Loft insulation and boarding without condensation problems.

[benawhile]

 

We have moved into a 1950's house with minimal insulation.

 

There is a conventional loft space between a hipped roof and plasterboard ceiling with 4” rafters. There is 4” mineral wool insulation between the rafters, which I want to increase by about another 6 inches, either using Celotex, Steico wood fibreboard or similar, or wool insulation roll, sheepwool or mineral, and then part board over for access and storage. If I use wool then I expect I will have to use plastic loft legs. I have also found insulated loft boards available from B & Q with 100mm insulation glued underneath, so we might try wool for the non accessible areas and insulated board for the storage areas.

 

I have read that wool insulation over the ceiling joists in a cold loft should be covered by a membrane to prevent the circulating cold air from wind etc from “wicking” off warm air at the top layer of the insulation.

What kind of membrane should be used for this, as I have also read conversely that a non permeable membrane will cause condensation on the underside as water vapour rises from the house through the ceiling?

 

Also, for the boarded sections, will the board itself act to prevent heat wicking away?

If I use Steico or Celotex, laid across the joists, should I put conventional loft board above that, to protect it, or below it to stop it being compressed where it stands on the joists?

 

For what it's worth, currently the eaves, with fascia and soffit boards, are fully enclosed with no ventilation, so the only loft ventilation is by any accidental gaps between soffit and brickwork, fascia board and felt, and at joints in roofing felt under the concrete tiles.

We have white coloured but probably aluminium framed double glazing, cavity walls but no c.w.i. yet.

 

[A_L]

hello and welcome,

 

Do not put celotex on the cold side of the existing mineral wool insulation, it has a relatively high vapour resistance and particularly if it has a foil face is liable to cause the interstitial condensation you refer to later. For any given thickness Celotex will have about 2x the insulating effect of wood fibre board and both will be relatively expensive compared to mineral/fibreglass wool rolls. 200mm of loft roll wood give the desired effect without the expense of sheep's, wool.

If you are using a membrane to reduce 'wind washing' it must be vapour permeable to prevent condensation. Putting above two 100mm layers and below one layer puts out of the way while still having the desired effect. It need not be 'airtight; windtight is sufficient, e.g. Tyvek Housewrap or equivalent.

Loft boards will reduce heatloss but the extent will be dependent on the insulation backing. Do not use Celotex or Steico across the joists unprotected, the compressive forces from standing on it are to high. Reasonable storage with board on top would be O.K.

Your first insulating task is to fit bonded bead CWI, try the Energy Saving Trust for local subsidised offers.

[Ferdinand]

I think that you need to decide whether you want a warm loft or a cold loft. If a cold loft then you may also be able to get free rockwool added to 250mm, but you would need to remove the existing such that there was under 100mm there when inspected. 

 

I would suggest putting the 100mm you remove under your suspended floors downstairs as underfloor insulation, if they are suspended floors. Messy but not difficult, as you only need to lift a floorboard every 750mm or so, then reach under with a staple gun.

 

And a cold loft will need adequate ventilation.

 

If a warm loft, then you will need teh rockwool out anyway, perhaps to use in the same way.

 

Loft Legs can add up quite quickly if you cover a large area, even though quite cheap each. I hve used them successfully for part boarding out a loft or two,

 

You also need to make sure that you have addressed air leaks from house to loft - far easier to do before insulating ?.

 

Ferdinand

 

Edited November 6, 2018 by Ferdinand

[benawhile]

Sorry to be late in replying, being new to the forum I didn't choose the right notification method.

Thank you for these very helpful replies, especially A_L, which concurs with what a surveyor friend advised after I posted. I'm going for a cold loft. I will be getting CWI, but not until I have arranged surveys of the cavities, as there are wall tie issues in some houses in my road.

The random sized graphite coated beads seem to have a good reputation. I have solid floors downstairs. I would have preferred suspended but heigh ho.

[Ferdinand]

15 hours ago, benawhile said:

Sorry to be late in replying, being new to the forum I didn't choose the right notification method.

Thank you for these very helpful replies, especially A_L, which concurs with what a surveyor friend advised after I posted. I'm going for a cold loft. I will be getting CWI, but not until I have arranged surveys of the cavities, as there are wall tie issues in some houses in my road.

The random sized graphite coated beads seem to have a good reputation. I have solid floors downstairs. I would have preferred suspended but heigh ho.

OK, so... just in case you have not covered these yet (bet you have, mind)

 

1 - Make sure all gaps with downstairs are well sealed to air (and runaway beads...). Well-insulated loft hatch?

2 - Ditto make sure that your loft has adequate ventilation - usually via slots at the eaves.

 

and

 

3 - My normal practice if I am insulating a loft is to work out where I will need to walk in future (eg if some joker has put the boiler up there in the past, or for an MVHR unit or similar) is to fix a walkway at rafter level over the first 100mm layer of insulation, and then putting the remaining 170mm of rockwool over it, congruent with the walkway, such that plumbers and gas men etc can just whip off a couple of pieces of rockwool and have a safe, secure walkway / environment to work with.

 

It also potentially saves me having to deal with elephant holes in the ceiling.

 

To do that with your graphite balls may need a little bit of thought.

 

Ferdinand