Hello everyone,
I want to insulate the attic in a house built in 1975. As far as I could analyze the existing structure of the top floor ceiling, there is wood or drywall on the room side, above that a wooden beam ceiling with glass wool insulation in between, and on top of that a 2cm (0.8 inches) tongue and groove wooden plank floor.
How do I properly insulate this floor from above? Vapor barrier on the wooden planks, then rigid insulation (such as PUR or XPS), and plywood boards on top?
Many thanks!
I want to insulate the attic in a house built in 1975. As far as I could analyze the existing structure of the top floor ceiling, there is wood or drywall on the room side, above that a wooden beam ceiling with glass wool insulation in between, and on top of that a 2cm (0.8 inches) tongue and groove wooden plank floor.
How do I properly insulate this floor from above? Vapor barrier on the wooden planks, then rigid insulation (such as PUR or XPS), and plywood boards on top?
Many thanks!
Thank you again for your detailed and very helpful response!
I am now only confused about which specific construction method is possible or recommended in my case (i.e., without a concrete ceiling and with an existing vapor retarder). What I believe I have understood so far:
Option 1) Remove the floorboards, take out the insufficient glass wool insulation, install a vapor retarder and properly seal it, then add cavity insulation with mineral wool up to the full height of the joists, rough-sawn boards, walkable mineral wool panels, then chipboard panels.
Option 2) Keep the existing structure, add walkable mineral wool panels and chipboard panels (but I am not sure about this, since you also mentioned being cautious with mineral wool panels without a vapor retarder, and chipboard panels are not vapor-permeable either?)
The first option seems like the clean/"proper" solution but is more complex. How can I insulate my floor structure without having to remove the existing setup and the currently minimal (but at least present) insulation? Since there is no vapor retarder, is only a vapor-open solution possible?
For the roof, we currently only have Heraklit boards under the tiles, so I could simply add cavity insulation with mineral wool there in the classic way and then install a vapor barrier foil on the inside, correct?
I am now only confused about which specific construction method is possible or recommended in my case (i.e., without a concrete ceiling and with an existing vapor retarder). What I believe I have understood so far:
Option 1) Remove the floorboards, take out the insufficient glass wool insulation, install a vapor retarder and properly seal it, then add cavity insulation with mineral wool up to the full height of the joists, rough-sawn boards, walkable mineral wool panels, then chipboard panels.
Option 2) Keep the existing structure, add walkable mineral wool panels and chipboard panels (but I am not sure about this, since you also mentioned being cautious with mineral wool panels without a vapor retarder, and chipboard panels are not vapor-permeable either?)
The first option seems like the clean/"proper" solution but is more complex. How can I insulate my floor structure without having to remove the existing setup and the currently minimal (but at least present) insulation? Since there is no vapor retarder, is only a vapor-open solution possible?
For the roof, we currently only have Heraklit boards under the tiles, so I could simply add cavity insulation with mineral wool there in the classic way and then install a vapor barrier foil on the inside, correct?
BugsBunny schrieb:
What I think I have understood so far: You have understood correctly. However, building on top of the existing floorboards only makes sense if the rafters are completely filled. Otherwise, there will be an air gap in between that will definitely ventilate somewhere – everything placed on top then loses its insulating effect.
BugsBunny schrieb:
The first option seems to me to be the clean/"tidy" one, but it is more labor-intensive. Yes, that is usually the case 🙂 Cutting corners is always easier.
BugsBunny schrieb:
How can I insulate my ceiling floor without having to remove the existing structure and the already present but limited insulation? If, as described, the cavity between the rafters is fully filled, you can simply apply wood fiber insulation with a compressed surface on top without additional chipboard panels. Or you can use non-load-bearing mineral wool or soft wood fiber mats, but those are not walkable at all – although at 100mm (4 inches) thickness, they might cost around 5€/m².
If the cavity is already filled (I estimate the rafters to be about 140–200mm (5.5–8 inches) thick), I would add another 100mm (4 inches) of wood fiber insulation that is walkable on top. That is the easiest solution – honestly, fully insulating 140mm (5.5 inches) is already quite good. Additional insulation will not make a huge difference.
BugsBunny schrieb:
Currently, under the tiles on the roof, we only have Heraklit boards. So I could just do a "classic" insulation with mineral wool between the rafters and then install a vapor control layer on the inside, right? Yes, exactly. However, the area to be insulated is significantly larger than the floor below, and you won’t warm up the gable ends that way. I would only fully insulate the floor cavities – unless you want a warm attic space.
Are there inclined ceilings in the upper floor? Are those insulated? Can you access them from the attic?
OK, so a clear picture is slowly coming together for me 😉
That means, to be completely sure: Once the cavity is filled, I can simply use wood fiber with a compacted surface and then don’t need a vapor retarder at all (which is recommended almost everywhere, so that surprised me)?
Sloped ceilings on the upper floor: Yes, unfortunately they exist and can only be accessed from the outside. I’m not sure if they are insulated. If so, probably only with Heraklit boards...
Below the slope, the upper floor also has a ventilated curtain wall facade with little or no insulation (I haven’t been able to check underneath yet, it was too risky to climb up). I will probably have to address this again (I have the same question about the build-up here), but apparently such a facade is generally quite advantageous in terms of insulation since it is durable and provides good insulating properties (once the 20cm (8 inches) insulation is installed, whereas we currently probably have only a few centimeters).
That means, to be completely sure: Once the cavity is filled, I can simply use wood fiber with a compacted surface and then don’t need a vapor retarder at all (which is recommended almost everywhere, so that surprised me)?
Sloped ceilings on the upper floor: Yes, unfortunately they exist and can only be accessed from the outside. I’m not sure if they are insulated. If so, probably only with Heraklit boards...
Below the slope, the upper floor also has a ventilated curtain wall facade with little or no insulation (I haven’t been able to check underneath yet, it was too risky to climb up). I will probably have to address this again (I have the same question about the build-up here), but apparently such a facade is generally quite advantageous in terms of insulation since it is durable and provides good insulating properties (once the 20cm (8 inches) insulation is installed, whereas we currently probably have only a few centimeters).
BugsBunny schrieb:
If the cavity is filled, can I simply use wood fiberboard with a compressed surface? And then I don’t need a vapor retarder at all (since it is recommended almost everywhere, that’s why I’m surprised)?!dertill schrieb:
A build-up that is vapor-permeable on the outside does not mean that a vapor retarder on the interior side is unnecessary.There are certainly roofs and floor ceilings designed to be vapor-permeable, but you need to fully understand what you are doing, know the physical background, temperature and dew point lines, and have competent workmanship with careful execution. Then it’s a great solution. It is important that there is no dew point condensation anywhere. This can easily happen with incomplete filling of cavities and corners, which causes serious problems. Using a vapor retarder is definitely safer. If you remove the old insulation or whatever is inside, do yourself a favor and carefully install a vapor retarder with a diffusion-equivalent air layer (sd-value) of around 3–10 m (10–33 ft). If you just place wood wool panels on top and nothing has happened so far, nothing will happen afterward either.
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