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Hi, I'm requiring advice/suggestions regarding protecting my timber soleplate from condensation/moisture between it and the DPC. I've read various threads how moisture can remain through lack of ventilation, and difference in temperature of timber and blockwork beneath, but not found a relevant solution yet, if necessary at all.

Please see attached drawing for the build up. The wall is built and i'm starting timber cladding in the new year. So, before cladding commences i'm wondering if there is something i can do at this stage to insulate the blockwork wall externally, without bridging DPC ???

Foundation detail.pdf

Edited by Blob the Builder

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My timber frame kit is just block, dpc plastic strip then timber sole plate.

 

I think that is just the standard approach.

 

 

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It's essential that some form of condensation risk assessment is done, something that is even more important now that insulation levels are significantly better than in times gone by.  One consequence of improved insulation is that the sole plate will be colder, as less heat will be lost from the house.  It was assumed at one time that including a vapour control layer on the warm side of the build up would eliminate the sole plate condensation risk, but this may well not be the case in the sort of variable weather we have. 

 

Water vapour can move into the structure from outside when humidity levels are high, and with luck will move back out again as humidity levels outside drop.  The problem arises when the temperature of the sole plate drops below dew point for any water vapour that has migrated inwards.  Because it takes, relatively, a lot of energy to turn liquid water back into vapour, and because the sole plate may well be being cooled by the ground beneath to the point where it doesn't get warm enough to drive the water out, there is a risk that it may end up wet for long enough to cause problems.

 

The mitigation measures that can be used include improving the insulation level of the foundation that the frame rests on, by using some form of insulated blockwork, or adding an insulating upstand to try and keep the wall/floor junction a bit warmer.

 

The key thing here is that the issue is a dynamic one, that isn't normally modelled in a standard steady state condensation risk analysis.  There's a thread here about the specific issue this presents to a SIPs build, but much of the content in that is equally applicable to timber frame construction where there is no external insulation around the wall/floor junction.

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10 minutes ago, JSHarris said:

It's essential that some form of condensation risk assessment is done, something that is even more important now that insulation levels are significantly better than in times gone by.  One consequence of improved insulation is that the sole plate will be colder, as less heat will be lost from the house.  It was assumed at one time that including a vapour control layer on the warm side of the build up would eliminate the sole plate condensation risk, but this may well not be the case in the sort of variable weather we have. 

 

Water vapour can move into the structure from outside when humidity levels are high, and with luck will move back out again as humidity levels outside drop.  The problem arises when the temperature of the sole plate drops below dew point for any water vapour that has migrated inwards.  Because it takes, relatively, a lot of energy to turn liquid water back into vapour, and because the sole plate may well be being cooled by the ground beneath to the point where it doesn't get warm enough to drive the water out, there is a risk that it may end up wet for long enough to cause problems.

 

The mitigation measures that can be used include improving the insulation level of the foundation that the frame rests on, by using some form of insulated blockwork, or adding an insulating upstand to try and keep the wall/floor junction a bit warmer.

 

The key thing here is that the issue is a dynamic one, that isn't normally modelled in a standard steady state condensation risk analysis.  There's a thread here about the specific issue this presents to a SIPs build, but much of the content in that is equally applicable to timber frame construction where there is no external insulation around the wall/floor junction.

Mmm, yes, what you explain here is what i was thinking, thanks. As my structure is already built i'm looking for possible solutions, so could one be to have some external insulation to the blockwork, i.e EPS, PIR, held on with battens, and render board on the battens? I'm not sure how this would fare when the finished ground level is made up around it?

Is the thread you mention in the mayfly blog?

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External insulaion, EPS or XPS, will do the job OK.

 

The SIPs thread is on this forum somewhere, IIRC, or may possibly have been on the now defunct Ebuild forum.  I seem to remember we had some input from a chap from Kingspan, who started off not realising there was a potential problem and ended up agreeing that there was a possible risk that could be mitigated with an insulation upstand.

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19 minutes ago, JSHarris said:

External insulaion, EPS or XPS, will do the job OK.

 

The SIPs thread is on this forum somewhere, IIRC, or may possibly have been on the now defunct Ebuild forum.  I seem to remember we had some input from a chap from Kingspan, who started off not realising there was a potential problem and ended up agreeing that there was a possible risk that could be mitigated with an insulation upstand.

Ok, thanks, i'll look into it....

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1 hour ago, Thedreamer said:

My timber frame kit is just block, dpc plastic strip then timber sole plate.

 

I think that is just the standard approach.

 

 

Ok, has it been up long?.......is it single skin blockwork, timber frame, and timber clad?

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Probably best to have a look at my blog. Yes it's a timberframe with a mixture of single blockwork and timber cladding.

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slightly different --but the same 

 I see very few ICF build diagrams for founds  that include any sort of DPC that makes a complete break in the wall

durisol say use water proof concrete below grade--

not sure i would believe concrete supplier than added  correct amount and no way to tell

most icf say nothing  and with build regs wanting you to build house low --for disabled ramp etc

surely there should be some mechanical dpc somewhere in the bottom layers --

seems a bit  simple to assume the eps is going to be perfect on its own 

or maybe you just have to use complete passiv house type eps raft under slab  to be sure and then stick on tanking ,as though its below grade , and join with under slab VPc

then up the wall a foot or so above grade to be bullet proof.

or maybe i,m just too anal 

 

 

 

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Did somebody not put a pic up recently of an external layer of xps that came from the footing up past the sole plate with a taper top on it and then the cladding coming down over that. 

 

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On 31/12/2018 at 11:19, Russell griffiths said:

Did somebody not put a pic up recently of an external layer of xps that came from the footing up past the sole plate with a taper top on it and then the cladding coming down over that. 

 

Hi Russell, i've found this detail on the forum, was it this you were thinking of?

I'm not sure how the brick slips support the earth in this detail thou.

Insulated foundation detail.jpg

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On 31/12/2018 at 09:21, scottishjohn said:

slightly different --but the same 

 I see very few ICF build diagrams for founds  that include any sort of DPC that makes a complete break in the wall

durisol say use water proof concrete below grade--

not sure i would believe concrete supplier than added  correct amount and no way to tell

most icf say nothing  and with build regs wanting you to build house low --for disabled ramp etc

surely there should be some mechanical dpc somewhere in the bottom layers --

seems a bit  simple to assume the eps is going to be perfect on its own 

or maybe you just have to use complete passiv house type eps raft under slab  to be sure and then stick on tanking ,as though its below grade , and join with under slab VPc

then up the wall a foot or so above grade to be bullet proof.

or maybe i,m just too anal 

 

 

 

Passiv raft maybe the way to go.....next time!

I grew up learning never to breach dpc, but i'm already doing that with my build as i have metal holding down straps connecting my single skin sub dpc blockwork to my timber panels!

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On 30/12/2018 at 22:30, Thedreamer said:

Probably best to have a look at my blog. Yes it's a timberframe with a mixture of single blockwork and timber cladding.

I've just managed some time for a quick look at your blog....looking good, very interesting...many similarities to my build; Cupa heavy slates also, all delivered and waiting on roofers arriving Monday. Interesting about your Klober Scotstrip, as this was suggested by my roofers, but on seeing it i didn't think it would work for me as i have no bargeboard, I'm attempting to run the horizontal siberian larch cladding to the underside of the dry verge. So i've gone with a Kytun T profile, which should arrive on Monday.

I see that you fitted your windows before doing cladding. I was intending this, but, today a 'touching up' guy came from Rationel who is also a fitter (we're fitting the windows ourselves), and he said to do cladding first as it makes it easier to achieve a watertight seal once you fit the windows, using a compressible tape between the frame and the cladding reveal. Did you use a compressible foam tape for your external weather seal?

Also, i couldn't quite tell from your photos, but what did you use for flashing at the head of your windows?

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@Blob the Builder

 

The dry verge was one of those areas I didn't think about until a decision was looming. I looked at the Kytun strip, one of the reason we didn't got for it was the size of the cupa heavies which are thicker than your average slates and where the three slates meet it can cause the strip to have bumps.

 

Discussed here Dry Verge

 

 

The thickness of strip we went for is the one designed for slates with battens rather than sarking boards and this provided some extra room. The bumps are not noticeable at all at ground level but if you stand on the scaffold you can see it.

 

Windows were from Nordan and didn't come with a flashing at the top, but we did fit a plastic strip similar to what you have for the DPC course over the top and has kept the water from coming in.

 

Not sure if different techniques exists for fitting windows in a timberframe house but our were attached to the firestops and then lifted into place. I've since seen the expanding foam tape looks like a good idea. 

 

 

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10 hours ago, Thedreamer said:

@Blob the Builder

 

The dry verge was one of those areas I didn't think about until a decision was looming. I looked at the Kytun strip, one of the reason we didn't got for it was the size of the cupa heavies which are thicker than your average slates and where the three slates meet it can cause the strip to have bumps.

 

Discussed here Dry Verge

 

 

The thickness of strip we went for is the one designed for slates with battens rather than sarking boards and this provided some extra room. The bumps are not noticeable at all at ground level but if you stand on the scaffold you can see it.

 

Windows were from Nordan and didn't come with a flashing at the top, but we did fit a plastic strip similar to what you have for the DPC course over the top and has kept the water from coming in.

 

Not sure if different techniques exists for fitting windows in a timberframe house but our were attached to the firestops and then lifted into place. I've since seen the expanding foam tape looks like a good idea. 

 

 

My windows are Rationel, who also don't supply head flashing. So i was wondering whether to get metal flashings made up, or use dpc over a filleted batten like yourself. How does the any moisture passing over the dpc at the head drain out?

Did you use 47 x 50 mm battens for firestops (and fixing cladding to) ?

And, therefore, how many mm do the face of your windows protrude from the timber panels?

Sorry for all these questions, just very relevant to me just now!

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No mention of the head flashing when I was ordering, but a day after the windows were fitted some water was coming over the top. Asked the joiner what could be done and he said he could fit some 9inch DPC over the top and he did this and no more water ever came in again.

 

Yes that was the size we used. The cladding was batten out when I was at work and I think they had to add a bit more to it as I spotted some additional strips of wood which were need for them to fix the cladding. 

 

The windows firestops were attached to the window and these were against the membrane so I guess you could deduct the size of these battens from the windows thickness and that would be how much the window protrude from the membrane.

 

Do you have any photos of your build?

 

 

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Isn't a strip of DPC over the top of the opening and up the back of the sheathing membrane SOP?

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The water would probably have done no harm, but was about 4 months before the blockwork could commence and they were the single most expensive item I have ever brought so was being a bit protective.

 

I noticed on your build that your blockwork has already started, for my build I was told that usually slating/tiling occurs before the blockwork, but was never sure why? 🤔

 

 

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11 hours ago, dpmiller said:

Isn't a strip of DPC over the top of the opening and up the back of the sheathing membrane SOP?

Alot of details out there.....I'm looking for something that's effective in relation to horizontal timber cladding, and not visible from the outside. This is presuming the dpc is lapped into membrane as you say, and directs water out over the face of the window.

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11 hours ago, Thedreamer said:

The water would probably have done no harm, but was about 4 months before the blockwork could commence and they were the single most expensive item I have ever brought so was being a bit protective.

 

I noticed on your build that your blockwork has already started, for my build I was told that usually slating/tiling occurs before the blockwork, but was never sure why? 🤔

 

 

I only have single leaf blockwork to dpc supporting my timber frame......and everything else. See photos of current state, slaters here tomorrow.

20190106_094058.jpg

20190106_094130.jpg

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Nice looking build. 

 

Are you getting involved in the labouring for the roofer?

 

My roofer spent a bit of the first day grading the slates and then left me to take them up the ladder. We were able to get three pallets onto the scaffold which saved some time.

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40 minutes ago, Thedreamer said:

Nice looking build. 

 

Are you getting involved in the labouring for the roofer?

 

My roofer spent a bit of the first day grading the slates and then left me to take them up the ladder. We were able to get three pallets onto the scaffold which saved some time.

Thanks. The roofers are coming as a team, i'll be ensuring protrusions go in the right place; flue outlet, SVP, MVHR exhausts. I've had issues with the roof membrane; It was laid a month ago on a very cold morning; the sarking boards were saturated and frozen; i had the solar pv guys coming same day to fit the panel array, needing the membrane down first. Then it rained all afternoon and evening. All seemed fine until more rain a couple of days later; it then became evident the membrane was letting in water all over the place. Subsequent storms pre xmas resulted in the same issue. My conclusion is that the membrane has failed due the conditions when it was laid,,,,,,,,,, therefore i'm opting for a fresh layer of membrane to be laid on top before the slates go down!

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5 minutes ago, Blob the Builder said:

My conclusion is that the membrane has failed due the conditions when it was laid

I have written on here before about roofsheild membrane..... this exact thing happened to me, if the material gets wet and then freezes it’s ruined. Get another sort to go over the top as there could be some serious cold weather still to come, don’t be like me and use the same shite twice..........  

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3 minutes ago, Cpd said:

I have written on here before about roofsheild membrane..... this exact thing happened to me, if the material gets wet and then freezes it’s ruined. Get another sort to go over the top as there could be some serious cold weather still to come, don’t be like me and use the same shite twice..........  

Good to know i'm not the only one who's experienced this, i've spent the last couple of weeks thinking about this situation.......new layer of membrane should be on Tuesday, immediately followed by slates, hoping for roof completion early next week....

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