U Value for two assumed building elements

[PeteThePen1]

Hi Everybody

We have a small three storey, two bedroomed terrace house that faces SSW. In summer the main bedroom on the south side of the top floor gets way too warm. As a solution I am proposing to line the inside of the bedroom on the exterior wall sides plus the cathedral ceiling with Aerogel Board (10mm Aerogell bonded to 9mm of magnesium board). My potential contractor warned that this would be of interest to the Building Regulations folk and he was right. I have had a couple of very helpful emails from them but they say that I need to have the U value for the exterior wall and for the cathedral ceiling.

I have attached a rought drawing to provide a cross section of the bedroom. The second drawing shows my calculations of the elements in the construction. These are in part hypothesised. The house was built in the late 1980s or early 1990s. The wall construction of medium density block, insulation batt (probably Kingspan type), air space, then brick is right as I saw that when the central heating boiler flue was changed. The dimensions of the insulation and air gap are derived knowing that the total wall thickness is 300mm and the bricks and blocks are 100mm wide each.. A similar approach was used for the cathedral ceiling. From an open Velux it was possible to measure the roof thickness as 180mm. The roof timbers are 92mm deep while the tile and batten must be at least 40mm leaving about 50mm for insulated plaster boaard. The latter is probably Kindspan type again as investigation with my endoscope showed nothing fibrous which whould have been fibreglass.

So my question is whether my hypothesised dimensions look right and then, if so, what would the U value of the exteriaor wall and the cathedral ceiling be?

Your comments and calculations would be gratefully received

Regards

Pete

 

[PeteThePen1]

I have an update for this question having had a good dig around on this site.

The BRE U value calculator mentioned back in 2011 is no longer available but BuildDesk's software is available. This can be downloaded and tested for 30 days free of charge. You can find it here:

http://www.builddesk.co.uk/our+software/builddesk+u

Using this software does require one to know a little bit about materials but is otherwise quite simple to use. Entering the criteria I provided in the first post the system yields a U value for the wall of 0.49 W(M2K) and RT 2.57 m2k/W. Usefully it allows one to choose brick size, block size, air gap width, etc. so that if you have an old house but are good at measurement you ought to be able to get a result.

Another useful site is Greenspec who have a section that gives the detals of all the common insulation materials:

https://www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/

For the cathedral ceiling the U value comes straight from the 50mm insulation backed plasterboard since the space above it is ventialted and thus at ambient temperature. From various adverts it looks as if the insulated plasterboard has a U value of about 0.486 W/m²K

By comparison the loft insulation was 160mm of fibreglass (500mm wide rolls squeezed into 400mm space!). Doing the necessary calcs that give a U value of 0.219 W/m²K.. It looks as if the builders were penny pinching where they thought they could get away with it. (They went bust not long after finishing the development).

If anybody spots a mistake, I willbe very happy to hear about it.

Regards

Pete