Suspended wood insulation

[MikeGrahamT21]

Hi,

 

Just wanting to run my plans past someone to ensure I'm on the right track...

 

We have suspended wood floor (using 4x2), with about 200mm void underneath the timbers and my plans are as follows:

 

• Clean out all of the rubbish from under the floor and hoover all the dust up
• Replace any bad timbers (we have had woodworm previously), and treat any remaining good timbers
• Re-do heating pipes and ensure fully insulated
• Apply a sand/cement parge around the joist ends, and then seal using airtight tape
• 100mm PIR friction fit in between the joists to fully fill them, and use expanding foam to fill in any gaps
• overlay the joists with breathable roofing type membrane, with the 'outside' side facing into the room, overlap by 100mm, and seal with airtight tape.
• Lap the membrane up the walls slightly, and bond to the wall (possibly using an airtight silicone??)
• Flooring to be 18mm OSB3 smart ply
• Skirting board to finish, and cover up the membrane lap

 

Is there any part of this which can be improved at all?

 

Thanks for your input.

 

Regards
Mike

[Russell griffiths]

Why do you need to seal joist ends if you are fitting a membrane on top of the joists. 

Any way you can use a batt type insulation instead. Have you seen the price of sheet type insulation, and also cutting and fitting is a pain. 

 

I have previously used rockwall type insulation suspended on a net system slung over and draped between joists. 

[MikeGrahamT21]

You are correct in terms of air leakage, not much point as it would be below the barrier, but my thinking was to stop any moisture migrating to the joist ends which would be a lot colder.

 

Thought about insulation type, I know fitting PIR is a pain, but with only 100mm to utilise, its the best type per £ i'm going to get. Also, if I use a wool based product, ideally i'd need the membrane below to stop air movement into the insulation, which isn't going to happen. I could lap the membrane over and under which would get the effect i'd need, but that would also be a pain to detail.

[bassanclan]

Is 4x2 the size of your joists? It seems small

[ProDave]

39 minutes ago, bassanclan said:

Is 4x2 the size of your joists? It seems small

Probably short spans between sleeper walls, used o be common.

 

Why use  breathable membrane, surely you want it to be air tight?

[MikeGrahamT21]

2 hours ago, ProDave said:

Probably short spans between sleeper walls, used o be common.

 

Why use  breathable membrane, surely you want it to be air tight?

 

Yeah thats the one, sleeper wall in the middle.

 

Are breather membranes not airtight within 50pa? I know they aren't fully airtight. I seem to remember asking someone before, and for suspended wood it had to be vapour open membrane, so that excess moisture didn't collect under the floor and rot the joists, is that not right? We have plenty of air bricks, and good flow underneath, so I guess it shouldn't be an issue, but was just erring on the side of caution. Plus i've got 2 rolls of the stuff! lol

[MikeGrahamT21]

breather membrane is tyvek supro by the way, not the cheap stuff.

[MikeGrahamT21]

Can't find anything online about Tyvek, but Kingspan Nilvent breather membrane claim "Airtight at normal building pressures", whatever that means?

 

EDIT, found it:

 

Air Permeability Kingspan nilvent ® is airtight, when tested at normal building pressures, i.e. 50 Pa pressure difference.

 

Pretty sure the Tyvek stuff will be the same, but i will check, and if its not airtight, i'll consider the Kingspan Nilvent stuff as a replacement for it, it will come in at some point for the roof.

Edited October 6, 2017 by MikeGrahamT21