Does it need to be diffusion barrier now after additional insulation?

[ccx]

Hello!

I mentioned this in another thread I created, but it's bothering me so much now and I need to find some peace with this. Really grateful for any answers.

We added insulation to our 1970s house. We took the opportunity to do this since we were already planning to remove our waist-height brick and switch to wood siding. We also took the opportunity to replace the sill.

To get over the concrete ledge, we added 95mm of insulation.

The total insulation thickness is now 95 + original wall of 95 = 190mm.

The house was built without a vapor barrier but with plastic-coated gypsum boards.

Should I act like nothing's wrong or do I need to take a comprehensive approach inside and add a vapor barrier everywhere?

Doing that work isn't particularly fun, and it's quite challenging in some places with intersecting interior walls and other issues.

What I can't quite see is the level of risk I'm exposing our house to. According to various guides, the rule of thumb seems to be a vapor barrier if more than 200mm. I assume this has to do with the dew point?

 

[ccx]

The house has mechanical exhaust which I forgot to mention.

 

[JohanLun]

What is the difference from before? If it was gullfiber but thinner, the dew point would still be in the middle of the wall in a material that doesn't dry.

As I understand it, it's massive wood houses being insulated that becomes a problem. In your case, if it's gullfiber, there must already be an adequate solution for the vapor barrier, otherwise, you should have already had issues, I think...

 

[ccx]

What is the difference compared to before? If it was fiberglass but thinner, the dew point still ends up in the middle of the wall in a material that doesn't dry.

As I understand it, it's solid wood houses where insulation becomes a problem. In your case, it must have already (if it's fiberglass) been a viable solution for the vapor barrier, otherwise, you should have already had problems I think...

It was only 95mm insulation before with oil-hardened board as wind protection. I thought that in this construction the dew point ended up outside the wall? Maybe it's not so?

 

[JohanLun]

I'm not an expert on this, far from it, but I've read a bit and I'm a physicist by training.

I would assume that if you draw the wall and have, say, -15 outside and +25 inside, the temperature in the wall will drop, if not linearly, then at least in such a way that zero ends up somewhere in the middle of the wall, regardless of how the wall looks. 1cm in from the outside, it's "almost" outdoor temperature, i.e., below freezing. 1cm from the inside, it's almost indoor temperature. This is true almost regardless of whether the wall is 3cm or 30cm thick. The idea is that no moisture should come out and condense in the wall. That is, even before, you had the dew point in the middle of the wall.

In a solid wood house without a vapor barrier, it's not a problem (or less of a problem) because moisture slowly penetrates the wood but also dries out from the wood. If you pour water on a piece of fiberglass, it behaves completely differently than a piece of wood. It becomes soaking wet. If it then lies against a piece of wood, mold and rot start. A wet piece of wood can often last a long time.

My assumption is that as long as fiberglass has been used, a vapor barrier has been needed. Even a 10cm thick wall that is soaking wet will mold. What might possibly affect is if you've changed the facade's structure. Some houses have facades that "vent" and others that are more sealed. That is, your house should have been built to have some kind of vapor barrier. Which hopefully works.

I wouldn't worry more than you already do with houses. Because all houses are at risk for moisture damage. Especially houses from the 70s. But it's probably not much worse than before your renovation.

 

[ccx]

Now, I am not an expert on this, far from it, but I have read a bit and I am a physicist by background.

I would assume that if you draw the wall and have, for example, -15 outside and +25 inside, the temperature in the wall will drop, if not linearly, then at least in such a way that zero is located somewhere in the middle of the wall, regardless of what the wall looks like. 1cm in from the outside, it's "almost" outdoor temperature, i.e., below freezing. 1cm from the inside, it's almost indoor temperature. This is almost regardless of whether the wall is 3cm or 30cm thick. The idea is that no moisture should come out and condense in the wall. That is, even before, you had the dew point in the middle of the wall.

In a solid wood house without a vapor barrier, it's not a problem (or less of a problem) as moisture moves slowly into the wood, but also dries out from the wood. If you pour water on a piece of glass wool, it behaves completely differently than a piece of wood. It becomes soaking wet. If it then lies against a piece of wood, mold and rot begin. A wet piece of wood often lasts a long time.

My assumption is that as long as glass wool has been used, a vapor barrier has been necessary. Even a 10cm thick wall that is completely wet will mold. What might possibly affect this is if you've changed the construction of the facade. Some houses have facades that "ventilate" and others that are more sealed. That is, your house should have been built to have some kind of vapor barrier. Which hopefully works.

I wouldn't worry more than one already does with houses. Because all houses are at risk of moisture damage. Especially houses from the 70s. But it's probably not much worse than before your renovation.

Externally, it is according to standard wind barrier and air gap. It is indeed vapor permeable.

 

[ccx]

Now I'm not an expert on this, far from it, but I've read a bit and have a background in physics.

I would assume that if you draw the wall and have, for example, -15 outside and +25 inside, the temperature in the wall will drop, if not linearly then at least in such a way that zero ends up somewhere in the middle of the wall no matter what the wall looks like. 1cm in from the outside it's "almost" outdoor temperature, i.e., below zero. 1cm from the inside it's almost indoor temperature. This is almost regardless of whether the wall is 3cm or 30cm thick. The idea is that no moisture should come out and condense in the wall. That is, even previously, you've had the dew point in the middle of the wall.

In a solid wood house without a vapor barrier, it's not a problem (or less of a problem) since the moisture slowly enters the wood but also dries out from the wood. If you pour water on a piece of fiberglass insulation, it behaves completely differently than a piece of wood. It gets soaking wet. If it then lies against a piece of wood, mold and rot start. A wet piece of wood often lasts a very long time.

My assumption is that as long as fiberglass insulation has been used, a vapor barrier has been needed. Even a 10cm thick wall that is soaking wet will mold. What might possibly affect it is if you've changed the facade's construction. Some houses have facades that "ventilate" and others that are more tight. That is, your house should have been built to have some kind of vapor barrier. Which hopefully works.

I would not worry more than one normally does with a house. Because all houses are at risk for moisture damage. Especially 70s houses. But it's probably not much worse than before your renovation.

Thanks for this answer. That's about how I've understood it too. Obviously, I do what I can. Tape windows from the inside, etc.