Ventilation Gap between loft floor and insulation

I'm flooring an attic on battens at 90 degree angle to existing joists.
The existing joists are insulated with a mixture of Kingspan 75 mm Kooltherm board that we had left over from an extension, and fibreglass.
The Kingspan board does not cover the full width of the joist, hence the need for fibreglass for the remainder.
I planned to run 2 inches fibreglass across the joists (the battens are 2 inches high) but now wonder if I should leave this for ventilation.

If you are converting this space for habitable use you need to do a lot more than this Joe - new joists, new proper stairs access [not a ladder], alternative fire escape/velux rooflights, upgraded fire protection for stairs enclosure and intermediate floors, better FDA - there's a whole host of stuff to do.

Even if its only for storage, I would caution you against what you're doing because of what has happened to many people last winter in highly insulated houses where the insulation was installed at ceiling level.
Attics froze and/or filled with moisture, especially where the 50mm continuous eaves vent was blocked.
You can find posts here an on the planning forum on Boards.ie telling of their horror stories.

Ideally the insulation should follow the envelope, i.e. the roof joists and the detailing should confirm with Part F of the building regulations, Diagram 6 P. 11.
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,22447,en.pdf

I am not in a position to comment on how this might interact with your wall insulation and you should take specialist advice from a competent architect or technician.

ONQ.http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,22447,en.pdf (<br /> <br />)[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

Thanks Onq,

It's not for habitual use - just light storage of items that won't be affected by the cold. We have a stira stairs for access. Our walls are pumped with warmfil.
Water tank and all pipework is already well insulated.
The eaves won't be blocked. The attic was already insulated for many years, we just beefed it up a bit with the strips of Kingspan board instead of dumping it.
We had no issues up there last year during the heavy frost.

Do I need the gap between insulation and boards ? I know its needed if insulating between the rafters but not clear about between joists.

JoeRoberts said:

Thanks Onq,

It's not for habitual use - just light storage of items that won't be affected by the cold. We have a stira stairs for access. Our walls are pumped with warmfil.
Water tank and all pipework is already well insulated.
The eaves won't be blocked. The attic was already insulated for many years, we just beefed it up a bit with the strips of Kingspan board instead of dumping it.
We had no issues up there last year during the heavy frost.

Do I need the gap between insulation and boards ? I know its needed if insulating between the rafters but not clear about between joists.

Click to expand...

Yeah, you're very welcome Joe.

As I said, its not the way I'd do it so I cannot comment on the need or otherwise for the gap you describe.

The more you insulate the ceiling the more you risk condensation forming on cold surfaces within the attic space.

According to problems reported online this can include the underside of the felt and the bottoms of boxes stored on the new flooring.

The lack of foil back plasterboard to the ceiling below or the presence of downlighters penetrating any vapour check will make this risk worse.

ONQ
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,22447,en.pdf (<br />)[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

You may not need a ventilation gap in your floor. I'm not 100% on what you are trying to do, or the proposed thickness of new insulation to be used.

I see the benifit of increasing the insulation to the undersude of rafters, to save heat while still keeping the attic storeroom warmer that outside.

Note also that electric wires / cables laid in the floor should not be over insulated due to risk of fire!
Have you considered this?

+ 1what RKQ has said.

The routing of cabling and pipe relocation/floor joist pipe relocating must be carefully considered - notching joists may not be a good idea.

You are trying to save money by flooring out on top of ceiling joists.
Ceiling joists can be up to 40% undersized in breadth and depth compared to floor joists

Ceiling Joists = 150 mm deep x 37mm wide instead of Floor Joists = 200mm x 44mm or 225mm deep x 44mm minimum wide.

The section strength also depends on the cross sectional area [CSA] and in the examples above this works out at

150 x 37 = 5550 sq.mm.

200 x 44 = 8800 sq.mm.

225 x 44 = 9900 sq.mm.

As you can see the largest standard floor joist is nearly twice the CSA of the standard ceiling joist.

Notching out will reduced the load carrying capacity of these ceiling joists which you are about to load up even more.
This will tend to make them deflect and "pop" the plaster nails used to fix the boards as well as lead to structural movements.
With a new attic floor it is advisable to space the new joists up off the existing wall plate by 25mm to allow for their final deflection to avoid this.

Also apart from wiring heat build up, the real risk of fire will arise from anything with a transformer becoming enclosed in insulation and unventilated.

As you can see from the comments above, technical knowledge is required for even the simplest of building activities, because there are a lot of factors to consider beside the task at hand.

The fact that a pal or a builder may advise you to the contrary based on their experience just shows there is a margin for error and they have been lucky thus far.

ONQ
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,22447,en.pdf (<br />)[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

Thanks for the many suggestions and advice.

There will be no notches in the joists for wires - that is the reason for the battens, to allow the existing wires run over the joists. Our electrican had agreed my plan, wires will not be covered and are all tacked to joists which he says dissipitates the heat. Heavy usage wires such as electric shower are located along join of joists and rafters which will not be insulated. There are only 2 downlights, which will be well ventilated. B & Q sell a v good fixture for this.
Attic floor will be carrying pretty much the same weight (bar the additional flooring weight) as has been for 15 yrs except now it is boarded with less risk of someone putting foot through ceiling. I have also braced the joists to rafters for extra support on advice of builder who did our extension.

Since my original post I have discussed the need for a ventilation gap with a BER asseror ( also a building engineer) and he sees no need. So my original question is now answered. Will monitor condensation levels in attic to establish if extra ventilation needed.

Appreciate the many suggestions. The purpose of these forums is to seek advice from other people more experieneced than oneself without having to always resort to paid for "professional advice". Many people are capable of good DIY tasks with a little bit of guidance and research.

The so called " professional advice" from a surveyor that I got and paid for on purchase of my house 15 yrs ago turned out to be lacking in very many areas so I now tend to do my own research.

I just want to make these comments so as not to put other people off doing a bit of well researched diy. Many people are insulating and boarding lofts.

"Note also that electric wires / cables laid in the floor should not be over insulated due to risk of fire!
Also apart from wiring heat build up, the real risk of fire will arise from anything with a transformer becoming enclosed in insulation and unventilated."

Have just done my bungalow attic with 300mm layers of insulation. However i never checked any wires, just put wagon pipe around downlighters and kept sky booster box away. I have no electric electric showers, all off the range.

Should I be worried and what do I need to do? Cheers.

Hi siteissues,

I'm not an expert on cabling or services, but in general if you insulate your attic at the level of the ceiling below you should make sure your cabling and pipework can't get damaged by being walked on, and is left clearly visible and labelled for maintenance purposes, doesn't create a trip hazard or a fire hazard and won't be affected by cold or heat build up in the attic space.

Lift them, insulate them, label them is my advice, not always easy to do with plumbing installations, which is why flooring out with battens can be a good idea.
It also makes the services a pain to get at if you need to afterwards.

For a definitive comment ask a friendly neighbourhood Mechanical and Electrical Consulting Engineer.

ONQ
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,22447,en.pdf (<br />)[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.