Wet Rafter! Advice please!

Southern Comfort

I recently increased the amount of insulation over the ceiling of my bathroom. I laid 100mm of Moy Metac between the joists, cross laid another 100mm of Metac over it and put 200mm of regular glass wool over this. This 200mm glass wool was the original insulation I put in there last year.

The insulated area is actually a section of the crawl space area to the side of my converted attic. Access is by the trap door in the bathroom. The day I did it (3 weeks ago), I noticed that the inside of the felt was slightly wet in places, as if caused by condensation, and the very tops of one or two of the rafters (where they meet the felt), were damp also. When I re laid the insulation, I left 25mm (at least) air gaps between the insulation and the eaves, even with the 2nd and 3rd layers of insulation.

I was back there yesterday to check the dampness and it appears to be worse. One of the rafters was quite damp but the one nearest the gable wall was actually wet. Wet!!! I cannot remember what it was like 3 weeks ago. Maybe it was the same. I just didn't notice.

I presumed that the flow of air from the eaves, up along the rafters and over the converted attic ceiling would keep it well ventilated and condensation free. The rafters are 4 inch and thete's only an inch of aeroboard insulation over the converted attic ceiling, so there's plenty of room for air flow.

But, on examination, I discovered that, because the builders had put in a Velux window in that area, the air flow space between the 1st and 2nd rafters and between the 2nd and 3rd rafters, over the attic room ceiling, has been completely blocked by the wood they put between the rafters when putting the Velux window in.

1. Should I be very concerned about this? Could the roof cave in? :eek: (The rafter is perfect apart from being very wet!).

2. What should I do? In the spring, I plan to take all the tiles off the roof and replace the felt with a breathable membrane. Will this solve the problem?

Thanks in advance.

P.S. All the other rafters on both sides of the converted attic are bone dry.

Lantus

This is porbably being caued by the extreme weather we are having.

Cold air entering the loft at near or well below zero conditions is cooling the felt and possibly there is some slight frost on the inside of the felt.

As the sun hits the roof and warms it the temperature rises in the loft and it condenses out to a liquid and drips down.

Have had this in my own loft in a house less than 5 years old and it was only when we had snow and freezing conditions.

Southern Comfort

Thanks, Lantus. That's a little reassuring.

That section of roof is north facing and doesn't get any sunshine on it.

What worrries me most, though, is the fact that the rafter next to the gable wall is wet, not damp, and may have been this way every year or every winter, at least, since the attic conversion ten years ago.

Poor Uncle Tom

Southern Comfort wrote: »

...... inside of the felt was slightly wet in places,....the very tops of one or two of the rafters (where they meet the felt), were damp...........thete's only an inch of aeroboard insulation over the converted attic ceiling,

My guess is that the converted attic is not insulated well enough allowing lots of the warm moist air through to the rafter/felt where it is condensing and running down to where you see it in the crawl space, in other words it isn't coming from the bathroom itself unless the bathroom has a vent opening into the crawl space.

muffler

Poor Uncle Tom wrote: »

My guess is that the converted attic is not insulated well enough allowing lots of the warm moist air through to the rafter/felt where it is condensing and running down to where you see it in the crawl space, in other words it isn't coming from the bathroom itself unless the bathroom has a vent opening into the crawl space.

+1 to this.

Given the extreme conditions at the moment with a substantial difference between external & internal air temps. its probably fair to say that it's more noticeable now.

creedp

Hi

This is similar to a query I have in relation to a new build where I have a reasonably sized attic which I want to insulate and slab for storage and occasional use such as an overflow playroom. I have already raised this point in another thread but as the OP's query here is exactly what I am considering I though I might seek a another opinion here. Apologies if this is inappropriate.

My point is I don't want to heat the attic space on a regular basis but will have radiators installed for the few occasions it will be used. Instead I want to insulate and airtighten at 1st floor ceiling level, including attic landing, in order to keep heat in the main house. I then propose to insulate and slab/skim the attic as a seperate space. I will maintain min of 50mm ventilation space from eave to ridge. Question is could this cause condensation problems both within the insulated attic space and/or at rafter level. As I said earler this attic space will not be heated regularly and will be sealed from the main house using a airtightness membrane at ceiling level.

Would puting an airtightness layer on the warm side of the converted attic insulation help or hinder with reducing potential condensation level at rafter level? Also would venting the attic space via a hole in the wall gable help with potential condensation in attic space. Alternatively, I have 3 veluxes in this space and these could be left on vented position.

All views greatly appreciated as I will be meeting with a SIGA rep in the next couple of days to discuss an appropriate approach to airtightness in the build.

Thanks again

Poor Uncle Tom

creedp wrote: »

Hi

This is similar to a query I have in relation to a new build where I have a reasonably sized attic which I want to insulate and slab for storage and occasional use such as an overflow playroom. I have already raised this point in another thread but as the OP's query here is exactly what I am considering I though I might seek a another opinion here. Apologies if this is inappropriate.

My point is I don't want to heat the attic space on a regular basis but will have radiators installed for the few occasions it will be used. Instead I want to insulate and airtighten at 1st floor ceiling level, including attic landing, in order to keep heat in the main house. I then propose to insulate and slab/skim the attic as a seperate space. I will maintain min of 50mm ventilation space from eave to ridge. Question is could this cause condensation problems both within the insulated attic space and/or at rafter level. As I said earler this attic space will not be heated regularly and will be sealed from the main house using a airtightness membrane at ceiling level.

Would puting an airtightness layer on the warm side of the converted attic insulation help or hinder with reducing potential condensation level at rafter level? Also would venting the attic space via a hole in the wall gable help with potential condensation in attic space. Alternatively, I have 3 veluxes in this space and these could be left on vented position.

All views greatly appreciated as I will be meeting with a SIGA rep in the next couple of days to discuss an appropriate approach to airtightness in the build.

Thanks again

A balance of heating and ventilation is the best option, imo. Since you will be using this space occasionally you don't want the contents to become damp and musty so the space will require an element of heating to stop it becomming outright cold and causing you problems, Also it is not a good idea to have a 'cold element' withing the 'heated shell' of a house. Large temprature differentials can lead to problems within the house instead of between the house and the outside, just my opinion.

creedp

Thanks PUT. Maybe I'm silly to be trying to seal off the attic from the main house and creating potential problems associated with temperature differences. Its just that the attic is a pretty substantial space and given the cost of heating, the fact that the only way is up for that cost and that I don't need this space especially in the short term, I would like to be able to maintain the attic space but not be lumbered with keeping it at 20c for the heating season. The real problem is that I didn't think I would have this space as when the plans were originally drawn up I was advised that the attic space would be very low and only suitable for storage, however this didn't turn out to be the case and I have yet to figure out how that happenned!!

I might just put rads in the space and have them come on for a couple of hours a day to keep the space fresh. The MHRV guy also put a fresh air duct into this space which might also assist with keeping it fresh and heated.

Just on the last point I mentioned earlier, would it be useful or harmful to place an airtight membrane on the inside of attic insulation? Will it help or hinder the development of condensation developing within the attic space or at rafter level like the OP has in his converted attic? I will also have a membrane at ceiling level and around the attic landing so as to limit the movement or warm moist air into the relatively cooler attic space. Is the attic membrane an overkill?

Again thanks for all comments. OP sorry to have hijacked your thread but hopefully the comments I receive will also be of benefit to you.

Poor Uncle Tom

creedp wrote: »

I would like to be able to maintain the attic space but not be lumbered with keeping it at 20c for the heating season.

You won't have to keep it at 20degrees just keep it from getting cold.

creedp wrote: »

I might just put rads in the space and have them come on for a couple of hours a day to keep the space fresh.

That's exactly it, or if the rads are thermostatically controlled keep them at, say 10degrees.

creedp wrote: »

Just on the last point I mentioned earlier, would it be useful or harmful to place an airtight membrane on the inside of attic insulation? Will it help or hinder the development of condensation developing within the attic space or at rafter level like the OP has in his converted attic?

If you put an airtightness membrane which also doubles as a vapour check you will be confining any moisture in the attic room air to the room itself instead of allowing it to pass through the walling and end up as condensation on the inside of the insulation or felt surface. It would also help if you are intending on only using this room 'part time' imo, as you are not risking interstitial condensation between this room and the rest of the house. If you are going to get condensation it will be surface condensation in the room itself and can be controlled with a balance of ventilation and heating.

Lantus

To creedP

If this space is inside the insulated fabric of the house then it needs to be treated as such. You may neded to add a thermostat and just regulate it to say 12-15 degrees to ensure the internal fabric is not damaged by cold and moisture.

If you dont have serious insulation between this room and the rest of the house then your may find you expend more heat as the attic is so cold.

To southern comfort

My house faces almost perfectly north south. It was the north roof that had the drips. However, as the rafter is wet and not just damp I would suggest someone taking a look just to be on the safe side if it is as wet as you say. Could be an underlying issue thats being exposed by the cold weather.