cavity insulation and DPC above lintels

[swimmydeepo]

AFAIK, a DPC installed above a lintel has the function of draining the water away from the cavity (via slope and weep holes). Now, if cavity insulation is to be installed, I assume that the panels (e.g. Celotex) need to be cut short before the base of the cavity (just above the lintel and the DPC) so that any instance of water can be carried away. This would result in an uninsulated area just above the lintels and the resulting cold bridges. what's the best way to tackle this issue?

I thought of installing the DPC only on the external leaf and sticking the other end of it to the aluminium foil of the Celotex panel. In this way the DPC function is maintained and the whole cavity is insulated.

Any comment? Please let me know.

Thanks

Alex

[Hugo Nebula]

On 21 Sep 2006 02:45:57 -0700, a particular chimpanzee named [e-mail] randomly hit the keyboard and produced:

Well, away from the inner leaf at least.

What lintels are you using? Catnic and IG steel lintels have a preformed cavity tray shape and, except for exposed locations, don't need a separate cavity tray. They have insulation in the 'cavity' part.

[Phil L]

Cavity lintels are insulated, usually with polystyrene but sometimes other materials are used, similar to foamy type stuff, so that your first course of exterior bricks on the lintel have the outlets (weep holes) inserted and are insulated behind (within the steel itself)

[swimmydeepo]

I forgot to mention that I am going to use prestressed concrete lintels (I got them for free). I've already used one internally and the cavity is too narrow for a catnic one anyway. I know the usual procedure (in this specific instance) would be to use a "L" shaped steel lintel for the external wall but I am going to use concrete blocks + rendering, so, I can get away with it. Now I've just realized that pre-stressed concrete lintels work in conjunction with the rows of blocks/bricks above (composite). It seems that using a DPC on top of the lintel would interfere with such composite action.

"there are too many dotty fools out there who think they are about be gods...GET OUT OF MY PATH!"

[Phil L]

Concrete lintels won't meet building regs if it's part of a dwelling...sheds and garages yes, dwellings, no.

The usual procedure is to use an insulated pressed steel lintel, angle iron (L shape) is neiither use nor ornament.

DPC? - what for?

Are you intending keeping the cavity open to the bottom of two seperate lintels, IE concrete and angle iron? - if the cavity is to remain open then no weep holes will be required as any moisture will just end up on top of the window/door casing, if you intend sealing the cavity at the bottom, then you need something more substantial than a piece of DPC....all this is academic anyway as it wouldn't meet any building regs that I'm aware of.

indeed

[swimmydeepo]

the insulated steel lintels I've found have a thickness of at least

50mm. As I've already mentioned the cavity is narrower (about 30mm). Regarding building regulations and concrete lintels; I need to check about this. If concrete lintels can be used on the inner leave and not externally then the problem seems to be mainly to do with aesthetics issues.

Well as I mentioned earlier, I thought that a DPC foil is used to carry any water away from the cavity (an alternative to cavity tray?)...if not the water would collect on top of the window creating cold bridges (and rot if timber window frame is used).

[Phil L]

This isn't really the problem, the problem is that you only have a 30mm cavity, which is also against building regs....what is the building and is there a building control officer involved? - have plans been drawn and passed or is it just a shed or garage?

DPC can be used for a cavity tray but a steel lintel does away with the need for this because the profile of it is sloping towards the exterior brickwork, meaning that any moisture will flow that way and out through the plastic weep holes.

Putting in two lintels, one one each skin, hasn't been common practice for at least 60 years when shaped concrete lintels were first used and later, steel. My only suggestion is that you use a concrete lintel the full width of the wall, but you may have to cast this yourself

[swimmydeepo]

This is an old terrace house the cavity is 30mm or less, like all terrace houses in the area. If I want a cavity of 50mm or more I would have to rebuild the whole house!

I am only rebuilding the external leaf of one side of the house...using a lintel for both leaves is not worth the trouble. There are "L" shaped and "C" shaped steel lintels which should do the job nicely.

Thanks for your suggestions