Dormer Insulation- what product?

Hi

hope someone will be able to help...

we live in a 1970's dormer bungalow with 2 bedrooms and a bathroom upstairs. In the winter the upstairs is freezing and in the summer it is unbearably hot.

there are eves that extend the whole length of the upstairs which we use for storage. When you go in the eves there is no insulation at all between the rafters (you can see the felt that backs the tiles). There is some rock wall type insulation between the eves and the bedrooms.

In order to retain winter warmth and reflect summer heat do i need to insulate between the rafters? I have looked at a kingspan type product that would be cut and put in between the rafters or a Gen X product that unrolls and staples onto the rafters them selves both leaving an air gap between the tiles and the insulation.

which of these is best?..the person at the suppliers said the gen x is a better product which will be much easier for me to fit but is more expensive (£440 compared to £300) to do the loft space. i don't mind paying more if it will do the job.

please can someone give me some advice on this before i commit to the job.

thanks very much and appreciate your time with this.

cheers

jack

 

What thickness of Kingspan are you looking at for GBP 300 ?

 

50 mm 8' x 4' sheets. I need 41 square meters. The sheets are £20 each. I don't think its actual Kingspan but the suppliers equivalent.

 

Your reply shows that the person at the merchant's needs his/her backside kicking.

The Gen X web-site gives an R value of 1.9 ( not sure if I believe it ) whereas 50 mm of Kingspan has an R value of 2.0 , so for less money the PIR board gives better insulation.

Neither will prevent the place getting hot in summer. It is simply not possible when the outside air is warmer than the inside temperature for a significant period of time.

 

Insulation will help to a point in controlling solar gain in summer. But you will also need to consider things like closing the curtains etc.

In my opinion, the more insulation you can get in the better, its not just about payback on the cost of insulation vs heating bills, its also about comfort.

I have looked in the past, but never specified it, the use of spray foam insulation, this according to the manufacturers bumph can be sprayed directly to the back of the tiles or felt between the rafters. An advantage I see of using something like that is that it will help with air tightness and that will help with your winter heating and if you decide to air-condition the house in summer it will also help with that.

 

This is something promoted by cowboy roofing-companies and is - I believe - contary to Building Regs. The roof cannot breathe, you cannot check for any faults such as leaks and it is impossible to undertake repairs easily . Avoid.

As mentioned above, you do not simply decide that a roof would be better being air-tight. You must leave an air-gap beneath the tiles - usually 50 mm - to allow for passage of air to prevent condensation and decay in the rafters

Unsure what advantages this wwill confer in respect of the questions posed.

 

Leaving a gap beneath the roof tiles really depends on the over all insulation calculation. The gap where required is to allow the space under the tiles to breathe. You can however fill this gap up completely providing the interstitial condensation calculation proves that no condensation will form.

So it's not a simple case of avoid, its a case of get your facts straight!

 

What I didn't mention in my last post, what that when constructing a new roof you would normally counter batten when constructing an unventilated roof detail to ensure that any water that gets past the tiles can drain away.

The spray foams for good or bad fill the entire void, leaving no room for condensation to form. But like I said, I have never actually specified them, so apart from the product data, I have no first hand information.

 

This is an existing situation where the OP wants to improve things a bit . are you seriously suggesting that a heat-loss calc is necessary or desirable ?


What is an interstitial condensation calculation ? I would be intrigued to see one . Are you saying this is necessary in normal roof-construction ?

sketch

Your posts seem to contain frequent " I specify .." " I have looked .." " I have no first hand information "

Not a lot there about actually doing anything you "specify " is there ?

If you want to know what people who actually do things think about foam spraying onto tiles, try searching "foam-spraying" in the roofing forum.

!

Precisely. I am sure you will try in future, even if only from a "specifying" point of view.

I hope you will reply to my questions about the need for the heat loss calc and the interstitial condensation calculation though, as it's always good to learn something new.

 

Mointainwalker, That be because I am a specifier, not on the tools, at least not that often cause I rarely get time to work on my own place.
The BRE 'U' value calculator does condensation risk analysis, not for everyone though as it costs, however some insulation suppliers do the calcs for you as part of specifying their product, so the person on the tools doesn't need to know the science behind it all, they just have to get on with the job to the manufacturers specification.

I wouldn't suggest that a heat-loss calc is done for a domestic extension, totally unnecessary, but in the UK you still have to comply with the building regulations, and that means making sure 'U' value requirements are complied with or exceeded and if you are proposing an unventilated insulation detail then the appropriate calcs are required for this. Don't have a clue about your part of the world.

 

For an existing situation you are , I believe, completely wrong.

My comments concern the UK. Please do not try to obfuscate the issue ( as is apparent in your other replies )

Still waiting for enlightenment about the interstitial condensation calculation / Is this about to be forthcoming or just a joke ?

 

OK Mointainwalker,
Seem like you are stuck in the world of being right all the time, so I really can't be arsed to argue with you. However, below is a link to the build desk software which funny enough, calculates condensation risk. But of course this can't be right as you must be.

http://www.builddesk.co.uk/our+soft...nsation+risk?gclid=CPqJ6IOh56oCFUgKfAodEFvU7Q

 

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