • This can only be true if there's an independent heat source in the bathroom. – isherwood Oct 17 '18 at 20:01
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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.

  • Be careful when doing this -- you need to watch your duct static pressures when you close dampers in the heating system to avoid blower or HX issues in your furnace.... – ThreePhaseEel Dec 25 '18 at 5:29
  • So common knowledge goes. But closing off the register on a typical bedroom, say 10x13, is only 10% of a 1300 sq.ft. house and proportionally less on a larger house. There will be more variation in static pressure from how the flex duct is laid down by the builder from house to house than that. There is proportionally so much volume in areas that can’t be closed off (like the kitchen, living room, dining room, hallways, etc.) that the areas that can are generally numerical noise in the design and construction. – George Shaw Dec 25 '18 at 16:37
  • Of course, if you have an evil ductopus lurking in your attic, all bets are off ;) but some HVAC systems actually have a semblance of proper duct design.... – ThreePhaseEel Dec 25 '18 at 17:36
  • There can be a large gap between design and construction, like tight turns (300 degrees in one case here) in flex duct. Poor practices ruin good designs all the time. – George Shaw Dec 27 '18 at 0:58
  • Well, proper duct design and construction -- which includes "don't use flex" ;) – ThreePhaseEel Dec 27 '18 at 1:14

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