Damp proof course for timber frame

[Kevan Marshall]

Hi,

just about to pour the subfloor for timber frame house, builder is worried about damp issues and suggests placing dpc below thermal block, I would be looking for something that render can adhere to as thermal block is only one block high, I have attached a drawing of how we are, only difference is the block we have laid to concrete up to is a 150mm x 100mm “soap bar” to increase concrete thickness, for additional dpc between soap bar and thermal block would something like sika blackjack work or is there any other product that would work or is my builder overthinking this, any advice would be greatly appreciated!

 

[Nickfromwales]

You can’t run cables in insulation. 
 

You’d want a couple of rows of common bricks, up to the height of the insulation, and then Marmox thermal break blocks on top of those, or cut the thermal block down to suit.  

[Kevan Marshall]

39 minutes ago, Nickfromwales said:

You can’t run cables in insulation. 
 

You’d want a couple of rows of common bricks, up to the height of the insulation, and then Marmox thermal break blocks on top of those, or cut the thermal block down to suit.  

Hi Nick, thank you, nearly all cabling or pipes shall be in behind the 50mm service baton behind the plasterboard, 50 + 100 was stated because it’s cheaper than 150mm, anything that was in the 50mm would be in a duct but we are trying to avoid going in there at all costs, my timber frame company suggested the DPM as per the blue line, my builder is used to building conventional brick/block built houses, at the rear of house we are about 350mm above ground level to FFL, I spoke to the block manufacturer and they said to render with Weber Pral D, we are up at bottom of thermal block level and this is where the question has come up about DPC at that point!

[Nickfromwales]

46 minutes ago, Kevan Marshall said:

Hi Nick, thank you, nearly all cabling or pipes shall be in behind the 50mm service baton behind the plasterboard, 50 + 100 was stated because it’s cheaper than 150mm, anything that was in the 50mm would be in a duct but we are trying to avoid going in there at all costs, my timber frame company suggested the DPM as per the blue line, my builder is used to building conventional brick/block built houses, at the rear of house we are about 350mm above ground level to FFL, I spoke to the block manufacturer and they said to render with Weber Pral D, we are up at bottom of thermal block level and this is where the question has come up about DPC at that point!

This should all land firmly in the lap of your Principal Designer, usually your architect, as they are covered by PI insurance and are the ones who would underwrite the solution as robust (60 years service life is typical from most reputable TF providers).

[Nickfromwales]

You should also install 150mm wide DPC across here too:

 

 

This protects the sole plates from water ingress, should anything get in behind and sit on the horizontal counter batten. 
 

I got chatting with Darren from MBC when he was on site doing the pre-airtightness test, test, and offered up this great bit of info, which he says is often overlooked; I conveyed this to the client and it was agreed in an instant that this was a good idea and to run with it. 
 

As it’s horizontal I’d recommend a 100mm tape along the top edge of the DPC strip also. Sole plate is the first point of failure if details are overlooked, and are ‘fatal’. 

[Kevan Marshall]

2 hours ago, Nickfromwales said:

You should also install 150mm wide DPC across here too:

 

 

This protects the sole plates from water ingress, should anything get in behind and sit on the horizontal counter batten. 
 

I got chatting with Darren from MBC when he was on site doing the pre-airtightness test, test, and offered up this great bit of info, which he says is often overlooked; I conveyed this to the client and it was agreed in an instant that this was a good idea and to run with it. 
 

As it’s horizontal I’d recommend a 100mm tape along the top edge of the DPC strip also. Sole plate is the first point of failure if details are overlooked, and are ‘fatal’. 

Hi Nick, yes that makes sense to run some sort of tape then membrane over the top, I’ll speak to my timber frame manufacturer tomorrow!