No insulation under the water tanks

[Steve059]

My house is a 1991 5 bed detached. It has concrete roof tiles with sarking under and factory made roof trusses. The header tanks for domestic hot water and the gas fired central heating are installed on a 4 foot square platform, about 18" above the ceiling joists. All this is as originally built. I have increased the loft insulation to the currently recomended level, except under the platform where there is none. Soffit ventillation has been maintained.

The ceiling (artexed) under the tank platform now has a beige tinge to it, caused presumably because the loft is now colder than before the insulation was increased, and damp is condensing in the plasterboard in this area. A max min thermometer showed the loft space got down to zero celcius last week when it was -3 outside.

Options which have occured to me so far are:

1. Build a box out of polystyrene slabs covering both tanks, the platform and the adjacent pipework, sealed to the ceiling, and bring the loft insulation tight up against it, so the temperature inside the box can rise due to the warm rooms below, but the temperature in the general loft area can go right down to exterior.

2. Insulate under the tank platform to the same standard as the rest of the loft and arrange some trace heating to stop the tanks and pipework from freezing.

I prefer the first idea but I don't know what conditions would be like inside the box. High humidity perhaps leading to mould? And what would happen when it is 35 degrees up there in summer?

Has anyone tackled this problem or got any better ideas?

 

[JohnD]

I've had loft freezes and bursts, but it has always been a pipe, never the tank. Such a large mass of water takes a long time to chill to freezing, and unless the house is empty, it will periodically have non-freezing water refilling it. My loft is also felted under the slates, so it doesn't get icy winds or snow blowing between the slates (have seen this in old houses).

My loft tank is also on a platform, and I am happy to insulate under it. If the pipes can be insulated and tucked under the loft roll, they will pick up a tiny amount of heat from the ceiling.

If I lived in a cold area, I might put a tubular heater up there. I have one by the incoming watermain in the unheated garage, with a stat set to 5C. It seldom gets that cold here.

 

[alan333]

Go with option 3. Leave it with no insulation under the tank and allow a marginal amount of heat to get to the base of the tank. As a side note when I got a report back after getting cavity wall insulation done, the report said that if I added the extra thickness of loft insulation it would've saved a whopping seven quid per year. Not worth bothering about within reason.

Btw my home is similar ie a new build approx year 2000

 

[Steve059]

Thanks for your quick responses.

John - I get the point about the tanks themselves not freezing. The pipes are my main worry. They are well insulated, but they are well above the joists and lots of free space around them. My concern is, if it got down to zero when -3 outside while 16 sq feet of uninsulated ceiling was keeping the chill off, how low will it go if -10 outside and 270mm of insulation everywhere?

The loft space is vast. 9 feet high in the centre, 22 feet wide by 33 feet long. I'm not sure a tube heater would make a lot of difference.

I've used trace heating cable under pipe insulation in an industrial application, but I'm reluctant to go that way, its another system to go wrong and its out of sight out of mind.

Decisions decisions.....

 

[foxhole]

While the tank may not freeze the water will be considerably colder entering the pipework making frozen pipes more likely hence tanks are rarely insulated underneath, except by misguided diyers.

 

[JohnD]

The header tanks for domestic hot water and the gas fired central heating are installed on a 4 foot square platform, about 18" above the ceiling joists.

 

[Steve059]

Thank you all for your posts. Still unanswered though is the question:
What do I do to stop condensation tainting the ceiling under the tank platform?

 

[JohnD]

I say insulate it.

The waste heat, because of the large loft and the airgap beneath the tank platform, will not have much effect on the tank.

The tank will tend to stay at the average day/night loft temperature of the previous few days.

Pipes lose their heat very fast.