Vapor barrier, loft above garage?

[Steeldragon]

I am building a double garage with a loft. The loft will function as a guest bedroom with space for a sofa and TV, plus storage.

The building has 120+45 insulation in the walls and 220 in the roof. The floors are then separated with an intermediate floor and insulation at 195mm. The loft will have maintenance heating with the help of an air source heat pump to about 15 degrees and sometimes higher if someone will be staying there. We don't plan to heat the garage itself, instead, it will be spill heat from cars and the heat that manages to move down from the loft.

Will I need to cover the loft with a vapor barrier?
Should the entire garage be covered in plastic? Or is it unwise to install plastic in the garage that doesn't have direct heating?

Additional consideration:
In the installation instructions for the pine flooring I will be laying in the loft, it states that there should be an "age-resistant plastic" under the floor. Should I have that even if I won't have a vapor barrier on the loft?

Grateful for any insights you can give me! I'm scratching my head a bit about this...

 

[sunnis]

Yes, with so much insulation you need to seal everything carefully so that it becomes airtight, otherwise there's a high risk of getting condensation in the insulation.

Then someone might write that it's not necessary if you're going to keep the garage unheated.

 

[Steeldragon]

Yes, with so much insulation, you must seal everything carefully to make it airtight, otherwise there's a big risk of condensation in the insulation.

Then someone might say it's not needed if you're going to keep the garage unheated.

Thanks for the reply!

Should I also seal the unheated garage section, or just the loft?

 

[sunnis]

Thanks for the response!

Should I also plastic the unheated garage section, or just the loft?

In my opinion, everything should be covered in plastic–the floor structure, ceiling, and all walls. Use overlapping plastic and tape the seams carefully. Perfect with 120+45, then you place the plastic on the 120 studs and then frame the installation layer with 45mm, the most optimal way to use the vapor barrier as it won't be unnecessarily punctured by boards/cables for electricity and everything else that runs within the installation layer.

You don't want warm and moist air from cars passing through the surface layers and eventually condensing in the insulation.

Also, make sure to ventilate effectively. The Mitsubishi VL-100 is a pretty clever device for a garage/loft. Otherwise, a small pax fan or a number of vents also work well for the garage.

 

[Steeldragon]

Thanks for the tip!
I'll check out VL-100 and see if it might be something.

 

[justusandersson]

Moisture in buildings is complicated. There are some basic physical conditions to start from. Moisture exists in three different states: solid (ice), liquid (water), and gaseous (water vapor). The ability of air to hold moisture in the form of water vapor is temperature-dependent; the warmer the air, the more water vapor it can hold. When warm air is cooled, some of the water vapor will convert to liquid form, i.e., water. This is what we call condensation. The idea behind using vapor barriers (vapor diffusion barriers are the same thing) in building constructions is to prevent condensation in an insulation layer. For such a risk to arise, the indoor air must be warm (with a large amount of bound water vapor), and there must be a significant temperature difference compared to the outdoor air. A good principle is not to use vapor barriers in buildings that are not permanently heated during the winter months. If the indoor air is always warmer than the outdoor air (except for high summer heat), using a vapor barrier does not lead to any problems, even though it does not provide any benefit. However, if there is a risk that the outdoor air is warmer than the indoor air for a long period, vapor barriers can lead to condensation on the wrong side.

Under no circumstances should both sides of a building component be equipped with a vapor barrier without there being an opportunity for ventilation.

 

[Steeldragon]

If the indoor air is always warmer than the outdoor air (apart from high summer heat), the use of a vapor barrier doesn't lead to any problems, even if it doesn't provide any benefit.

Thanks for a detailed response! What I interpret from your quoted text above is that a vapor barrier is unnecessary in my case?

If we use a heat pump like the one recommended in some earlier posts, and use it to cool down the space in the summer, the outdoor air could very well become warmer than the indoor air.

But doesn't that apply to all buildings that have some form of heating/cooling pump?

 

[justusandersson]

It is common for it to be warmer outside than inside during the summer. However, these are small temperature differences and temporary conditions, which generally don't require much thought.

Regarding your garage, you can equip the walls and roof with a vapor barrier, although it won't be particularly beneficial, so you can also disregard it. However, the floor structure between the garage and loft should not have plastic on both sides. That could cause issues.

 

[sunnis]

Thank you for a detailed answer! From what I interpret from your quoted text above, is it unnecessary to have a vapor barrier in my case?

If we use a heat pump like the one recommended in some posts earlier, and use it to cool down the space in the summer, the outside air can indeed become warmer than the indoor air.

But doesn't that apply to all buildings that have some form of heat/cool pump?

The question is, what should be saved on? Plastic, tape is not particularly expensive, and the task of applying plastic is not very time-consuming either. Then the building is correctly built, and you don't need to worry about moisture forming in the insulation.

 

[justusandersson]

One cannot say that a building is correctly constructed if a vapor barrier is used without regard to heating and temperature conditions. The problem is complex and requires deeper knowledge in building physics.

 

[sunnis]

One cannot say that a building is correctly executed if one uses a vapor barrier without regard to heating and temperature conditions. The problem is complex and requires deeper knowledge in building physics

In a garage, you should have a vapor barrier. Quite simple. The building has partly heated spaces and vehicles should be able to evaporate freely in the garage. If there is 200mm insulation in the walls, it is not correctly executed if you skip the plastic.

 

[justusandersson]

I am trying to conduct a principled discussion. The threads in Byggahus have many readers.

Unheated spaces (regardless of what they are used for) do not need a vapor barrier; in some cases, it can be directly inappropriate. In an unheated garage, moisture from a car will not turn into water vapor in the air, as this requires a heat source. An intermediate floor in a space where exterior walls and roof have been fitted with a vapor barrier should not have a vapor barrier. In particular, it should not have plastic on both the top and underside.