underfloor insulation

[messimagician]

I would to insulate under my lounge that is suspended timber and I am in two minds what to use, celotex or sheep's wool ? I am thinking sheep's wool since will be easier to use. I plan to use that netting used on scaffholding to create baskets for the sheep's wool, I know i will also need a vapour membrane to go over the joists with floorboards then put on top.

My concerns are that the other ground floor rooms wont have any insulation or vapour membrane, will this cause problems with moisture condensing under the lounge ?

Another potential issue is i plan to run cables in some conduit under the lounge floor for AV equipment, will having the conduit come up through the floor cause problems ? with warm air travelling down the inside of the conduit ?

 

[AronSearle]

Warm air wont travel down, more likely cold air will be sucked up (warm air will escape upwards through gaps in the building fabric, light fittings, extractors, loft hatches, gaps in or around windows).

Insulating one bit of the floor and not the other should not cause a condensation problem.

What will is if the path for ventilation is blocked. Normally there is a sufficient air gap *under* the joists, and gaps in any intermediate brick up-stands to allow through ventilation, these gaps are again typically under the floor joists. If through ventilation depends upon airflow between the joists rather than under them, then you need to rectify that regardless of insulating bits or all of it.

I've heard anecdotal cases of sheep's wool going mouldy, I reckon in cases where the ground is particularly damp. So probably fine for most people, but if you have damp ground conditions then rigid foam may be more robust.

 

[messimagician]

Ok thanks, that's put my mind a bit at ease

The joists run 90 degrees to the air bricks located at the front and back of the property, do you also have vents at the joists ends as well ? I believe sheep's wool is suppose to breathable so it should allow air to pass though it unlike celotex which is another thing that put me off it

 

[AronSearle]

You don't want air passing through your insulation, as that just means cold air blows through it, it is part of the reason why rigid insulation materials are better as the air is trapped.

Breathable means water vapour can pass through it, celotex without the foil facing is breathable.

You want airflow under the joists and insulation. If practicable it's good to install a breather membrane under the joists, as this maintains breathability, but reduces airflow going through the insulation.

 

[mointainwalker]

celotex without the foil facing is breathable.

I disagree: no foams be they PUR, PIR or PEX are breathable. I think the foil is just a marketing idea.

I recall seeing a report of an experiment that said that after 30 days immersion in water, a sheet of foam (don't remember type) added 0.07% of its original weight due to water-absorbtion.

Celotex (and similar) insulate by having closed-cell structures which are filled with an insulating gas. Since they are closed-cell, air cannot penetrate the cells and there are no voids between the cells which would permit air passage.

 

[AronSearle]

I disagree: no foams be they PUR, PIR or PEX are breathable. I think the foil is just a marketing idea.

Celotex (and similar) insulate by having closed-cell structures which are filled with an insulating gas. Since they are closed-cell, air cannot penetrate the cells and there are no voids between the cells which would permit air passage.

You are confusing vapour passage with air-tightness, breathability means allowing the passage of vapour.

Rigid foam values do vary depending on the type, but typical vapour resistivity is IIRC around 300 MNs/gm, which is similar to hardwood.

 

[mointainwalker]

You are confusing vapour passage with air-tightness, breathability means allowing the passage of vapour

I think it would be rather difficult to confuse those concepts, so no, I am not.

Products like Tyvek are breathable, are designed to be breathable and insulation products should, ideally, be sealed against movement of gases.

Whatever transmission is allowed by Celotex - and your own illustrations indicate that it would be miniscule - I do not think anyone could ever reasonably describe it as breathable as not only would it be wrong, but would be a negative for the product.

 

[AronSearle]

You are confusing vapour passage with air-tightness, breathability means allowing the passage of vapour

I think it would be rather difficult to confuse those concepts, so no, I am not.

Products like Tyvek are breathable, are designed to be breathable and insulation products should, ideally, be sealed against movement of gases.

Whatever transmission is allowed by Celotex - and your own illustrations indicate that it would be miniscule - I do not think anyone could ever reasonably describe it as breathable as not only would it be wrong, but would be a negative for the product.

Wood allows the passage of vapour, but not liquid water (think wooden cups).

'breathable' in the context of building materials refers to the ability to allow moisture vapour through, not air.

FYI Tyvek do air-tight breather membranes, as well as air permeable breather membranes..