epsilonGreedy Posted July 22, 2018 Share Posted July 22, 2018 (edited) Most diagrams of foundation/footing designs show a continuous dpc/tray at dpc between the inner and outer masonry walls. Why is such a tray preferable to two independent dpc's particularly in view of the obvious risk that the tray promotes the formation of mortar damp bridges? All I can think of is that it creates a barrier to noxious gasses rising out of the ground and into the upper cavity. Before posting I did my own reading and found this NHBC advisory about the subject but in fact it covers complex cases and not ground level dpc. http://www.nhbc.co.uk/NHBCpublications/LiteratureLibrary/Technical/TechnicalExtra/filedownload,58010,en.pdf Edited July 22, 2018 by epsilonGreedy Link to comment Share on other sites More sharing options...
Russell griffiths Posted July 22, 2018 Share Posted July 22, 2018 Why does the tray promote mortar damp bridges? my feelings are it’s there to stop them, the inner leaf is normally higher than the outer, so any moisture build up in the cavity is directed to the outer leaf and towards any weep holes 1 Link to comment Share on other sites More sharing options...
Declan52 Posted July 22, 2018 Share Posted July 22, 2018 It's there as Russell says to stop any moisture traveling through. It won't stop gases either as your b&b floor will have vents to get rid of any build up here. We call it tanking in NI and I done it on 2 sides of my build due to the differences in my ground levels. My house is cut into a 2m high bank on 2 sides. With the dpc folded up I could put in a slight gradient on my level access instead of a shorter ramp. Put it simply a 30m roll of either 450mm or 600mm dpc will cost less than £20 and will be of benefit to you to build it in. Link to comment Share on other sites More sharing options...
Russell griffiths Posted July 22, 2018 Share Posted July 22, 2018 https://goo.gl/images/pZ4Fwn a bit like this. You don’t have to have a tray, but if you want to insulate below the inner floor level to stop any cold bridges then you will need to demonstrate a way of preventing moisture passing to the inner structure. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted July 23, 2018 Author Share Posted July 23, 2018 20 hours ago, Russell griffiths said: Why does the tray promote mortar damp bridges? my feelings are it’s there to stop them, the inner leaf is normally higher than the outer, so any moisture build up in the cavity is directed to the outer leaf and towards any weep holes Because it catches dropped mortar above dpc. When such a lower damp proof course/tray is level (as it often shown in technical diagrams) there is obvious potential to create a mortar bridge. See another thread where @Brickie describes a bricklaying process to sweep out dropped mortar on a daily basis. An incline in the tray makes sense but gravity would not counteract capillary action upward to the inner wall should a bridge form. The incline is there I understand to direct condensed moisture or rain driven seepage outwards. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted July 23, 2018 Author Share Posted July 23, 2018 (edited) On 22/07/2018 at 13:57, Declan52 said: It's there as Russell says to stop any moisture traveling through. We must be picturing different situations because in my build I am wondering why an air gap would not be as effective as a dpc fitted as a tray. Given a level site why is a tray dpc preferable to two independent 100mm dpc's in each wall? Edited July 23, 2018 by epsilonGreedy Link to comment Share on other sites More sharing options...
Russell griffiths Posted July 23, 2018 Share Posted July 23, 2018 I will try to give my view but it’s a bit tricky as all our build ups are different old school method. Two damp courses level with each other. No insulation in the cavity at below floor level. Cavity core filled to two courses below dpc. Moisture droplets form and drop off wall ties as it’s meant to. It falls down the cavity and lands in this two course void and then wicks to either the inside or outside but cannot climb above dpc. Winner winner chicken dinner. New school thinking. Fill the void below floor level in the cavity with a solid insulation to stop a cold bridge at floor level. Moisture drops down cavity and hits top of insulation, bounces onto inner skin of blockwork, KABOOOOM. Damp shows on inner wall at skirting level. So filling the cavity with a ridgid insulation has just created another issue. Remedy. Create a tray damp course. if you have found pictures of a level tray dpc ITS FOOKING WRONG. my personal opinion of course 1 1 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted July 23, 2018 Share Posted July 23, 2018 https://goo.gl/images/yG8wE4 in my opinion it should look like this. 2 Link to comment Share on other sites More sharing options...
epsilonGreedy Posted July 24, 2018 Author Share Posted July 24, 2018 13 hours ago, Russell griffiths said: old school method. Two damp courses level with each other. No insulation in the cavity at below floor level. Cavity core filled to two courses below dpc. Moisture droplets form and drop off wall ties as it’s meant to. It falls down the cavity and lands in this two course void and then wicks to either the inside or outside but cannot climb above dpc. Winner winner chicken dinner. New school thinking. Fill the void below floor level in the cavity with a solid insulation to stop a cold bridge at floor level. Moisture drops down cavity and hits top of insulation, bounces onto inner skin of blockwork, KABOOOOM. Damp shows on inner wall at skirting level. So filling the cavity with a ridgid insulation has just created another issue. Remedy. Create a tray damp course. NHBC should employ you to write their technical bulletins. This stuff is much simpler to understand as an historical timeline of ideas rather than a point in time absolutist statement of right and wrong. Now for some over the horizon thinking. What would be wrong with no ground level cavity tray, independent dpc's in each wall and blown beads that have fused into a lattice at the very bottom of the cavity below ground level? One conclusion I have come to is that I do not like the idea of cavity batts sitting in the cavity within 6 inches of ground level, above or below dpc. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted July 24, 2018 Author Share Posted July 24, 2018 (edited) 14 hours ago, Russell griffiths said: https://goo.gl/images/yG8wE4 in my opinion it should look like this. Are you aware that the image you chose to illustrate your point comes from the anti radon portion of the Cavity Trays web site? www.cavitytrays.com = great site for an education on all types of tray scenarios. Edited July 24, 2018 by epsilonGreedy Link to comment Share on other sites More sharing options...
PeterW Posted July 24, 2018 Share Posted July 24, 2018 2 hours ago, epsilonGreedy said: One conclusion I have come to is that I do not like the idea of cavity batts sitting in the cavity within 6 inches of ground level, above or below dpc. Cavity Batts aren’t certified under the BBA cert for below ground use Link to comment Share on other sites More sharing options...
ADLIan Posted July 24, 2018 Share Posted July 24, 2018 Beg to differ. You will find just about all built in cavity insulation products are BBA certified for use below DPC Link to comment Share on other sites More sharing options...
PeterW Posted July 24, 2018 Share Posted July 24, 2018 10 minutes ago, ADLIan said: Beg to differ. You will find just about all built in cavity insulation products are BBA certified for use below DPC Rockwool state starting 150mm below DPC and the BBA shows a clear air gap below the lower insulation layer before any infil. 150mm would also be just above ground level, anything below that would make it borderline for use anywhere below ground i.e. where weak mortar is normally used. Link to comment Share on other sites More sharing options...
ADLIan Posted July 24, 2018 Share Posted July 24, 2018 Rockwool data I have shows cavity batts below DPC starting just above the lean mix cavity fill - I believe this is common to just about all manufacturers and BBA certs 1 Link to comment Share on other sites More sharing options...
epsilonGreedy Posted July 24, 2018 Author Share Posted July 24, 2018 39 minutes ago, PeterW said: Cavity Batts aren’t certified under the BBA cert for below ground use Did I make a statement to that effect? There is an intermediate zone below the dpc to around ground level where I have seen cavity batts used to perform edge of floor insulation to avoid a cold bridge. I do not like that usage approved or not. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted July 24, 2018 Author Share Posted July 24, 2018 13 minutes ago, ADLIan said: Rockwool data I have shows cavity batts below DPC starting just above the lean mix cavity fill - I believe this is common to just about all manufacturers and BBA certs Quite, which correlates to what I have seen at some sites and could take the batts just below ground level depending on floor make up. Link to comment Share on other sites More sharing options...
PeterW Posted July 24, 2018 Share Posted July 24, 2018 20 minutes ago, ADLIan said: Rockwool data I have shows cavity batts below DPC starting just above the lean mix cavity fill - I believe this is common to just about all manufacturers and BBA certs I don't disagree - what I was referring to was taking a cavity batt below ground to infil the cavity instead of using lean mix infil. I think we are discussing the same point ! 1 Link to comment Share on other sites More sharing options...
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