Hi, first post, so be gentle! I have read lots of posts on this great site of information but still not 100% if I have this right? I wish to fit insulation to my loft and then board over the top of it all. The joist's seem quite shallow (75mm) as its a 1930's house. I understand that I need to leave a gap between the top of the insulation and the underside of the boards I will be placing on top for ventilation. Is this right? If so, that doesn't leave me much room for insulation. Am I looking at fitting additional joists across these to get around this problem? Any info would be of great help, cheers!
Ventilation gap required between loft insulation and boards?
- Thread starter Mivvi
- Start date
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if you are using fibreglass quilt type insulation then you will need a hefty amount of depth in which to accommodate the thickness of inso' (250mm) and be able to overboard.
you can compress it if you wish but this will only serve to lessen the effect of the insulation.
thinner, rigid board type insulation is a good solution.
you can compress it if you wish but this will only serve to lessen the effect of the insulation.
thinner, rigid board type insulation is a good solution.
Hi, thanks for the reply! A rigid board was a route I was thinking, I don't seem to have many options really. Where you say I could compress it, wouldn't that mean leaving 'no' gap for ventilation between the insulation and the boards?
there is no need for a gap.
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I think you might be confusing insulating the joists that form the floor of your loft and the rafters that form the apex of the roof.
When insulating the loft floor there is no need for a gap between the boards and insulation. It is however necessary not to push the insulation under the eaves (where the roof meets the floor) as this is where the air comes in to the loft space to keep it dry.
When insulating the rafters there needs to be a gap between insulation and the tiles.
So if you want to improve your loft insulation you would probably be best fitting 75mm Cellotex, Kingspan, or Xtratherm boards as they offer higher insulation properties than rockwool or fiberglass. However it is a lot more expensive and you must cut it accurately to fit snug between each joist. Typically a 1930's house will be built by eye, not measured so you will find that the gap between joists will be odd sizes so you will have to cut funny shapes from the board. You will then need to use foil tape to seal the joins.
If all you are doing is using the loft for storage your easiest and cheapest option is probably to fill the existing joists with rockwool or fiberglass and then lay some new joists across the existing ones. lay a second layer of rockwool between these and then put your floor boards down.
When insulating the loft floor there is no need for a gap between the boards and insulation. It is however necessary not to push the insulation under the eaves (where the roof meets the floor) as this is where the air comes in to the loft space to keep it dry.
When insulating the rafters there needs to be a gap between insulation and the tiles.
So if you want to improve your loft insulation you would probably be best fitting 75mm Cellotex, Kingspan, or Xtratherm boards as they offer higher insulation properties than rockwool or fiberglass. However it is a lot more expensive and you must cut it accurately to fit snug between each joist. Typically a 1930's house will be built by eye, not measured so you will find that the gap between joists will be odd sizes so you will have to cut funny shapes from the board. You will then need to use foil tape to seal the joins.
If all you are doing is using the loft for storage your easiest and cheapest option is probably to fill the existing joists with rockwool or fiberglass and then lay some new joists across the existing ones. lay a second layer of rockwool between these and then put your floor boards down.
Hi 'goingdigital', you were right, I was getting confused with the rafters after reading so many posts! Well you have pointed me in the right direction now. I will price up Kingspan to see how that comes out price wise first. If not I will go down the route of rockwool and additional rafters on top of that. I understand that if in the future I decide to dorma the attic, I would have to strenthen the existing joists. Can I incorporate this into the task ahead? Is there a specific thickness of joists that must be fitted? If I need to create a specific post regarding this, I will do. Thanks again all for your input, much help.
There seems to be a lot of confusion re boarding of lofts over insulation. A lot of people advise cross battening over the joists leaving a continuous air gap over the insulation for ventilation purposes. Is this now not considered wise?
There is sadly quite a lot of misinformation on this thread, and I've seen people on YouTube recently believe what's been written. To clarify, there absolutely IS a need to leave a gap between the top of your insulation and the underside of the boards.
The British Board of Agrement (the BBA - UK's authority for approving building products) says this should be 50mm (for example, see Section 8 on the approval certificate for the LoftZone raised loft boarding product here: http://www.bbacerts.co.uk/CertificateFiles/52/5269PS1i1.pdf) though I've seen a report from the Buildings Research Establishment (BRE) that says it can be only 29mm if you have excellent ventilation from eave to eave.
The point of the gap is to remove moist air. How much you have depends on how humid your house is and how well sealed your ceiling is, so not all houses will suffer this, but water vapour moves through the plasterboard ceiling, even if there are no gaps, and will cool as it passes through the insulation. So there's a risk that it will condense on the underside of the boards, causing damp and mould, which is why the BBA and BRE insists on the air gap. It's particularly bad in the winter, when the loft is cold. I've seen plenty of cases of boards getting so sodden that both them and the insulation have to be thrown away, and where fungus/water stains the ceiling and leaks down into the room below.
As a DIY'er it's up to whether you want to take that risk, but it wouldn't be allowed for a professional builder. There are plenty of ways to raise the boards above the insulation, it's not hard to do.
The British Board of Agrement (the BBA - UK's authority for approving building products) says this should be 50mm (for example, see Section 8 on the approval certificate for the LoftZone raised loft boarding product here: http://www.bbacerts.co.uk/CertificateFiles/52/5269PS1i1.pdf) though I've seen a report from the Buildings Research Establishment (BRE) that says it can be only 29mm if you have excellent ventilation from eave to eave.
The point of the gap is to remove moist air. How much you have depends on how humid your house is and how well sealed your ceiling is, so not all houses will suffer this, but water vapour moves through the plasterboard ceiling, even if there are no gaps, and will cool as it passes through the insulation. So there's a risk that it will condense on the underside of the boards, causing damp and mould, which is why the BBA and BRE insists on the air gap. It's particularly bad in the winter, when the loft is cold. I've seen plenty of cases of boards getting so sodden that both them and the insulation have to be thrown away, and where fungus/water stains the ceiling and leaks down into the room below.
As a DIY'er it's up to whether you want to take that risk, but it wouldn't be allowed for a professional builder. There are plenty of ways to raise the boards above the insulation, it's not hard to do.
@Mivvi have a look at loftzone store floor. This is the route I plan to go down.
Ideally you need 270mm of loft insulation plus some breathing space. The loftzone kit comprises of some plastic triangles that you screw to the joists. You then screw down some aluminium beams and then screw the loft boards to this.
The kit allows you to raise the floor to the minimum level, allowing for 270mm insulation plus breathing space.
Alternatively you could screw down some larger beams at 90 degrees to the previous joists (after laying down 100mm insulation), lay down some extra insulation and the screw the boards to the joists.
Ideally you need 270mm of loft insulation plus some breathing space. The loftzone kit comprises of some plastic triangles that you screw to the joists. You then screw down some aluminium beams and then screw the loft boards to this.
The kit allows you to raise the floor to the minimum level, allowing for 270mm insulation plus breathing space.
Alternatively you could screw down some larger beams at 90 degrees to the previous joists (after laying down 100mm insulation), lay down some extra insulation and the screw the boards to the joists.
I expect Mivvi has sorted it now, nearly a decade on .....
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