Subfloor of fiber cement. Windproofing or not?

[Slitvarg]

I have used byggmax fiber cement boards as a blind bottom. According to the manufacturer's product declaration, the board is wind and weather-resistant.

Should I, despite the board being "wind-resistant," supplement with a diffusion-open wind barrier like AC 350 to further reduce the risk of drafts?

 

[Derbyboy]

I didn't do it. Right or wrong, I have no idea. I don't feel any pull.

It should probably depend a bit on what's underneath, like warmgrund or outdoor vented crawl space.

I have a closed crawl space on rock that is never colder than 10 degrees.

 

[justusandersson]

Even though the boards themselves are windproof, the construction may not be. It requires a precision that is unlikely. It is incorrect to describe AC 350 as diffusion-open, it is rather a vapor barrier. The classic material in blindbottnar is AC 150.

 

[Slitvarg]

The question was not whether wind barrier AC 350 is diffusion-open or not.

 

[justusandersson]

Threads on Byggahus have many readers beyond the most closely concerned. That kind of information should be accurate, even if it's not the main point.

 

[Slitvarg]

You have probably misunderstood the concepts of diffusion-open and vapor barrier. In material contexts, it also becomes relative. Manufacturers of wind barrier paper, such as Tectis, classify their 350 as diffusion-open for the simple reason that it allows diffusion. Unfortunately, it is also not uncommon today for people to not differentiate between a vapor barrier and a diffusion barrier or to confuse them.

It's absolutely correct that others read these threads and the information should be accurate. It's incorrect to classify a conventional wind barrier paper as a "vapor barrier" when it is not intended to be used as a vapor barrier.

 

[justusandersson]

With 50 years of professional experience and a technical university education as a foundation, I don't believe I have misunderstood the concepts of diffusion openness and vapor brake. Fifty years ago, AC 350 was the diffusion barrier of the time, before plastic films, which were even more vapor-tight, came a bit into the 1970s. Between diffusion-tight (which is not an absolute term) and diffusion-open, there are many intermediates, but describing these as diffusion-open without further specification is misleading. If you want to use a fully diffusion-open material for the blind bottom, there are many variants of wind barriers.

I'm not saying it's wrong to use AC 350 for the intended purpose, but the material is unnecessarily thick and cumbersome. AC 150 is significantly more manageable and usually comes in a more practical width.

 

[vectrex]

Why bother with heavy and rigid asphalt roofing felt when there is a wind barrier made of thin polyethylene plastic. It's simpler, faster, and it's also vapor permeable.

 

[tensiden]

My carpenters installed fiber cement boards as the foundation lining and only taped all the joints. However, we have a dehumidifier in the foundation and a well-defined climate there that helps, which made everyone agree not to do more with the foundation lining.

 

[Slitvarg]

According to the manufacturer's assembly instructions that I finally obtained, the seams should indeed be taped. No form of additional wind protection is prescribed.

justusandersson:
I understand your point. However, I believe you have initially had a logical fallacy about what is "right and wrong." I don't know where your need initially comes from to assert yourself on a matter that wasn't originally related to the question. The crucial point, however, is that it is just as "misleading" to call a wind paper a vapor barrier as to say it is open to diffusion. Today, it is generally accepted that vapor brakes, like vapor barriers ("construction plastic"), are used on the inside of the building envelope. Variable vapor brakes are also something that has recently entered the industry. Technically, they serve different functions and are adapted to different types of insulating constructions. It is unfortunately common today for them to be confused, which quickly leads to devastating damages in constructions, something I have experienced more than once in my work. In that aspect, it is important to call a spade a spade. It is confusing enough already, so it's unnecessary to start mixing in wind paper into the terminology. If you think it's important to correctly define materials in vapor resistance aspects, you can start with yourself.

Hereby, I conclude the discussion.

 

[Pelletompa]

According to the manufacturer's installation instructions that I eventually got hold of, the joints should indeed be taped. No additional wind protection is prescribed.

justusandersson:
I understand your point. However, I believe you initially made a fallacy in arguing about what is "right and wrong." I do not know where your need originally comes from to assert yourself in a matter that was not related to the original question. The crucial point is, however, that it is just as "misleading" to call a wind barrier an air barrier as it is to say it is open to diffusion. Today, it is generally accepted that air barriers, like vapor barriers ("construction plastic"), are used on the inside of the building envelope. Variable air barriers have also recently entered the industry. Technically, they serve different functions and are adapted to different types of insulating constructions. It is unfortunately common today for them to be confused, which quickly leads to devastating damage in constructions, something I have experienced more than once in my work. From that perspective, it is important to call a spade a spade. It is confusing enough as it is, so it is unnecessary to start mixing wind barrier into the concepts. Therefore, if you think it's important to correctly define materials in vapor resistance aspects, you can start with yourself.

With that, I will end the discussion.

According to the manufacturer’s installation instructions that I eventually got hold of, the joints should indeed be taped. No additional wind protection is prescribed.

justusandersson:
I understand your point. However, I believe you initially made a fallacy in arguing about what is "right and wrong." I do not know where your need originally comes from to assert yourself in a matter that was not related to the original question. The crucial point is, however, that it is just as "misleading" to call a wind barrier an air barrier as it is to say it is open to diffusion. Today, it is generally accepted that air barriers, like vapor barriers ("construction plastic"), are used on the inside of the building envelope. Variable air barriers have also recently entered the industry. Technically, they serve different functions and are adapted to different types of insulating constructions. It is unfortunately common today for them to be confused, which quickly leads to devastating damage in constructions, something I have experienced more than once in my work. From that perspective, it is important to call a spade a spade. It is confusing enough as it is, so it is unnecessary to start mixing wind barrier into the concepts. Therefore, if you think it's important to correctly define materials in vapor resistance aspects, you can start with yourself.

With that, I will end the discussion.

Hi,

I have bought Byggmax fiber cement board for the foundation but wonder which side should be facing down. The "shinier" with print or the "rougher"?

Tomas