Insulation in Attic

[spiidey]

Have been looking through some old threads for advice - we're installing insulation in our attic, rooms below are 4 bedrooms, the upstairs landing and 2 bathrooms (one with shower, one with bath).

We're using Rockwool Prorox SL 960 100mm in between the joists (up to the full height of the joists), and then the usual roll insulation on top of this. Total depth 300mm.

Do we need to have a plastic sheet / moisture barrier directly above the plasterboard i.e. between the top of the plasterboard and the bottom of the Rockwool, to guard against condensation issues e.g. as referred to in the 4th post of the link below.

//www.diynot.com/forums/viewtopic.php?t=251263

 

[freddiemercurystwin]

Not in my opinion but if you're worried then go ahead, it's not a big expense.

 

[spiidey]

Not in my opinion but if you're worried then go ahead, it's not a big expense.

So we won't get condensation issues i.e. the Rockwool becomes damp/wet over time?

I'm confused on the whole condensation issue I have to say!!!

 

[spiidey]

Anyone else able to advise on this?

Having done some research and tried to understand the water vapour / condensation issue, I now know we've got what's termed a "cold roof" (pitched).

However, am I right in assuming the following:-

1. Water vapour can pass through all materials except metals, plastics and glass?

2. The water vapour will be able to pass through all of the following (all of which we're using in our loft insulation job):-

Timber joists
Rockwool Prorox SL 960 (100mm depth)
Normal yellow roll thermal insulation (200mm depth)

3. The key is to ensure that:-

(a) there is adequate airflow in the loft (i.e. the insulation isn't blocking where the air will come in from the soffits) and
(b) we take the normal precautions in our living parts of the house (running extractor fan, not leaving washing draped about the house, opening windows in the morning, etc)

4. Therefore based on all of the above we should have no condensation issue in our loft?

5. Or is there something else I'm missing?

 

[spiidey]

Also, what type of the thermal roll insulation would others recommend? i.e. that does a good job?

Is it worth getting roll insulation that is also acoustic in nature?

 

[Grenage]

Most homes in the UK have simple plasterboard ceilings, and they don't have soaking lofts - you're worrying about it too much. Any plastic you put down under the insulation would be going on top of the joists, so if there was actually any moisture, it would be trapped against the wood.

I'd only bother with a membrane if I was replacing/over-boarding a ceiling.

As long as you've got ventilation up there, throw down whatever insulation you fancy - I wouldn't bother with acoustic values.

Wet clothes etc tend to be more of an issue in the living space, where there's often no real airflow - but it's good practice to avoid.

 

[spiidey]

Most homes in the UK have simple plasterboard ceilings, and they don't have soaking lofts - you're worrying about it too much. Any plastic you put down under the insulation would be going on top of the joists, so if there was actually any moisture, it would be trapped against the wood.

I'd only bother with a membrane if I was replacing/over-boarding a ceiling.

As long as you've got ventilation up there, throw down whatever insulation you fancy - I wouldn't bother with acoustic values.

Wet clothes etc tend to be more of an issue in the living space, where there's often no real airflow - but it's good practice to avoid.

Thanks, I'll dispense with the idea of any membrane!

So in terms of ventilation in the loft itself, how big a gap do I need to leave around the edges of the house (i.e. to allow airflow to come up from/through the soffits and "remove" any water vapour that has travelled up into the loft space)?

 

[freddiemercurystwin]

50mm min, assuming you have vents in the soffits/eaves somewhere.

 

[spiidey]

50mm min, assuming you have vents in the soffits/eaves somewhere.

The soffits overhang the house (i.e. from the top of the joists in the attic, the soffits hang down around 30-50cm (difficult to measure exactly). They have slitted vents in the middle of the soffits, all around the house.

I presume this is standard building practice and doesn't give us any issue in terms of ventilation?

So where does the 50mm gap need to be from/to? Is it from the pitched roof to the insulation, measured vertically down?

 

[Grenage]

From the top of the insulation, up to the roof. Provided you do that, and the insulation isn't on the soffit vents, you'll be fine.

 

[spiidey]

From the top of the insulation, up to the roof. Provided you do that, and the insulation isn't on the soffit vents, you'll be fine.

Brilliant, thanks.

 

[spiidey]

Is there any specific type of the roll insulation that others would recommend?

We're looking for something with a high thermal insulation value.

 

[HawkEye244]

I used the same prolox you mention as it's got good acoustic and thermal properties so can recommend that. I think you'll be happy with it. It's a horrible job though, best to use long sleeve clothing or one of those protective overalls and a dust mask is essential. You'll find yourself itchy as hell if you man handle it too much. I actually insulated the joists of my attic but didn't have the heart to tackle the actual pitch of the roof so I got loads up there waiting to be fitted As for rolls they are all inferior to the rockwool, I've used Isover in places but really anything in that price range will do. Most rolled insulation is pretty much the same quality. You'll find the rolled insulation says 100mm but actually unrolls to perhaps half that so you may end up doubling up in places, this is why it's cheap