Another loft/roof insulation Question

I have nearly cleared the loft of the limited old loft roll and the crap left by the roofers who installed the new roof.

The loft is spacious, I want to deck the floor for storage use and install lighting and power up there. The joists and rafters are 4x2's which seem very small but obviously years ago that is what they used.

So I now have no insulation in the roof/loft. Given that there is ample space up there I had planned on fixing 50-70mm PIR to the rafters. The idea being that it would give a nicer environment in the loft and that i wont have to cross batten the "floor" to install 200-300mm loft roll.

Is there any issues with this approach and if I go down that route is there any point in installing 100mm loft roll between the joists as well? I have seen loft storage convertor videos online that do both but I have never understood the reasoning, if you insulate the joists why then insulate the rafters?

 

You'll be heating a largely unoccupied space that could have a volume of maybe 50% of the first floor of your house, and with 50-70mm PIR it will lose heat quite quickly. A slightly better compromise might be 200mm in the floor (stilts or blocks would be better to raise it) and 100mm under the rafters.

 

100mm under the rafters is fine. What is the normal depth of quilt to a loft space is it 300mm? Would 100mm of quilt and 100mm of PIR do the job or leave me light. I would rather not cross batten the loft IF it can be avoided.

 

Rafter insulation over an unheated loft is a waste of time, effort and money.

Heat will still rise from the house, and due to heat loss of the insulation, it wont magically mean that the loft is then warmed and that in turn keeps the house warm. The house stays just as cold as with an uninsulated loft.

Likewise for insulating the ceiling and the rafters, only its actually worse - just insulating an even colder space.

 

So woody you are suggesting just go with loft roll to the joists forming the ceiling void?

I assume 100mm is not enough to stop heat loss from the house to keep it warm so in that case i will need to cross batten for more depth. Realistically what depth of mineral roll is a good depth.

 

If you just insulate the rafters surely the heat will rise from the house and warm the loft space. This heat will not be lost as the rafters are insulated and the house will then remain warm. More juice required to heat as you are heating a larger area but once it is warm the heat wont be lost. Am i misunderstanding the entire concept?

 

Yes it's misunderstanding. The loft wont just fill with heat from the house and stay that way like a thermos flask.

Heat will be escaping from the rafter insulation and loft draughts faster that it will be rising from the house, so the loft will stay just as cold as it was before. In the meantime the same amount of heat is escaping from the house into the loft, so there is no actual gain in terms of a warmer house.

 

Interesting. So what is the optimal depth of loft roll. Is cross battening the best way to install if more than my 100mm joists is required.

 

Normally 2 layers of 150mm. Obv if you want to use the space you’ll need to build a deck up

 

That's not quite right.

if heat is escaping from the house into the loft, the loft will be warmed.

if the loft heat loss rate is reduced by insulating the slopes, the loft will be warmer than before. It will not be "just as cold as it was before."

Heat transfer rate is proportion to temperature difference between the hot and the cold body. So less heat will be lost into a warmer loft.

 

It's exactly right for the reasons I stated.

This heat escaping from the house is not like heat given off from a heater warming the loft. It's heat conducting into the ceiling structure and then trying to rise into a cold draughty void that does not have time to heat up by any significant degree because the heat convecting in to it does not have the thermal energy to start with.

 

All I can say is I did that some years ago.
100mm roll in the joists, 75mm PIR on the rafters.
The loft is now a little warmer but is a very useful light storage area. The wife and I are happy with what we have done (in fact the wife is happy to store infrequently used iteks from her wardrobe up there, in suit cases).
It's not as warm as the rest of the house but definately not as cold as it prior to PIR on the rafters.

With the size of timbers you ahve be careful just how much weight you put up there in any one area and in total.

 

wgt52. Would you say it is cold up there still or now warm (even if not as warm as the house)?

Do you feel that the 100mm roll retains sufficient heat within the house itself?

Noted and agreed on the limitations to storage weights up there. Very small timbers for both joist and rafters compared to modern day construction..

 

I have seen new homes where they only insulate the rafters. This would seem strange if there is such heat loss from the roof (as i would expect there to be due to the throughdraft). How do they make this work or are they just more airtight in new homes?

 

As I said earlier "It's not as warm as the rest of the house but definately not as cold as it was prior to PIR on the rafters." It is warmer and dryer so suitable for storage. It isn't as warm as the rest of the house. Most important to get it fully insulated and sealed. Not all houses are suitable as you may not get PIR down to the Wall Plate which is what is needed to retain the warmth in the loft area.

What you are asking did we notice any reduction in heating costs? Probably - our house is open plan but I started working from home around the same time. Guess the best thing to advise is the house warmed up quicker when we returned from work in the evenings.