DIY Doctor

Adding joists to loft for insulation depth and storage load

Postby anakin » Tue Nov 13, 2012 12:45 pm

Hi All

At the moment I have regular ceiling joists ( 75mm ?) in the loft of my 1930s semi. The roof joists are supported on purlings. I have just had eaves ventialtors fitted along with ventilator tiles fitted near the ridge to avoid condensation problems. I now intend to update the insulation from the old thin insulation currently fitted to 200mm-300mm

The ceiling joists run from the front to the rear and a section is boarded over for storage. This section occupies half the width of the loft spacebetween the party wall and the load-bearing dividing wall running down the centre of the house from front to back. The boarding sits astride another wall running across half the width of the house (ie the dividing wall between the front and rear bedrooms - also a proper brick wall rather than a partition ). The loft boarding extends 5 - 6 feet either side of this wall.

I want to get a decent thickness of insulation and improve the load bearing of the boarded area. I was considering running a few joists across the existing ceiling joists at 90 degrees and supporting the ends on joist hangers at the party wall end and sitting on the front-to-back wall at the other end. Then I would put nice fat insulation between the joists and board over.

In addition, I was thinking of laying a vapour barrier below all the insulation that would end up under the boaring to prevent any condensation under the loft boards.

Does this seem reasonable?

Thanx in advance for any advice
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Simply Build It

Postby Perry525 » Sun Nov 18, 2012 7:01 pm

In the 1930's most houses were built using 4x2 inches 100x50mm.

Joists are under a lot of stress from the weight of the roof, this weight pulls them like a violin string and as such they are strong enough for most loads, floor boards, people, insulation, boxes etc. that people put in their loft.

Your best bet, is to remove the floor boards and to put closed cell insulation under them between the joists, cut it to a push tight fit and fill any gaps with spray foam. Use the removed insulation, plus more to bring the rest of the loft up to standard.

Keep in mind that from time to time the thickness required will no doubt increase. The more you put in now, taking care to avoid any gaps the more heat heat you will save over the years and the lower your heating bills.

You probably understand that the water vapour in your home is created by cooking, washing, breathing, sweating animals, children, indoor flowers etc.

Water vapour is programmed to move from hot to cold, so it is always trying to escape to the outside.

A normal plaster ceiling with fiberglass insulation is transparent to water vapour and it passes through without problem. If you install a water vapour proof membrane you will then need to deal with the build up of water vapour in the home. Letting the water vapour find its own way out through the plaster/fiberglass part of the ceiling will work.

Some form of mechanical ventilation or remembering to open the windows from time to time will otherwise be called for - better to stick with the plasterboard and fiberglass and forget the water vapour proof membrane - you don't add to the cost, nor do you need to let your warm wet air out and let cold air in. This only adds to the heating cost.
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