Interior paint on exterior walls

[Peasea]

Hi there

I have an outbuilding which I suspect the dud builders had used interior paint on the exterior walls. It’s a double blocked structure with cavity insulation, but I’ve noticed that there has been some damp in the room and humidity is about 77% when I go into the room in the morning. The outbuilding is just over a year old.

Is it possible that rain is somehow penetrating the walls and making inside damp? Ideally RH in that room needs to be 55% or is that wishful thinking for an end of garden outbuilding that doesn’t have any water supply connected to it?

 

[DIY_Doctor]

Where is the damp?

My block outbuilding with insulation would have RH between 60-80% in the winter or when it rained, but in the summer its 40-50% so possible. Good ventilation is the key.

 

[opps]

I think you need to be more precise about the building materials used.

I built my garage with thermalite blocks, I spoke to the manufacturers, they told me that the blocks are waterproof but warned me that the mortar could potentially allow some moisture to be sucked in.

At the time I wasn't overly concerned because I knew that it would eventually be rendered (and painted).

If regular emulsion had been used, after a year, I would expect it to be in a pretty poor state.

 

[JohnD]

Tape some clingfilm or other clear plastic tightly to the walls and floor. If water appears on the room side, it is condensation. If it appears under the plastic, the wall or floor is damp. Outbuilding floors will be damp if there is not a good DPM.

Humidity inside a well ventilated, unheated outbuilding will be similar to outdoor humidity, except there will be a lag in the building warming up in the morning and cooling at night. If you have windows there should be enough solar gain to keep it slightly warmer and therefore humidity slightly lower.

You say it has insulated cavity walls which is unusual for an outbuilding. Is it a workshop, garden office, living accommodation?

 

[Peasea]

Tape some clingfilm or other clear plastic tightly to the walls and floor. If water appears on the room side, it is condensation. If it appears under the plastic, the wall or floor is damp. Outbuilding floors will be damp if there is not a good DPM.

Humidity inside a well ventilated, unheated outbuilding will be similar to outdoor humidity, except there will be a lag in the building warming up in the morning and cooling at night. If you have windows there should be enough solar gain to keep it slightly warmer and therefore humidity slightly lower.

You say it has insulated cavity walls which is unusual for an outbuilding. Is it a workshop, garden office, living accommodation?

This is useful advice. The room is to be used as an outdoor office and maybe a gym at some point. I think I’m being naive in thinking that the outdoor room will be the same as inside the house in terms of RH. When it rains, the overnight RH gets as high as 78% in the outbuilding but then when it’s sunny and warm, RH stays at around 60 overnight. It has glass bifold doors on the front and I am wondering if it’s creating that humidity in a sort of greenhouse effect.

 

[JohnD]

What ventilation does it have?

 

[Peasea]

What ventilation does it have?

It has a 20cm x 120 cm window at one end and 3m wide triple bifold on the other side.
I’ve also installed a humidistat fan which in my opinion hasn’t done much.

 

[JohnD]

It has a 20cm x 120 cm window at one end and 3m wide triple bifold on the other side.

Are they shut?

 

[Peasea]

Are they shut?

Ha! The bifolds will be closed overnight but the window is open slightly to let air in and out, but so far with the window open it let’s cool air in overnight and it raises humidity