Silicone Injection DPC / DPC Cream - Damp Issue Prevention

[bearsparko]

Hi,

We are renovating a 1930's detached house with solid 9" brick walls. Our goal is to make this house as good as it can be and then stay here for a long time.

We intend to make sure that the house is as well insulated as possible and on the west side of the house which doesn't get much sunlight the plan is to put studding against the external wall and insulate with something like SF40 SuperFoil.

We had a timber and damp survey carried out and it identified some damp issues along the wall which we want to insulate, and in one corner of the room the plaster did come off in big chunks, higher up it was pretty well fixed. All the plaster is now off the whole wall and we didin't want to lose room space with the studding.

I need to treat the damp that is visible, even if it is not very serious yet and I want to make sure that we never have any damp issues in the future as we are "going to town" on this house. For the sake of £100 and a bit of time I intend to put a silicone injection DPC 1 course above the the slate DPC which is already there, along the entire length of the house, so that in 15 - 20 years eventually when the slate breaks down we don't have a big problem. Before anyone tells me I know this is "belt and braces", but it's a great house and at the moment we are taking all the plaster off walls and ceilings in some rooms, rewiring, new heating, extension etc, etc. so a bit of DPC is no big deal.

Questions;
1. Should I drill the holes in the mortar course or in the bricks themselves (different sites give different advice)?
2. Is there any guide to how long/how much to inject?
3. Is there any danger to the existing slate course doing this?
4. I plan to inject the course above the existing slate DPC, is this OK or should I try for the course below the existing slate DPC?
5. Am i right in thinking that the new type of "creams" won't work as they require a fair bit of moisture in the brick to be activated?

Thanks for your help. Marco

 

[ajrobb]

If you want to be confused, read Heritage House. He shows pictures of silicone injection with damp (from condensation) above the layer and dry bricks below.

These are comments of Pete Ward:

"I prefer to use steel framing, simply because it doesn’t rot – and introduce a clear gap between wall and frame – rather than attaching stud to wall. Insulate with 50mm sheepswool or fiberglass… (although fiberglass seems to be a bit doubtful – it disintegrates over time) . Otherwise, open to floor and ceiling cavities – no problem. I don’t think it needs to be heated – just to breathe with the seasons. It only gathers moisture if it is trapped by plasters that hold condensation.. My house is done this way – medieval stone walls and 18 / 19th century brick, and not a skerrick of damp anywhere."

I haven't shown my questions as I think his answers are quite clear. Now you have removed the damp plaster and hygroscopic salts, Pete seems to suggest the wall will dry out. He does not like surfaces that don't 'breathe' so avoid concrete.

 

[kevintaylore]

1. Should I drill the holes in the mortar course or in the bricks themselves (different sites give different advice)?

Creams are always injected into the mortar course. DPC injection is generally injected into the brick. I am told that this is because it is difficult to get a good seal on soft mortar.

2. Is there any guide to how long/how much to inject?

No really relevent for creams, but I would imagine BS6576 gives advice for chemical injection.

3. Is there any danger to the existing slate course doing this?

Hammer drilling close to the existing slate DPC could probably damage it. But it won't matter if you inject an effective chemical DPC.

4. I plan to inject the course above the existing slate DPC, is this OK or should I try for the course below the existing slate DPC?

BRE say you should aim for 150mm above ground level to avoid splashing rain bridging the DPC. In practice you may choose to go lower (e.g. to protect embedded joists) or higher for practical reasons.

5. Am i right in thinking that the new type of "creams" won't work as they require a fair bit of moisture in the brick to be activated?

Creams are injected into the mortar rather than the brick. Formulations of creams vary widely, so all will behave differently in different types of mortar (wet, dry, old, new, lime, cement etc...). Most manufacturers have technical departments who will be able to provide you with test data to demonstrate which conditions and mortar types their products will work in. I have less experinece with liquid injection fluids, but I would imagine there is quite a difference in performance between types - e.g. water-based, solvent-based, silicone, stearate, etc... so it is probably safest to ask the manufacturer.

 

[mointainwalker]

Can you please say what evidence you have that that your slate DPC is going to break down ?

 

[hotrod]

I intend to put a silicone injection DPC 1 course above the the slate DPC which is already there, along the entire length of the house, so that in 15 - 20 years eventually when the slate breaks down we don't have a big problem.

As mointainwalker says what evidence do you have that the existing slate DPC is failing? Natural slate is a highly durable medium that will, in the correct environment, outlast you and your house. Please, please, please don't go injecting remedial chemical DPCs into your walls unless you are sure your existing DPC has failed and you have a rising damp problem.

Rising damp does exist [cue barrage of yes/no arguments] but is often misdiagnosed. So before you go to all the disruption of hacking the plaster off your walls and drilling / injecting make sure the damp isn't being caused by something else.

 

[ajrobb]

I want to make sure that we never have any damp issues in the future

Please look at a similar thread.

Condensation can be mistaken for rising damp as they both affect the bottom of the wall mostly.

Yesterday I got back a house I rented out to the same tenants for 10 years. It was built in 1978 and should not have the damp problems associated with solid-walled houses. However, the tenants had covered all the windows with polythene so there was no ventilation. The house is now riddled with mould from condensation.

I lived there throughout the 80s and the house was always warm and dry. It just goes to show that damp is not always about the house but often about how it is used.

The modern trend is to attempt to build a sealed, insulated box inside the existing solid-walled structure. Where mineral wool is used in lofts and under floors, condensation is prevented by improving ventilation of the cold areas through the introduction of extra air bricks, soffit vents and ridge vents etc. A vapour barrier is used on the walls also to prevent condensation.

The treatment is rarely complete however. For instance, what do you do between ceilings and floors? These areas are unventilated(1), uninsulated, unheated(2) but still exposed to water vapour through the ceiling and floor. This sounds like a perfect recipe for condensation. It thus remains important to limit humidity in the dwelling, which is usually through ventilation.

1. there might be considerable air leakage through the walls

2. a lot of heat rises through the ceiling

I grew up in a 1930s solid-walled house and it didn't have damp problems until after my father died at 69. My mother lived on in the house for another 11 years but kept the heating turned down and the windows shut. It was only then the house started to get damp.