Insulation and Loft Boarding - Condensation Questions

[Blaggins]

Hey guys,

I've read that you shouldn't board directly onto the joists with insulation inbetween them because of condensation.
I've also read that you should use the loft legs (or similar) to provide a gap, which you can then fill with more insulation up to the recommended minimum of 270mm.

If you do this, aren't you still boarding over with no gap?? What's the difference (condensation-wise)?

How much gap is needed between insulation and loft boards (for storage) to avoid condensation?

Also, is the same true regardless of the insulation material? thinking about using insulation board under the boarded area to retain more head-height.

Thanks.

(If this question seems familiar it's because it was part of my huge post here which was a bit too wordy to get much of a response!)

 

[Why Not Indeed]

Perhaps you need to consult whoever suggested that you shouldn't board on to joists. I can't think why this would be a condensation issue.

 

[Blaggins]

Perhaps you need to consult whoever suggested that you shouldn't board on to joists. I can't think why this would be a condensation issue.

Thanks for the reply.

No one specifically told me, I’ve just seen it written a bunch of places, including on this forum. Some of those places were presumably trying to sell loft leg solutions and similar, but also seemingly impartial blogs and posts in here mention not boarding directly onto insulation because it needs an air gap?

 

[Blaggins]

Perhaps you need to consult whoever suggested that you shouldn't board on to joists. I can't think why this would be a condensation issue.

to be clear, it's not the boarding onto joists that I think might be an issue, but the fact that the 70mm joists have 70mm of insulation between them, so insulation would be touching the loftboards.

 

[Why Not Indeed]

As you're tight for head height and want to use rigid foam insulation, you don't need a gap. Rigid foam is usually coated on both sides with foil and you can tape the joints with foil tape to give you a vapour/air barrier, thus preventing moisture coming through from your living space into your loft.

 

[Blaggins]

As you're tight for head height and want to use rigid foam insulation, you don't need a gap. Rigid foam is usually coated on both sides with foil and you can tape the joints with foil tape to give you a vapour/air barrier, thus preventing moisture coming through from your living space into your loft.

Wouldn’t that be a problem on top of the existing wool then? Condensation trapped by the solid board next to the wool? Or are you suggesting I ditch the wool and replace with solid board too? my initial thought was leave the wall, solid board over the top, then loft boards over that.

but now I wonder if that’s a bad idea?

basically, my primary objective is storage space. Ideally I’d like to increase the insulation up a Bit from the 70mm of wool I currently have between the rafters. I’d obviously also like to avoid any problems like condensation, if it’s a concern. It’s not the whole loft, just a 1.5m x 3m area in the centre. Bedrooms underneath.

 

[foxhole]

Hey guys,

I've read that you shouldn't board directly onto the joists with insulation inbetween them because of condensation.
I've also read that you should use the loft legs (or similar) to provide a gap, which you can then fill with more insulation up to the recommended minimum of 270mm.

If you do this, aren't you still boarding over with no gap?? What's the difference (condensation-wise)?

How much gap is needed between insulation and loft boards (for storage) to avoid condensation?

Also, is the same true regardless of the insulation material? thinking about using insulation board under the boarded area to retain more head-height.

Thanks.

(If this question seems familiar it's because it was part of my huge post here which was a bit too wordy to get much of a response!)

I think you are confusing rafter insulation with Joist insulation.

 

[Blaggins]

I think you are confusing rafter insulation with Joist insulation.

Yep, sorry - not sure why I switched to saying rafters then when I’ve said joists previously! Apologies, I’m not sure of the right terms.
It’s the horizontal lengths of wood running across the house, above the upstairs ceiling, 70mm deep with glass fibre insulation in between, that I would be looking to board on.
I’m guessing the rafters are the diagonals that directly support the roof?

 

[foxhole]

Yep, sorry - not sure why I switched to saying rafters then when I’ve said joists previously! Apologies, I’m not sure of the right terms.
It’s the horizontal lengths of wood running across the house, above the upstairs ceiling, 70mm deep with glass fibre insulation in between, that I would be looking to board on.
I’m guessing the rafters are the diagonals that directly support the roof?

Yes and subject to condensation if air gap not left when insulating.

 

[Blaggins]

Yes and subject to condensation if air gap not left when insulating.

So back to my original question, how much air gap is needed?
And is there a problem using board insulation on top of glass fibre with loft boards over?
Thanks

 

[foxhole]

So back to my original question, how much air gap is needed?
And is there a problem using board insulation on top of glass fibre with loft boards over?
Thanks

As stated no condensation with joist insulation only rafter .

 

[Blaggins]

As stated no condensation with joist insulation only rafter .

Ah ok, sorry - I misunderstood your response. I thought if the roof/rafters weren’t insulated but the joists/ceiling was with boards above, then you had a cold area (loft) with warm air trapped beneath the board, hence condensation? Have I completely got that wrong? If it really isn’t a concern then happy days!

thanks

 

[Jonathan1111]

Ah ok, sorry - I misunderstood your response. I thought if the roof/rafters weren’t insulated but the joists/ceiling was with boards above, then you had a cold area (loft) with warm air trapped beneath the board, hence condensation? Have I completely got that wrong? If it really isn’t a concern then happy days!

thanks

Condensation can be a problem, warm moist air will come up through the quilt, cooling as it goes, and hopefully reach the loft space before it reaches dew point and the moisture condenses back to liquid water, so it can vent away through the eaves gaps. By overboarding the quilt, the air is trapped and is more likely to condense in the quilt if there is no gap, as the board will be a cold surface. Using a foam board with taped joints can make an airtight layer, so there is no moist air coming up in that area, and will need a thinner layer for the same effect. The downside is the higher cost.

 

[Blaggins]

Condensation can be a problem, warm moist air will come up through the quilt, cooling as it goes, and hopefully reach the loft space before it reaches dew point and the moisture condenses back to liquid water, so it can vent away through the eaves gaps. By overboarding the quilt, the air is trapped and is more likely to condense in the quilt if there is no gap, as the board will be a cold surface. Using a foam board with taped joints can make an airtight layer, so there is no moist air coming up in that area, and will need a thinner layer for the same effect. The downside is the higher cost.

Thanks so much, that’s what I thought!

So would laying insulation board above the joists/wool cause also trap the condensation against the wool then? In which case is it advisable to use board OR wool, not both?

Honestly I’m tempted to just not worry about any additional insulation and create an air gap by cross battening and boarding on that. I’ve lived here this long without the extra insulation! But maybe that’s mad, I don’t know!