Dealing with the DPM on the outside of timber frame on passive slab

[MikeSharp01]

I need to now tidy up the DPM (Violet line) / DPC (Green line) on the outside of our frame. As I see it I have two options, see picture. I prefer the first (left hand) option as any splashing off the ground cannot get into the DPM/DPC as it is not accessible. Option 2 (right hand) feels more traditional but I am not sure it is as sound as the first in that the there seems to be a moisture path, by capillary action, between the two DPM/DPC layers.  

 

 

Any thoughts gratefully accepted as ever.

[Ian]

Option 2 is by far the better detail.

[Onoff]

Just out of interest, that 316 st/st trim...does it not form a cold bridge where it tucks up on the warm side of the 140mm insulation?

 

Could that affect the bottom edge of the 15mm OSB?

[MikeSharp01]

6 hours ago, Ian said:

Option 2 is by far the better detail.

Thanks that is the traditional approach but I am not sure why - can you perhaps explain?

[MikeSharp01]

4 minutes ago, Onoff said:

Just out of interest, that 316 st/st trim...does it not form a cold bridge where it tucks up on the warm side of the 140mm insulation?

It is 40mm insulation but there 300mm on the inside and although it fixes to the 300mm sole plate that is split by 50mm of insulation ( the greeny / yellow lump) so it is well protected cold bridge wise. I do have a condensation worry but I figured that I could expect the whole mass of the trim to move slowley temp wise but I will review it again.

[Ian]

20 minutes ago, MikeSharp01 said:

Thanks that is the traditional approach but I am not sure why - can you perhaps explain?

Because option 1 has the potential to trap water against the bottom of the timber sole plate and OSB.

[Onoff]

 

1 hour ago, MikeSharp01 said:

It is 40mm insulation but there 300mm on the inside and although it fixes to the 300mm sole plate that is split by 50mm of insulation ( the greeny / yellow lump) so it is well protected cold bridge wise. I do have a condensation worry but I figured that I could expect the whole mass of the trim to move slowley temp wise but I will review it again.

 

Sorry, read it as 140mm (small screen, bad eyes ? ).

[Pete]

9 hours ago, MikeSharp01 said:

I need to now tidy up the DPM (Violet line) / DPC (Green line) on the outside of our frame. As I see it I have two options, see picture. I prefer the first (left hand) option as any splashing off the ground cannot get into the DPM/DPC as it is not accessible. Option 2 (right hand) feels more traditional but I am not sure it is as sound as the first in that the there seems to be a moisture path, by capillary action, between the two DPM/DPC layers.  

 

 

Any thoughts gratefully accepted as ever.

I had to use option 1. My B/insp wanted me to fold the membrane up and behind the breather membrane on the outside of the panelvent or the OSB sheathing in your case. My frame make up from the outside is render board, batten, breather membrane and then panel vent board and then timber frame and insulation. Hope this helps.

[MikeSharp01]

4 hours ago, Ian said:

Because option 1 has the potential to trap water against the bottom of the timber sole plate and OSB.

True if any water gets in there. If the breather membrane is outside it I am not sure how water can get there and any water that does will be dried out by the permiability of the structure will it not. As ever its the little details that frustrate. In the traditional case can water not get along between the two membranes if any splashes up there in a storm.

[Ian]

2 hours ago, MikeSharp01 said:

True if any water gets in there. If the breather membrane is outside it I am not sure how water can get there and any water that does will be dried out by the permiability of the structure will it not. As ever its the little details that frustrate. In the traditional case can water not get along between the two membranes if any splashes up there in a storm.

that SS trim will protect it from any splashes. (btw, does the trim have slots or perforations in the angled end section to allow for air ventilation up the cavity?)

[MikeSharp01]

47 minutes ago, Ian said:

(btw, does the trim have slots or perforations in the angled end section to allow for air ventilation up the cavity?)

Yes it has perforations.

[gavztheouch]

On 17/09/2019 at 14:10, MikeSharp01 said:

Yes it has perforations.

Which option did you go for in the end?

[MikeSharp01]

We went for option 2 in the end - BC suggested best as did a few here.