ᐅ Floor structure and subfloor construction in the attic with loose fill?
Created on: 20 Dec 2017 19:40
M
Markus31
Hello dear forum members,
As part of a renovation, we are currently refurbishing our attic. The new roof is now in place, and we are starting with the interior work, beginning with the floor. Below the attic is an occupied apartment. The attic will eventually house various rooms (office, bathroom, living room, etc.).
We have already installed 20cm (8 inches) high joists. Beneath the joists, there is a layer of clay from earlier construction that we want to keep as it is (see image 2). The next step is to fill the gaps between the joists so we can then build up the floor structure (OSB boards, etc.).
1. Question:
Right now, we are unsure about what to use to fill the spaces between the joists.
- Loose fill?
- Mineral wool?
- Something else?
As mentioned, the height to be filled corresponds to the joist height of 20cm (8 inches). The total attic area is 60 sqm (8 m x 7.50 m / 86 sq ft), and the spacing between the individual joists is 55cm (22 inches).
What would you recommend here and why?
2. Question:
How thick would you make the floor build-up?
We plan to install OSB boards (screwed) onto the joists, followed by impact sound insulation and then the final floor covering (e.g., vinyl, hardwood, tiles in the bathroom, etc.).
- What thickness of OSB boards would you recommend for this situation? (They need to support considerable weight, such as a bathtub in the bathroom.)
- How much impact sound insulation?
We are extremely grateful for any answers and recommendations.
Greetings from Würzburg,
Markus
As part of a renovation, we are currently refurbishing our attic. The new roof is now in place, and we are starting with the interior work, beginning with the floor. Below the attic is an occupied apartment. The attic will eventually house various rooms (office, bathroom, living room, etc.).
We have already installed 20cm (8 inches) high joists. Beneath the joists, there is a layer of clay from earlier construction that we want to keep as it is (see image 2). The next step is to fill the gaps between the joists so we can then build up the floor structure (OSB boards, etc.).
1. Question:
Right now, we are unsure about what to use to fill the spaces between the joists.
- Loose fill?
- Mineral wool?
- Something else?
As mentioned, the height to be filled corresponds to the joist height of 20cm (8 inches). The total attic area is 60 sqm (8 m x 7.50 m / 86 sq ft), and the spacing between the individual joists is 55cm (22 inches).
What would you recommend here and why?
2. Question:
How thick would you make the floor build-up?
We plan to install OSB boards (screwed) onto the joists, followed by impact sound insulation and then the final floor covering (e.g., vinyl, hardwood, tiles in the bathroom, etc.).
- What thickness of OSB boards would you recommend for this situation? (They need to support considerable weight, such as a bathtub in the bathroom.)
- How much impact sound insulation?
We are extremely grateful for any answers and recommendations.
Greetings from Würzburg,
Markus
Hi KlaRa,
thanks for the prompt reply. I’m happy to wait until next week for my follow-up question as well.
A few more points:
1:
2 layers of OSB (what thickness?) versus 2 layers of 18mm gypsum fiberboard BRIO 18:
--> Which of the two would you personally prefer? OSB is considerably cheaper, so what advantage does BRIO offer?
2:
I definitely want to use a 10mm (0.4 inch) wood fiberboard as the first layer directly on the joists (because fleece strips with a build-up of 30mm (1.2 inches) would increase my total height by 20mm (0.8 inch) and create a step).
In your opinion, would I have to nail/screw this wood fiberboard directly to the joists rather than installing it floating? Would it still be guaranteed that the panel can "work" (i.e., expand and contract over the years without negatively affecting the floor)?
3.
Regarding your second suggestion:
(Quote)
"Nail fleece strips onto the joist layer,
then one layer of OSB or BRIO,
then install the REGUPOL mat floating on top,
then the second layer of BRIO or OSB glued tongue and groove."
- Could I replace the fleece strips with the 10mm wood fiberboard?
- Should the first layer of OSB or BRIO be installed floating?
- What thickness is the REGUPOL mat?
- Should the second layer of BRIO or OSB also be installed floating (tongue and groove) on the REGUPOL mat?
thanks for the prompt reply. I’m happy to wait until next week for my follow-up question as well.
A few more points:
1:
2 layers of OSB (what thickness?) versus 2 layers of 18mm gypsum fiberboard BRIO 18:
--> Which of the two would you personally prefer? OSB is considerably cheaper, so what advantage does BRIO offer?
2:
I definitely want to use a 10mm (0.4 inch) wood fiberboard as the first layer directly on the joists (because fleece strips with a build-up of 30mm (1.2 inches) would increase my total height by 20mm (0.8 inch) and create a step).
In your opinion, would I have to nail/screw this wood fiberboard directly to the joists rather than installing it floating? Would it still be guaranteed that the panel can "work" (i.e., expand and contract over the years without negatively affecting the floor)?
3.
Regarding your second suggestion:
(Quote)
"Nail fleece strips onto the joist layer,
then one layer of OSB or BRIO,
then install the REGUPOL mat floating on top,
then the second layer of BRIO or OSB glued tongue and groove."
- Could I replace the fleece strips with the 10mm wood fiberboard?
- Should the first layer of OSB or BRIO be installed floating?
- What thickness is the REGUPOL mat?
- Should the second layer of BRIO or OSB also be installed floating (tongue and groove) on the REGUPOL mat?
Regarding point 1)
Lower OSB board 15mm (0.6 inches), upper board 22mm (0.9 inches) thick.
Advantages of BRIO: none.
Regarding point 2)
Wood fiberboard crumbles quickly under load: not a good idea!
Suggestion: "Knauf Insulation." Cut these into strips and nail them concealed onto the supporting beams.
Available thicknesses are 15mm (0.6 inches), 20mm (0.8 inches), and 25mm (1 inch).
20mm (0.8 inches) would be recommended.
To avoid height issues, lay the two OSB boards directly on top of each other with staggered board edges, floating on the insulation strips, but screw them together firmly. Tongue and groove joints should be well glued with D3 adhesive!
Regarding point 3)
Not applicable with the above approach.
Alternatively, two 15mm (0.6 inches) boards (instead of 15mm + 22mm) can be used.
Height saving: 7mm (0.3 inches).
Regards, KlaRa
Lower OSB board 15mm (0.6 inches), upper board 22mm (0.9 inches) thick.
Advantages of BRIO: none.
Regarding point 2)
Wood fiberboard crumbles quickly under load: not a good idea!
Suggestion: "Knauf Insulation." Cut these into strips and nail them concealed onto the supporting beams.
Available thicknesses are 15mm (0.6 inches), 20mm (0.8 inches), and 25mm (1 inch).
20mm (0.8 inches) would be recommended.
To avoid height issues, lay the two OSB boards directly on top of each other with staggered board edges, floating on the insulation strips, but screw them together firmly. Tongue and groove joints should be well glued with D3 adhesive!
Regarding point 3)
Not applicable with the above approach.
Alternatively, two 15mm (0.6 inches) boards (instead of 15mm + 22mm) can be used.
Height saving: 7mm (0.3 inches).
Regards, KlaRa
1)
What arguments are there against OSB boards and in favor of, for example, Knauf BRIO? After all, OSB panels are significantly lighter and less than half the price of Knauf BRIO.
2.1)
Regarding the crumbling of the Knauf BRIO board.
In the Knauf video, this product is specifically recommended for such purposes (see from 1:35)
[MEDIA=YouTube]-fOyo44p6uc[/MEDIA]
2.2)
If I were to, for example, nail "Knauf Insulation" in 20mm (0.8 inch) strip form only onto the joists, then my height on the joists would be 20mm (0.8 inch) higher than between the joists where the mineral wool ends flush with the joist height. This means there is a small gap there if the boards (BRIO or OSB) are laid floating on top. Isn’t that suboptimal, since the boards might bend slightly over time under heavy load?
What arguments are there against OSB boards and in favor of, for example, Knauf BRIO? After all, OSB panels are significantly lighter and less than half the price of Knauf BRIO.
2.1)
Regarding the crumbling of the Knauf BRIO board.
In the Knauf video, this product is specifically recommended for such purposes (see from 1:35)
[MEDIA=YouTube]-fOyo44p6uc[/MEDIA]
2.2)
If I were to, for example, nail "Knauf Insulation" in 20mm (0.8 inch) strip form only onto the joists, then my height on the joists would be 20mm (0.8 inch) higher than between the joists where the mineral wool ends flush with the joist height. This means there is a small gap there if the boards (BRIO or OSB) are laid floating on top. Isn’t that suboptimal, since the boards might bend slightly over time under heavy load?
Hello Markus.
Regarding point 1), I have already responded.
Regarding point 2): a layperson can only infer information from product advertisements, which requires some understanding. Experts in the field, however, must also consider the background details that are inevitably hidden from non-professionals.
The soft fiberboards are load-bearing over an area, as shown in the video. However, when installed as strips above wooden beams, they will fail due to the high point loads. Aside from that, the impact sound insulation will only be marginal. We need a resilient underlay to achieve sufficient decoupling, which we cannot achieve with soft fiberboards placed on load-bearing beams!
That is why my solution represents the best possible outcome in terms of impact sound insulation in timber construction.
The filling of the cavities will logically only be carried out once the substructure of the floating floor is completed. Then there will be no hollow spaces. Even if small cavities remain within the cavities, their negative effect is marginal. The resulting resonance space can be neglected!
As a metaphor:
When considering the maximum permissible total weight of your car, it is irrelevant whether the passengers are also wearing hats or not.
It is similar with hollow spaces...
In public forums, we can only provide generally valid advice.
I am currently working on specific projects in Frankfurt and Berlin, where the load-bearing capacity of all floor slabs is limited, but still must accommodate a high surface load of up to 5kN/m² (104.6 psf).
These situations require on-site inspection, as I am responsible for the construction specifications. With local conditions unknown to me and only reported secondhand, this is obviously not possible. Therefore, the responsibility lies solely with the contractor performing the work. In this case, that responsibility is yours.
This also highlights the significant risk when any supposed "expert" gives non-binding (and often technically unfounded) recommendations.
What is often overlooked:
Anyone undertaking a trade must not only know the technical regulations but also comply with them.
Thus, any homeowner acting as a craftsman must later bear the risk of success or failure. Installation videos can certainly be useful, but they do not replace the necessary background knowledge.
Best regards, KlaRa
Regarding point 1), I have already responded.
Regarding point 2): a layperson can only infer information from product advertisements, which requires some understanding. Experts in the field, however, must also consider the background details that are inevitably hidden from non-professionals.
The soft fiberboards are load-bearing over an area, as shown in the video. However, when installed as strips above wooden beams, they will fail due to the high point loads. Aside from that, the impact sound insulation will only be marginal. We need a resilient underlay to achieve sufficient decoupling, which we cannot achieve with soft fiberboards placed on load-bearing beams!
That is why my solution represents the best possible outcome in terms of impact sound insulation in timber construction.
The filling of the cavities will logically only be carried out once the substructure of the floating floor is completed. Then there will be no hollow spaces. Even if small cavities remain within the cavities, their negative effect is marginal. The resulting resonance space can be neglected!
As a metaphor:
When considering the maximum permissible total weight of your car, it is irrelevant whether the passengers are also wearing hats or not.
It is similar with hollow spaces...
In public forums, we can only provide generally valid advice.
I am currently working on specific projects in Frankfurt and Berlin, where the load-bearing capacity of all floor slabs is limited, but still must accommodate a high surface load of up to 5kN/m² (104.6 psf).
These situations require on-site inspection, as I am responsible for the construction specifications. With local conditions unknown to me and only reported secondhand, this is obviously not possible. Therefore, the responsibility lies solely with the contractor performing the work. In this case, that responsibility is yours.
This also highlights the significant risk when any supposed "expert" gives non-binding (and often technically unfounded) recommendations.
What is often overlooked:
Anyone undertaking a trade must not only know the technical regulations but also comply with them.
Thus, any homeowner acting as a craftsman must later bear the risk of success or failure. Installation videos can certainly be useful, but they do not replace the necessary background knowledge.
Best regards, KlaRa
Hi KlaRa,
Regarding point 1:
It's still not clear to me. You mentioned there are no disadvantages of OSB compared to Knauf BRIO. If that were true, then everyone would buy OSB since it’s lighter and considerably cheaper. So why would anyone still choose Knauf BRIO, for example?
Regarding point 2:
We will definitely be installing mineral wool insulation between the floor joists instead of loose fill.
Did I understand you correctly that I should avoid laying the wood fiber insulation boards in strips, but I can install them as full boards, floating over the entire surface (including on top of the mineral wool between the joists)? And then place my precast screed panels above that?
So the wood fiber boards are not recommended only as strips in this case, but they are suitable as full boards covering the entire area?
Best regards,
Markus
Regarding point 1:
It's still not clear to me. You mentioned there are no disadvantages of OSB compared to Knauf BRIO. If that were true, then everyone would buy OSB since it’s lighter and considerably cheaper. So why would anyone still choose Knauf BRIO, for example?
Regarding point 2:
We will definitely be installing mineral wool insulation between the floor joists instead of loose fill.
Did I understand you correctly that I should avoid laying the wood fiber insulation boards in strips, but I can install them as full boards, floating over the entire surface (including on top of the mineral wool between the joists)? And then place my precast screed panels above that?
So the wood fiber boards are not recommended only as strips in this case, but they are suitable as full boards covering the entire area?
Best regards,
Markus
S
stefanc848 Jan 2018 16:23Post deleted, initial situation overlooked.
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