Insulate an unused roof without a vapor barrier?

Hello everyone,

we live in a house from the 70s and have a half-height uninsulated attic. The floor below gets unbearably hot in the summer, and I have been considering insulating the roof myself.

It is supposed to be an insulation between the rafters. I have already found a lot of information about it and think it should be no problem to do it myself.

However, one question is bothering me: if I do not use the attic as living space and the insulation wool does not disappear behind drywall panels, do I even need a vapor barrier? Or can I save the cost and especially the effort by simply clamping the insulation wool between the rafters and fixing it a bit with battens?

I hope someone can help me so that I can start this project as soon as possible and also endure the summer in the living room.

Thanks in advance for your help

Ben

I would definitely install a vapor barrier in any case. I also wouldn’t know why not, the effort is very manageable. Especially with insulation between the rafters, simply tack a corresponding foil as the last step and tape the joints with approved adhesive tape, just like the edges and penetrations, done.

Even if the attic is not currently intended for use, who knows what the future holds?
And moisture from the house also reaches the attic and condenses in the insulation without a vapor barrier in cold weather. This makes it wet and reduces the insulation effect.

If you install batt insulation properly, you do not need battens underneath to hold it in place. If desired, you can also install the battens after the vapor barrier is installed.

Don't you have any collar beam insulation of the top floor ceiling?

Such a roof insulation must be carefully planned so that you gain more benefits than medium-term damage. The effort or the component structure depends on the respective existing construction and cannot be specified generally. The installation situation must be assessed on site.

As a rule, the roof trusses from the 50s to 70s lack the ventilation layer (counter battens), which ensures that the moisture from the indoor air can be removed from the insulated roof layer. Without this layer and a functional, vapor-permeable second drainage layer (underlay membrane/roofing membrane), cavity insulation between rafters cannot be installed without medium-term damage. The insulated construction must also be convection-tight, i.e., it must not be penetrated by warm, moist indoor air. Therefore, you need a material on the interior side with a vapor-retarding effect. Unfortunately, the way you have planned the conversion will not work.

I would recommend having a building survey conducted by a planner who calculates a suitable structure for you (pay attention to the Energy Saving Ordinance 2016!), possibly points out structural interrelations and additional work, and accompanies the entire construction project at least in the form of construction consulting if you wish to carry it out as DIY.

Basically, I would also rather consider insulating the collar beam layer. Otherwise, you will always be heating the unused attic.

Thank you for your answers

I will probably also install the vapor barrier. I just thought I could save some work and a few euros.

I think I will also insulate the collar beams. But maybe I am misunderstanding something. Collar beams and floor joists are not the same, right?
I drew a picture of how it looks in my case.
So I simply place the compressed insulation between the wooden rafters and the collar beam, then the vapor barrier in front of it (absolutely tight, of course), and then, if necessary, a counter batten.