Warm Deck Dormer Roof - Is this correct?

ritesh

andyuk2005 wrote: »

I am surprised that building control didn't pick any of this up. I was here when they came round but apparently they just had a very quick look inside the room and that was it! What's even more surprising is we had that independent surveyor come round and he missed that too!

Speaking with Steve Dickinson from London Flat roofing (you can see his videos on YouTube he mentions that a big issue with BC is thet they do not pick up on the cold bridging at the fascia point. He makes a very good argument as to why you need to have insulation at this juncture, but alas BC tend to miss it

andyuk2005

Hi Cyclone!

I was told warm decks were more thermal efficient? I'm not sure how true this is. I just had a quick google search and there are a few references saying that it is considerably better to have a warm deck!

Obviously it all depends on different factors. In our case we had plenty of room for a thicker roof. Warm decks are usually easier to install and you don't have to worry about condensation etc.

cyclonebri1

andyuk2005 wrote: »

Hi Cyclone!

I was told warm decks were more thermal efficient? I'm not sure how true this is. I just had a quick google search and there are a few references saying that it is considerably better to have a warm deck!

Obviously it all depends on different factors. In our case we had plenty of room for a thicker roof. Warm decks are usually easier to install and you don't have to worry about condensation etc.

Ok, but NO, they aren't. The rules for condensation are simple, moisture laden air hits cold face, water gets deposited. It can happen in any type of roof space and this is what the battle is all about.

I'll throw you another curved ball. Given what you said and I then said, where did the builder place the water vapour shield membrane in your roof?

Go back, see what coeficient you have with the insulation thickness and type you have.

Then see how much (more) you could get in the same space.

Then take away the affect of heating a roof void only you know exists, ;)

andyuk2005

Hi Cyclone,

Here's a video which helps explain the difference between cold and warm deck, really helpful. It confirms that warm decks are cheaper, quicker, less prone to error and therefore easier to install than cold decks. When I mentioned the condensation in the previous post I didn't explain it very well and its not right. With a warm deck the point of condensation is different than a cold, that's pretty much what I meant to say!

I would assume the builder put the vapour barrier between the deck and insulation as its a warm. I can't check this however as there's no point of access for me. However the firm sent round a another builder who knew his stuff and they've fixed our problem now (free of charge of course). He knew his stuff and said he would check for any other problems on the build. There was a significant difference in temperature last night, just showed how much heat was escaping behind that fascia!

Although you are heating a roof void, because of less heat bridging, as in the video which demonstrates it, it is more efficient and therefore cheaper to heat a warm deck rather than a cold deck.

System

I can't see the video but...
A warm roof is not necessarily cheaper/easier/faster to build there are many variations of each type of roof, numerous finishes/build up options/products to be used - don't believe everything you see in a youtube video people!

cyclonebri1

the_r_sole wrote: »

I can't see the video but...
A warm roof is not necessarily cheaper/easier/faster to build there are many variations of each type of roof, numerous finishes/build up options/products to be used - don't believe everything you see in a youtube video people!

I totally agree. No video would convince me of the advantage of heating a roof void.
There is also the question of how much more difficult the warm deck is to repair when or if the flat roof leaks. The insulation would likely also need replacement or be damaged at the same time even though it will be rigid foam, so, no, not a believer unless as I said it has to be that way.

And yes the vapour barrier goes on the warm side, always has to whether warm or cold decking so it should be on the roof deck and under not over the insulation sheeting.;)

andyuk2005

https://www.youtube.com/watch?v=EsYj-a3SMDc&feature=youtube_gdata_player

Here we go folks! This guy knows his stuff, his phone number is there too as he runs what seems to be a pretty successful roofing company which even uses heat detection cameras! I'm sure he'll be happy to go through any pros or cons with you. If there's any specific information on how cold decks are better than warm, please put the sources here for people who would be interested!

cyclonebri1

I'm sorry andy, I remain unconvinced for the reasons in my post above. The other point is that given you still need constructional timbers for the roof, so you can get around twice as much insulation in without raising the roof height, and again, you are heating about 10% extra volume with a warm room, space you don't even see.

This isn't new technology, and it's always been a bone of contention, nothing has changed other than more efficient insulation, and this can be used in either method.

Anyway, you don't need to convince me and I'm not likely to change your opinion of something you have spent your hard earned on;);)

andyuk2005

As I said if you have any proof rather than theory of how a cold deck is more energy or heat efficient, then please put it on here as I'm sure people who are looking into choosing either one will be interested. So far I have found no proof that a cold deck has much advantage over a warm except for space saving.

cyclonebri1

andyuk2005 wrote: »

As I said if you have any proof rather than theory of how a cold deck is more energy or heat efficient, then please put it on here as I'm sure people who are looking into choosing either one will be interested. So far I have found no proof that a cold deck has much advantage over a warm except for space saving.

Sorry andy, but you asked for help, we tried to help. To be honest the only ""proof"" you have is the advertising blurb from the guy on youtube. That's not proof by any measurement.