Loft - vapour barrier - fit between joists or blanket over?

[Dhillon]

Hi,
I have literally read hundreds of posts on the need to fit a vapour on the warm side of loft insulation at floor level. I.e. plaster board, vapour barrier (not needed if foil backed plasterboard), joists and loft insulation.

Background - Coming to the end of refurbishing our 1940's 3-bed semi and unfortunately I found out to late it is a good idea to either have foil backed plasterboard to ceilings or a CONTINUOUS vapour membrane.

The loft is 8mx6m with lots of headroom, dry with no signs of previous condensation. The roof has no felt above or below the rafters, only tiles. So there is good ventilation at the eaves and roof.

Currently no insulation as the old stuff was unrecognisable and binned. The plan is to lay 100mm roll insulation between the existing 4x2 joists, cross batten, lay another 100mm of insulation and board over the top. Bingo.

Now back to the vapour barrier, before any insulation goes down. Remember I am not pulling any ceilings down so it has to be a retrofit if anything. Some say the membrane needs to be cut into strips, laid between the joists and stapled every 6 to 8". Others say it can be simply draped over the joists?

I can see the strips working to some extent, even though they are far from a continuous barrier perhaps better than nothing? As for draping the membrane over the joists and infilling with insulation, not sure about this. Will this not trap condensation against the joists causing problems?

I welcome your thoughts and suggestions.

Oh if it helps I intend to use a non-itch loft roll insulation made from 85% recycled plastic bottles . Not sure if I'm allowed to mention the exact product so I will not for now.

 

[freddiemercurystwin]

There is a school of thought that thousands upon thousands of houses accross the country do not have a vapour barrier under their loft insulation yet do not suffer any condensation problems in the loft or within the insulation.

 

[Dhillon]

Hi, yes I know what you mean. The thought has crossed my mind to go without a vapour barrier... suck it and see as it were. Or at a minimum some sort of barrier over the bathroom.

 

[Dhillon]

Found some very useful Vapour Barrier information on a US website

Installing Vapor Diffusion Retarders in Existing Homes
Except for extensive remodeling projects, it's difficult to add materials like sheet plastic as a vapor diffusion retarder to an existing home. However, many existing homes don't really need a more effective vapor diffusion retarder than the numerous layers of paint usually on their walls and ceilings. These multiple layers are quite effective as a vapor diffusion retarder in all but the most extreme northern climates.

Vapor Barriers...
http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810
How Moisture Moves through a Home...
http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11760

I am going to forgo trying to retro fit vapour barrier to the whole loft floor. Instead concentrate on getting a good seal over the bathroom from where moisture is most likely to enter the loft.

 

[mointainwalker]

Since the loft is empty, why are you vacillating over spending twenty or thirty quid after having refurbished the whole house ?

Surely just on the basis of better safe than sorry, it;s worthwhile putting down a vb over the whole loft ? I am on the side of one continuous sheet.

 

[Dhillon]

Hi mointainwalker,
I did not mention anything about cost being an issue. In fact I was looking into products costing a lot more than £20 to £30. My concern is that it will not be one continuous barrier when laid between the joists. On the other hand laying one continuous sheet over the joists will trap condensation against the joists which defeats the purpose does it not? I hope that's not what you were suggesting

I am waiting to take delivery of loft insulation and am still open to suggestions.

 

[mointainwalker]

The vb stops the moist air penetrating the insulation and therefore traps it on the warm side of the insulation i.e. in your house.

If you are not already suffering from condensation in your house., how is it going to start forming now ?

 

[Dhillon]

I am struggling to understand what you are suggesting with regards to my situation. As I have explained the ceilings are already plasterboarded how can I possibly fit a continuous vapour barrier? Laying it over the joists would not be an option as it would trap moisture against the joists.

If you are not already suffering from condensation in your house., how is it going to start forming now ?

it may not form, but by increasing the insulation from virtually nothing to 200mm and boarding over the top it increases the chances of it forming.

 

[mointainwalker]

Laying it over the joists would not be an option as it would trap moisture against the joists.

I don't believe so but am tired of going through this so signing off here.

I welcome your thoughts and suggestions.

I think this from your opening post was not really correct as you obviously had your mind made up.

 

[Dhillon]

What do you mean you don't think so? Work it out. Your advocating enveloping joists in a non breathable membrane. Any water vapour rising through the plasterboard or any gaps will have no where to go, it will condense on to the joists.

Just because I do not agree with your suggestion does not mean I am not open to taking advice. I really appreciate your suggestion but I think in this case it is misguided.

Healthy debate though and I hope it stokes some more interest

 

[freddiemercurystwin]

Ignoring mointainwalker's hissy fit if you were to fit a vapour barrier (though as previously mentioned I fail to see any reason to do so) then just fit it between the ceiling joists.

 

[marsaday]

Why bother fitting a VB ? It is just not needed. Currently any warm air does manage to find its way up and out to the roof space above.

Fitting it between the joists is the only solution you have and will need to be stapled into the joist.

I really dont think it is needed though.

If you were to fit a new ceiling in the bathroom (old ceiling maybe) then in that instance i would fit a VB before the PB goes on.

But really your VB in this case is just not required like M has stated

 

[Gerrydelasel]

I wish people would report back and say how it all turned out...