Attic Insulation + Sound Proofing

philboy

Hi,

I am converting my attic currently and am also soundproofing it. I have a question regarding the proper method of insulation as I'm worried I may create a mositure problem trying to both insulate and soundproof it.

I will be using the area behind both purlins for storage and would like this area insulated to keep it warm and help prevent mold on the stored items.

I have 5x2 rafters and the plan is 50mm airgap with 75mm Rockwool RW3 between them. Then counter batten and add 50mm Metac. In the storage void behind the queen post, I will simply put 12.5mm fire line board over the Metac and then airtight membrane.

In the ceiling area of room, I will add MLV on top of the 12.5mm plasterboard and then add 50mm insulated plasterboard over it.

I wonder can I also add 75mm RW3 to the queen post too for soundproofing? I'm worried that because the storage area will have insulation on all 3 sides (roof, floor and queen post/knee wall) that this may cause moisture problems.

My reason of adding Rockwool to the knee wall is to prevent noise coming from the void as a school is being built behind us. I'm referring to the areas in green in the picture but I will also be insulating the red area. Thanks!

10-10-20

Okay... I for one are slightly lost on the buildup there. Might need a more detailed pictoral on the matter.

But if I get the short version of the long part 😀 then I'd be concerned about your plan to insulate both directly behind the queen/knee-wall and up against the rafters as there will be a temperature difference between that and the living space, potentially leading to some element of condensation (all depending on the average RH/DP of the air masses involved).

What are the options to treat the rafters with soundproofing all the way down to the hoists and have a single envelope rather than a problematic hybrid approach?

philboy

My reasoning to have the extra Rockwool RW3 in the knee wall in addition to the rafters was to try and isolate some of the noise from the cold water storage tank in one side of the attic.

If I insulate the rafters all the way down to the eaves with the RW3 and counterbatten and add the Metac. I could then leave out the insulation on the knee walls (RW3 and insulated plasterboard). The knee wall would then have 2 layers of 12.5mm fireline plasterboard with the 5mm Mass Loaded Vinyl sandwiches in between for soundproofing.

The reason I am not running the soundproofing down to the eaves and using the knee walls instead is because the floor space of the attic conversion is treated with multiple layers of soundproofing. Behind the purlins is not treated with these multiple layers so I don't want noise leaking up from downstairs from behind the purlins and through the knee walls. This is the main reason I had added in the RW3 behind the knee walls. Although, adding this as an acoustic barrier creates the extra layer of insulation which might cause the mold problem.

I hope this makes sense. I wonder is there a free tool I can use to draw out the plan and explain it properly via a diagram/pic.

Thanks

Lumen

If I understand correctly, the volume in green is going to be a ventilated, unheated space. It will therefore essentially be outdoors.

You therefore need insulation and air tightness on the knee wall and ceiling or else you'll get heat loss and condensation on the other side.

I'm unclear on whether insulating the slope of that space is worthwhile or a good idea.

philboy

Ho Lumen,

For the part in green what I had hoped to do was the following:

RAFTER

• 50mm airgap
• 75mm RW3 Rockwool
• Counterbatten
• 50mm Metac

FLOOR

• 125mm RW3 Rockwool
• Counterbatten
• 50mm Metac

KNEE WALL

• 75mm RW3 Rockwool

The problem I think may be that the green part is not heated except from heat rising from bedroom below and it's insulated and sealed on all 3 sides. The ventilation is only going to pass behind the insulation of the rafters to keep the rafters dry. There won't be any ventilation on the actual green storage area. I do intend to airtight membrane both the storage area in green and the living area. The living area in red will be heated by a large electric radiator.

Padre_Pio

Just insulate right down to the wall. It doesn't use much more material, less bevel cuts and you have a warm storage space. Also means your whole house has double insulation which will help with your heating bills.

Scr55

If I may share my experience with noise and the attic I would recommend looking into the type of window used. I thought that 3-ple glaze windows will reduct noise significantly. Unfortunately, I have noticed that attic windows equipped with windows vents will pass a lot of noise. My windows are Fakro so top class and there is significant difference in noise transmission when vents are open or closed.