PIR inbetween rafters in storage space, but should I block the vents?

[bnsbts]

Hi all,

I'm in the process of planning an upgrade to my loft insulation. I already have rockwool inbetween the joists in the ceiling/loft floor and I'm planning on adding another 250mm at 90 degrees, following guidence, and avoiding the loft ventilation areas.

However I use this attic space to store some items that can be prone to damage from external tempatures. Vinyl records etc.

Because of this i'm looking to insulate between the rafters with PIR (and gapotape) leaving a 25mm gap between the insulation and the felt under the roof tiles for ventilation. This in theory, from my research will be acceptable, were it not for the fact that I'm not introducing an alternative ventilation while blocking the existing ventilation in the loft space with this new layer of insulation.

My question is, what's the best option to ventilate this new "room" I'm creating?
As as far is it goes for now, the loft is basically like being outside, great ventilation and temperatures matching the other side of the tiles.

 

[harrythefurrysquid]

Skipping forward to when you've introduced the extra PIR:

What temperature are you aiming for inside the space? How will you heat it?

 

[freddiemercurystwin]

Because of this i'm looking to insulate between the rafters with PIR (and gapotape) leaving a 25mm gap between the insulation and the felt under the roof tiles for ventilation. This in theory, from my research will be acceptable,

Assuming you have a non breathable felt underlay, rather than a breathable one, you would need to introduce vents at the ridge for this plan to be effective, whilst presumably you already have vents at the eaves, unless there is also a vent at the ridge then the space will not be able to vent and may well be prone to a build up of condensation.

 

[Notch7]

Hi all,

I'm in the process of planning an upgrade to my loft insulation. I already have rockwool inbetween the joists in the ceiling/loft floor and I'm planning on adding another 250mm at 90 degrees, following guidence, and avoiding the loft ventilation areas.

However I use this attic space to store some items that can be prone to damage from external tempatures. Vinyl records etc.

Because of this i'm looking to insulate between the rafters with PIR (and gapotape) leaving a 25mm gap between the insulation and the felt under the roof tiles for ventilation. This in theory, from my research will be acceptable, were it not for the fact that I'm not introducing an alternative ventilation while blocking the existing ventilation in the loft space with this new layer of insulation.

My question is, what's the best option to ventilate this new "room" I'm creating?
As as far is it goes for now, the loft is basically like being outside, great ventilation and temperatures matching the other side of the tiles.

the insulation won’t alter temperatures as it’s an unheated space.

However it should stop rapid temperature change

your risk is condensation if you get the ventilation wrong.

if you reduce ventilation you need to be very careful vapour isn’t getting into the loft from airing cupboard, bathroom, bedrooms, F/ E or water tank.

 

[bnsbts]

Skipping forward to when you've introduced the extra PIR:

What temperature are you aiming for inside the space? How will you heat it?

Anywhere between 10 and 25'C.

It won't be heated beyond any transfer of heat from below.

Perhaps insulation is pointless without a heat source.

 

[bnsbts]

Assuming you have a non breathable felt underlay, rather than a breathable one, you would need to introduce vents at the ridge for this plan to be effective, whilst presumably you already have vents at the eaves, unless there is also a vent at the ridge then the space will not be able to vent and may well be prone to a build up of condensation.

Thanks, my plan is to ventilate that space created with ridge vents along with the current sofit vents.

My main worry is that condensation will build within the insulated space.

 

[harrythefurrysquid]

Anywhere between 10 and 25'C.

It won't be heated beyond any transfer of heat from below.

Perhaps insulation is pointless without a heat source.

Like @Notch7 says, it should help reduce temperature swings, but I'm not sure how much warmer it'll make it unless you compromise the floor insulation. Cold air can hold less moisture than warm air, and the PIR and tape will make it hard for any moisture to escape, so unless you have a super-effective VCL underneath your existing insulation, I feel like it's going to be a quite-cold damp space instead of the current cold dry space.

I believe the migitation would be to properly insulate between and over the rafters, including a VCL on the warm side, and actively heat and condition the space so it's warm and dry. The VCL would minimise moisture transfer into the cold space which combined with soffit and ridge vents should then stay dry.

But IMHO it does sound a lot simpler and cheaper to just leave it as a cold dry-ish space and just not store temperature-sensitive stuff up there.

 

[old duffer]

I have boarded my all of my lofts (5 separate spaces with different access points) and recently I laid an old carpet over the floors (the hassle of taking it to the tip was the motivator). I also applied the foil insulation over the rafter insulation (150mm) and sealed the joins with tape. I can tell from the ice/snow on the roof and how long it remains on the roof that its worked a treat and its much cooler in the loft spaces now.