Under rafter loft insulation advice

wpp34

I have been quoted a lot for spray on foam insulation directly on to the roofing felt in my loft as I cannot add more layers over the ceiling.  I have looked into this and I do not like the fact that once applied it is very difficult to remove. I also don’t like the cost. It seems that I could insulate  the roof by using super quilt stapled into place horizontally. This would be possible but I wondered if anyone had done something similar and if there might be problems if not done correctly.

Jeepers_Creepers

Why do you want to do this? Do you think it'll make your house warmer, or are you trying to keep your loft warm?

How many layers are already on your ceiling, and why can't you add more?

Grenage

Don't do it; keep the loft space cold and just insulate the floorspace.  You don't really want anything that can trap moisture or impede air flow.

FreeBear

wpp34 said: I have been quoted a lot for spray on foam insulation directly on to the roofing felt in my loft as I cannot add more layers over the ceiling.

There is also anecdotal evidence of some mortgage providers refusing to lend on properties with spray foam insulation. It also traps moisture within the rood timbers which leads to rot - A very expensive repair bill when (not if) that happens.

A superquilt, whilst looking good on paper, is actually not a very good insulator - It is good for reflecting heat back into a space, but a loft is inherently cold & draughty in the first place.

If you can't increase the thickness of the current loft insulation (presumably fibreglass or rockwool), your options are limited. Replacing the fibre insulation with well fitted Celotex/Kingspan type boards will increase the thermal performance by some 30% for the same thickness. Expensive, but it has the advantage of being easily removed if it does cause a problem.

wpp34

Thanks  for the replies, very helpful. A few answers:
1 house is 3 storey Victorian semi so poorly insulated. Loft partially boarded over and the rest has around 200 mm of insulation. Want to reduce heat loss.
2 difficult to lay more insulation because of boards so cannot add more layers over the floor space.
3. I am worried about moisture build up so have noticed some materials claim to be breathable.
My theory is that heat escaping through current insulation is then dissipated through the roof as it is just slates over roofing felt. If I put a layer of insulation over this it would mean the loft space was warmer. If the loft is warmer then less heat will flow into it as the temperature difference will be smaller. Therefore more heat stays in the rest of the house. 

Jeepers_Creepers

wpp34 said:

Thanks  for the replies, very helpful. A few answers:
1 house is 3 storey Victorian semi so poorly insulated. Loft partially boarded over and the rest has around 200 mm of insulation. Want to reduce heat loss.
2 difficult to lay more insulation because of boards so cannot add more layers over the floor space.
3. I am worried about moisture build up so have noticed some materials claim to be breathable.
My theory is that heat escaping through current insulation is then dissipated through the roof as it is just slates over roofing felt. If I put a layer of insulation over this it would mean the loft space was warmer. If the loft is warmer then less heat will flow into it as the temperature difference will be smaller. Therefore more heat stays in the rest of the house. 

Yes, it sounds good in theory; nice warm loft = nice warm house below.

In practice, it should be as Grenage says. The loft must be kept well ventilated in order for it to be dry and have no risk of damp. That means it will be cold up there (in winter at least...), and that's fine - it should almost be thought of as an external space, open to as much air as possible.

If you restrict the ventilation up there - and adding insulation to the undersides of the slates will help to do this - then the loft will still be the coldest part of your house in winter, but now all the warm moist air which percolates from your house up into the loft (and this is a continuous process) will not be ventilated away, but will instead condense out on the colder surfaces up there and will almost certainly cause long-term damage.

It seems counter-intuitive to begin with, unless you look at what actually happens up there.

You cannot temporarily lift the boards up there to check/add insulation to the top of your ceiling?

If you do add more loft insulation, make sure it doesn't block any ventilation along the front and back eaves, where the roofline comes down to the walls - you want an air gap along there on both sides.

wpp34

Good points Jeepers. I have checked the loft for ventilation and it is good and there are also vents in the roof. Have decided to leave well alone but in the future may move the junk around, lift a board and add a bit more insulation. 

(I was looking forward to having an excuse to buy a cordless stapler to fix the quilt to the rafters but that will have to wait as well!)

TELLIT01

Would fitting foil backed insulation board between the rafters do the job?  Leaving a gap at eaves level for ventilation purposes should avoid any damp issues.

TELLIT01

Watching 'Grand Designs' and other home improvement programmes, spray insulation seems to be coming back into use.  I wouldn't discount it out of hand based on possible problems in the past.  Technology moves on.

Mojisola

wpp34 said:

This would be possible but I wondered if anyone had done something similar and if there might be problems if not done correctly.

A previous thread on this -

https://forums.moneysavingexpert.com/discussion/6225109/homebuyers-report-came-back-with-spray-foam-insulation-issue/p1

FreeBear

TELLIT01 said: Watching 'Grand Designs' and other home improvement programmes, spray insulation seems to be coming back into use.

If the building is designed an constructed with spray foam in mind from the outset, it shouldn't be a issue. Spray foam is used quite extensively in North America where timber construction is pretty much standard. Loft spaces are usually insulated between the joists (with foam rather than fibre), so the roof space remains cold & ventilated. It still causes problems when retrofitted - Just do a search for "spray foam fishy smell"