Condensation on mdpe pipe in kitchen

[ectoplasmosis]

You'll be gaining a dry pipe!

 

Don't know why it's so difficult to grasp... Just fit the pipe lagging, and stuff a load of mineral wool into the hole until it's flush with the pipe lagging, job done!

[andyscotland]

The air under your house will also be slightly warmer than outside (because it is sheltered and being heated by the house above) and will still have humidity (from the house, and from the ground, and from the outside air).

 

At this time of year, the water pipe itself will likely be colder when water is flowing.

 

So you will potentially get condensation on the MDPE anywhere it is not lagged, but particularly as it gets closer to the house air as the air temperature will rise but the pipe will be the same.

 

With your ideas (taping foil over the top / putting PIR over the top) you are allowing that condensation to form at the top of your floor surface but attempting to seal it there. Hence why your foil was wet underneath. This will eventually be bad for the flooring/structure of the house because that is not designed to be permanently damp.

 

With the pipe lagging solution that everyone else is suggesting, you are *preventing* the warmer air from touching the pipe, so it will not condense in that area.

 

Even if this is only a short length down to just below your joists, that is enough to stop condensation forming where it's a problem.

 

You might still get condensation on the section below the lagging, but this will either evaporate off (due to your subfloor ventilation) or drip onto the solum and eventually evaporate from there. Neither of those is a problem - the solum is expected to be slightly damp at times and the house is designed to accommodate that.

[gambo]

On 22/10/2025 at 12:49, ectoplasmosis said:

You'll be gaining a dry pipe!

 

Don't know why it's so difficult to grasp... Just fit the pipe lagging, and stuff a load of mineral wool into the hole until it's flush with the pipe lagging, job done!

It's not difficul to graspz but i see gaps in what some suggest. I don't see how filling the hole with mineral wool can be any better than covering it with foil+ PIR. Im trying to understand the whole process of forming the condensation to make sure Im bit causing bigger issue later on.

 

[gambo]

@andyscotland wow! That's a great explanation!! Thank you! I'll keep it wrapped (did it a few days ago) a bit longer and will unwrap it just before covering with units to see where and how much if condensation is there, and will give an update.

[JohnMo]

9 hours ago, gambo said:

That's a great explanation!! Thank you! I'll keep it wrapped (did it a few days ago) a bit longer and will unwrap it just before covering with units to see where and how much if condensation is there, and will give an update.

Why, just get some pipe insulation on there - job done.

 

See no advantage making stuff damp for the sake of it.

[andyscotland]

11 hours ago, gambo said:

@andyscotland wow! That's a great explanation!! Thank you! I'll keep it wrapped (did it a few days ago) a bit longer and will unwrap it just before covering with units to see where and how much if condensation is there, and will give an update.

 

Condensation in buildings varies significantly over time depending on outside and inside environmental conditions (temperature & humidity) and the difference between them - and in this case also the temperature of the outside water supply and the amount of time that water is flowing/not flowing, since that will control the surface temp of the pipe.

 

Small differences in any of those factors can be enough to make condensation appear or not at any particular moment.

 

I'd be surprised if you'd get enough evidence over a few days/weeks to understand the long-term performance of that junction.

 

Whereas there's plenty of evidence that the usual pipe lagging solution will work.

 

11 hours ago, gambo said:

I don't see how filling the hole with mineral wool can be any better than covering it with foil+ PIR. Im trying to understand the whole process of forming the condensation to make sure Im bit causing bigger issue later on.

 

Because the mineral wool is breathable, allowing a temperature and humidity gradient across it, so the condensation doesn't form at that interface.

 

Big condensation issues in buildings are almost always caused by constructions that trap moist air in the wrong places.

[gambo]

But mineral wool will slow down the process of transferring moisture from one side to another, correct? Will this not cause an issue long term (build up of moisture at the bottom side)?

 

My other thread was going to be about usage of the foil below mineral wool in the loft but I guess I would need a very well specified mvhr to avoid build up of moisture beneath the foil.

[Nickfromwales]

59 minutes ago, gambo said:

But mineral wool will slow down the process of transferring moisture from one side to another, correct? Will this not cause an issue long term (build up of moisture at the bottom side)?

 

My other thread was going to be about usage of the foil below mineral wool in the loft but I guess I would need a very well specified mvhr to avoid build up of moisture beneath the foil.

I’m coming to your house and I’m confiscating all of your foil.

[andyscotland]

11 hours ago, gambo said:

But mineral wool will slow down the process of transferring moisture from one side to another, correct? Will this not cause an issue long term (build up of moisture at the bottom side)?

 

All air contains moisture(*). What matters is the combination of how much moisture there is and how hot the air is. The warmer the air, the more moisture it can carry.

 

If slightly warmer air is next to slightly colder air, it's easier for the moisture to stay as moisture and move to the warmer air, so everything stays in balance.

 

Think of it like a big wedge made of toilet paper, the "cold" end is a single sheet thick, the "warm" end is several sheets thick.

 

If you slowly pour water on the thin end, the paper will start to soak it up until it is saturated. If you pour more water on, the paper will draw it across to the thicker pile (even though that means the water is flowing upwards!). Only when all the paper is saturated will you start to see liquid water lying on top/overflowing the edges.

 

But if slightly warmer air suddenly touches something cold, the air right next to the cold thing will be much colder and the moisture will have to condense into water.

 

(Here imagine you rapidly press down with your finger on a wet bit of the thick toilet paper, so it is suddenly only the thickness of the "cold" end. If there is still some dry paper, you'll see it get wet. If all the paper is wet you'll see water leaking out)

 

The moisture in the air under your house will mostly come from three places: the outside air, evaporation from the solum, and warm moist air leaking from your house.

 

The outside air will be cold but by definition not be saturated (unless you are literally inside a rain cloud).

 

So the air will be able to absorb some of the house moisture even as the house air cools.

 

The solum (generally the coldest surface) then acts as a kind of buffer.

 

If the air under the floor gets saturated (because it is cold and/or humid outside, or the house is humid inside), water will condense onto the solum and soak in making it damp. Nothing will evaporate from the solum because there is nowhere for it to go.

 

But then on days when it is warmer/drier/you've had the windows open in the house, the air will not be saturated so the moisture in the solum will be able to evaporate.

 

So - assuming your ventilation etc is as it should be - over time and across the seasons you will get a variation in dampness down there but it will balance itself out.

 

* Well, not air in deserts or up mountains, but...

 

11 hours ago, gambo said:

My other thread was going to be about usage of the foil below mineral wool in the loft but I guess I would need a very well specified mvhr to avoid build up of moisture beneath the foil.

 

If you use foil just below the wool, it will do very little because moisture can still get round it and will go to the places that are weakest.

 

If you use foil across the whole loft (under the wool and over the joists) it will trap moisture at the top of the joists where they are cold and that will cause problems.

 

You either need a fully sealed and continuous vapour barrier below the whole loft construction (e.g. between joists and ceiling) or you need to design the construction to breathe and then ventilate the loft, which is the standard detail for loft insulation in existing construction.

[Nickfromwales]

14 minutes ago, andyscotland said:

 

All air contains moisture(*). What matters is the combination of how much moisture there is and how hot the air is. The warmer the air, the more moisture it can carry.

 

If slightly warmer air is next to slightly colder air, it's easier for the moisture to stay as moisture and move to the warmer air, so everything stays in balance.

 

Think of it like a big wedge made of toilet paper, the "cold" end is a single sheet thick, the "warm" end is several sheets thick.

 

If you slowly pour water on the thin end, the paper will start to soak it up until it is saturated. If you pour more water on, the paper will draw it across to the thicker pile (even though that means the water is flowing upwards!). Only when all the paper is saturated will you start to see liquid water lying on top/overflowing the edges.

 

But if slightly warmer air suddenly touches something cold, the air right next to the cold thing will be much colder and the moisture will have to condense into water.

 

(Here imagine you rapidly press down with your finger on a wet bit of the thick toilet paper, so it is suddenly only the thickness of the "cold" end. If there is still some dry paper, you'll see it get wet. If all the paper is wet you'll see water leaking out)

 

The moisture in the air under your house will mostly come from three places: the outside air, evaporation from the solum, and warm moist air leaking from your house.

 

The outside air will be cold but by definition not be saturated (unless you are literally inside a rain cloud).

 

So the air will be able to absorb some of the house moisture even as the house air cools.

 

The solum (generally the coldest surface) then acts as a kind of buffer.

 

If the air under the floor gets saturated (because it is cold and/or humid outside, or the house is humid inside), water will condense onto the solum and soak in making it damp. Nothing will evaporate from the solum because there is nowhere for it to go.

 

But then on days when it is warmer/drier/you've had the windows open in the house, the air will not be saturated so the moisture in the solum will be able to evaporate.

 

So - assuming your ventilation etc is as it should be - over time and across the seasons you will get a variation in dampness down there but it will balance itself out.

 

* Well, not air in deserts or up mountains, but...

 

 

If you use foil just below the wool, it will do very little because moisture can still get round it and will go to the places that are weakest.

 

If you use foil across the whole loft (under the wool and over the joists) it will trap moisture at the top of the joists where they are cold and that will cause problems.

 

You either need a fully sealed and continuous vapour barrier below the whole loft construction (e.g. between joists and ceiling) or you need to design the construction to breathe and then ventilate the loft, which is the standard detail for loft insulation in existing construction.

Amen. 
 

My tired thumbs were NOT looking forward to typing out similar.

 

Beers on me. 🍻 

[gambo]

I've now removed rockwool mineral wool from the hole and the pipe insulation from mdpe. The mineral wool is wet at the bottom. Not very wet but enough to assume that it may get quite wet if left for longer. I know it's breathable but will this not cause an issue if it attracts condensation quicker than it breaths? I wonder if just leaving it open is more sensible. I can cover the pipe partially as the whole and angle of the pipe doesn't allow to push it all the way to the ducting.

[JohnMo]

Are you sure you haven't got a leak? 

 

If you aren't drawing water off, you won't get condensation forming normally. So overnight it should be perfectly dry. If wet you have a leak at one of the joints or at the valve stem. Most days it should remain dry all the time .

[Nickfromwales]

Assuming this is a continuous piece of MDPE to the stopcock, the only place a dribble could be occurring and going unnoticed is from that ungodly stopcock which is brass compression onto the MDPE. I hate them, as the least amount of excuse you provide will result in this failing.

 

I always go MDPE > copper via an MDPE fitting which is specific for this job.

 

@gambo are you 1000% sure this MDPE > brass connection is 100% leak-free? 

[gambo]

Yes, unfortunately there is no leak at stopcock. I've checked it in number of ways. 

I can feel and smell damp further down and it smells like when you are inside bristol-bath two tunnels path.

 

 

[Mr Punter]

I would hoover out as much crap as I could then slide some pipe insulation down as far as it will go.  That will stop the condensation.  A bit naughty using gas ducting but it could be worse.  

[Kelvin]

What’s at the other end of the duct? Could water have got into it from outside? One of my ducts filled up with water before I sealed and buried it as rain was running down the cable that ran into it. I had to make up an extension for my hoover and suck it all out. 

[Mr Punter]

14 minutes ago, Kelvin said:

What’s at the other end of the duct? Could water have got into it from outside? One of my ducts filled up with water before I sealed and buried it as rain was running down the cable that ran into it. I had to make up an extension for my hoover and suck it all out. 

 

It is perforated gas duct.

[gambo]

It's late 80s house build, I think, by David wilson. This has ducting is not the worse I've seen during the renovation. 

1 hour ago, Kelvin said:

What’s at the other end of the duct? Could water have got into it from outside? One of my ducts filled up with water before I sealed and buried it as rain was running down the cable that ran into it. I had to make up an extension for my hoover and suck it all out. 

The other end is probably around the water meter 12m from this place and around 4m from the fron of the house, although I can't see it due to an amount of mess/soil around the meter. I spent 1hr digging and removing the soil just to be able to turn off water.

 

If the condensation is from either some sort of leak (would need to be very small if the meter doesn't move when turned off) or rain/soil then isn't it better to let it escape by not pushing the pipe insulation to deep?

If I put pipe insulation that doesn't go inside the hole than that covered pipe doesn't attract condensation and it just sits and breath on the pipe inside the hole.