Loft insulation and boarding

3 Jan 2008
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United Kingdom
Hi Guys,
I have just bought a house which has around 50mm loft insulation which I want to increase. I also want to board a portion of the loft for storage.

I have done a search on here and picked up some advise, but couldn't find an answer to my questions.

When I lay the first layer of loft insulation this will come to the top of the roof joists. I then wanted to board the centre of the loft for storage and then insulate the rest of the loft with a second layer of insulation around the boards. So I will have 150mm insulation under the boards in the middle of the loft and 250mm everywhere else. Will this cause any problems?

Also when laying the boards should there be an air gap between the insulation and the boards? or is it ok for the insulation to touch the underside of the boards when resting on the joists?

Sorry for the long winded post. Any help appreciated.
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Also when laying the boards should there be an air gap between the insulation and the boards?
Yes leave the old insulation underneath the boards providing there's enough air gap approximately 50mm for air flow, the boards will act as insulator
I was hoping to put some more insulation on top of what is currently there and then board. but if there has to be an air gap then i won't be able to do this, and I will only have 50mm of insulation.

Also what do people do about ceiling roses? do you cut insulation around them? or will the wires be ok?
In my opinion you do not need and do not want any air-gap between the ceiling and the boards: air-gaps are for roofs.

Standard advice is "Do not insulate over electrical wires".

Having said that , this is most important for cables feeding devices like electric-showers which require a lot of current. A lighting-point is not going to be significant.
So I will have 150mm insulation under the boards in the middle of the loft and 250mm everywhere else. Will this cause any problems?

Don't know. What kind of problems are youthinking about ?
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I was worried about any condensation problems as one part of the loft space will have more insulation than the part in the middle.
Obviously I will leave a gap at the eves for ventilation, as there currently is.

With regard to the wiring, I only have lighting wires in the loft, they look like they are 1.5mm cable, which I believe should not cause any problems.

Many thanks for your advise regarding the boards.

I will lay new insulation to the top of the ceiling joists then board the middle of the loft for storage, then lay more insluation around the boards to make up to 250mm.
However, I am not sure there will be enough slack in the wires to put over the top of 250mm insulation. So not sure how to do this. Most are clipped to joists anyway.
The most popular method for boarding the loft is cross battening with 25mm timber which allow for air gap and cable/pipe on top of the ceiling joists, this way you add more insulation under the loft boards to the height of the ceiling joists, my theory is the heat rises and hit the underside of the board and got nowhere to go creating moisture whereas the insulation is breathable without the boards, this is my own opinion.

Also they do recommend not to insulate under water tank
Thanks for the advise about the water tank..
I didn't want to add extra joists for the extra insulation and the loft isn't very high anyway, and I would lose height that way.
There are not many wires in the part I wish to board so this should not be much of a problem anyway.
In order to be certain of preventing condensation , you must have a vapour barrier that prevents warm, moist air from your living area permeating into the loft. If you do that the thicknes of insulation is irrelevant.

If you do not already have a vapour barrier under your existing insualtion, you should seriously consider installing one. this need be nothing more than a plastic sheet laid dirctly onto the plasterboard and over the joists.

I know that with the problems with the wires and now this, it becomes a real pain but..........................
Thanks for that. This just has insulation straight on top of the plasterboard ceiling.
My previous property was like this also, and was around 15 years old.

Maybe I need to look at doing this too. Does the vapour barrier need to be put between the joists and then insulation on top?
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Has to be a continuous - or taped - sheet over the whole ceiling, between and then over joists and then lay insulation on top/
Manythanks I will look at doing that too then.
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I would be interested in your views freddymercurystwin.

I can sort of see how it would work, but also, most houses I know of don't have a vapour barrier from new.
Perhaps if you use square edge boards rather than T&G and leave small gaps you'll avoid the possibility of condensation under the boards.

Alternatively, a couple of coats of acrylic paint on the ceiling under the boarded area should be more than adequate as a VPL. I suppose this is most important over bathrooms.

I think a well ventilated loft will naturally dry out down at the ceiling (due to solar heating during the day and heat retention at night). The trick with mineral wool in the loft is for dry air to diffuse into the insulation to dilute the water vapour coming up through the ceiling and prevent condensation. As wood is probably less permeable than plasterboard, boarding tightly over the insulation could inhibit this exchange and allow the humidity under the boards to build up to levels of the room below and cause condensation.

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