The Naked Scientists

The Naked Scientists Forum

Author Topic: Does insulating a roof ABOVE the ventilated airspace make much sense?  (Read 7227 times)

Offline Karsten

  • Hero Member
  • *****
  • Posts: 701
    • View Profile
    • Fortunately still only a game
I am in the process of purchasing a little house. I want to insulate the roof more even though it is already insulated well according to US standards (R30; not that much). People keep telling me to insulate with rigid foam panels above the existing air-space. I keep thinking that not eliminating the airspace between the old and new insulation makes little sense. Air space may be good for ventilation but what is the point of adding 5-10 cm of foam above a layer of fresh and flowing air? The ventilated air space should be on top of all of the insulation and there should be no air exchange with the outside withing the insulation layers.

Right? Wrong? It depends?

Karsten


 

lyner

  • Guest
I think you should add an extra layer of insulation directly on top of the existing but not take it out to the edges, so that air can circulate in and out over the sides. You need a free path for air over the top but none underneath, as you say.
 

Offline BenV

  • Neilep Level Member
  • ******
  • Posts: 1503
    • View Profile
I need to better insulate my loft soon, there's only about 15cm of fiberglass up there now, and I'd like to replace that with something a bit more friendly (and a lot more of it!).  It's partially floor boarded, so I guess it's a case of taking up the boards, laying as much down as will fit and boarding it back up.  Does anyone know if the "space age" foil-lined insulation is any good?  If not, I think I'll look at saving up and using wool.  Also, should I put extra insulation up in the eaves?  If so, how do I got about it without impacting ventilation?

And while we're at it, would anyone like to come and insulate my loft?  I'll offer a constant supply of tea!
 

Offline Karen W.

  • Naked Science Forum GOD!
  • *******
  • Posts: 31653
  • Thanked: 5 times
  • "come fly with me"
    • View Profile
Thanks for asking such a good question...
I for one have zero insulation anywhere in my house. I need to buy some someday when the money tree in my back yard blooms! LOL....
I have been told the space age foil kind is good but also there is a blow in kid that actually sound proofs your house as well as is fireproof..as I recall. some old friends of ours does it for a living...hopefully I will learn a thing or two from your questions.... I am also interested in the amount you end up with....I am a northern Californian I freeze in the winter..my house in the hot summer stays cool til nightfall then its bloody hot on those days when it was 105 D.Fehrenheit.. winter the house stays cold and dampunles you have good heat going 24 / 7.
 

lyner

  • Guest
Quote
And while we're at it, would anyone like to come and insulate my loft?

I think you're insulating our intelligence.
 

Offline Karsten

  • Hero Member
  • *****
  • Posts: 701
    • View Profile
    • Fortunately still only a game
I am certain that in a house with no insulation (difficult to believe that this still exists in places where you need to heat) ANY insulation will make a big difference. It will keep the house warmer and prevent it from getting hotter. It may cost you some money to install it, but if energy costs go up again where they should be, you will quickly save more money than you spent. A very good start to keep you house from loosing heat in the winter would be to get it more air-tight. That is easier to do than insulating. The majority of heat loss is the warm air just flowing away through cracks in the walls. You can search for holes with your bare hands on a windy day, or with a incent stick. The smoke gets sucked out of the building where warm air escapes or stirred up where cold air enters.

I read on a website once that the best cure for an old, inefficient, Vermont house is a gallon of kerosene and a match. I assume the same applies to old, inefficient, Northern Californian homes.  ;)

Karsten
 

Offline Karsten

  • Hero Member
  • *****
  • Posts: 701
    • View Profile
    • Fortunately still only a game
Why REPLACE fiberglass? It works, it is there, why not leave it where it is and add more of something else if you want more? Throwing it out seems wasteful unless you are exposed to it directly.

Karsten
 

Offline Maxtra

  • First timers
  • *
  • Posts: 1
    • View Profile
Having a space above the existing ceiling or current insulation would make good sense. Creating air flow and the ability for it to breathe. Foam is not a common insulating material here in Australia with the preference being glass wool batts or wool. If only you guys had an insulation rebate like in newbielink:http://www.perth-insulation.com [nonactive] is actually subsidized by the government and with the cost mostly paid for with no out of pocket expense to the homeowner. It is apart of the new commitment to reducing carbon footprints.
 

Offline Karsten

  • Hero Member
  • *****
  • Posts: 701
    • View Profile
    • Fortunately still only a game
Having a space above the existing ceiling or current insulation would make good sense. Creating air flow and the ability for it to breathe. Foam is not a common insulating material here in Australia with the preference being glass wool batts or wool. If only you guys had an insulation rebate like in perth insulation is actually subsidized by the government and with the cost mostly paid for with no out of pocket expense to the homeowner. It is apart of the new commitment to reducing carbon footprints.

The question was whether it makes sense to insulate ABOVE the already existing airspace (in ADDITION to the insulation in the roof) in a cold climate. Basically what was suggested by builders was: Insulation, airspace, insulation. Made no sense to me.

While I have in the meantime purchased the house, I have not gotten much closer to the insulation project. Too many other things to do there. I don't think I will insulate the outside air space. I will put more insulation inside under the ceiling.
« Last Edit: 31/08/2009 00:06:29 by Karsten »
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
I can't think of a good reason to insulate free moving outside air from other, slightly less, free moving outside air (inside the loft). However, I expect the air in the loft will not move very freely, certainly not as much as the outside air on a windy day, so if your main insulation (loft to ceiling) was not perfect, then there may be some slight benefit from insulting the roof.

It will depend on how "ventilated" your roofspace is. It was not uncommon in the UK to have the roof constructed of tiles laid on laths (fixed horizontally across rafters), with no barrier between the laths and the outside tiles. The house I live in (built circa 1923) is like this. The ventilation is 100% and the loft is thick with accumulated dust on top of the ceiling-loft fibreglass insulation. The loft does not need this much ventilation and this construction has the disadvantage that a slipped tile means water ingress too. Most later houses use roofing felt (tarred paper) below the laths (or are close boarded) which stops the wind whistling through but still allows some ventilation via gaps in the eaves. Modern houses use this construction but usually seal the gaps under the eaves too; ventilation is provided via grilled ventilators in the soffits giving the advantage of letting in the air without the wildlife.

So, to get back to the original question, if the construction is such that the roof space has very limited, but hopefully sufficient, ventilation then there may be some advantage to further insulation of the roof, though personally I wouldn't do it. You would be better off putting that insulation between the ceiling and roof and adding to the thickness. If the loft is overly well ventilated it would be, even more, a waste of money.

As a matter of interest I do wonder about being overly concerned with ventilation. It seems to me that most loft conversions often create sections of roof areas that are not well ventilated at all.

 
 

Offline Karsten

  • Hero Member
  • *****
  • Posts: 701
    • View Profile
    • Fortunately still only a game
As a matter of interest I do wonder about being overly concerned with ventilation. It seems to me that most loft conversions often create sections of roof areas that are not well ventilated at all.

There is some research that says that ventilation of the roof cavity is not be equally important in all climates. A lot also depends on how much humidity you create in the home and how you get rid of it before it creeps into the insulation and roof. And it might be interesting to look at those unventilated roof sections in 20 years.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length